Sequential EFI vs. Bank-To-Bank & Paired EFI

[Danny Cabral]

Bank-to-Bank or Paired EFI injects half the amount of fuel (lb/hr - PW), twice for each combustion cycle (4-stroke engine). Sequential EFI injects the total amount of fuel (lb/hr - PW), once for each combustion cycle (4-stroke engine). So even though the net amount of fuel injected is the same, sequential EFI does it with half as many injector events, by injecting double the amount of lb/hr - pulse width fuel. The total injector duty cycle remains the same (two injector events per cycle with Bank-To-Bank, Paired), but with double the amount of pulse width from sequential injection. The increased injector pulse width is why large injectors can still idle good under sequential control but not under Bank-to-Bank control (or Paired). Different injection method, but essentially the same amount of fuel consumed.

Sequential injection will always get slightly better fuel economy and cleaner exhaust emissions. Also, according to page 152 of "How to Tune and Modify Engine Management Systems" (book by Jeff Hartman), sequential injection always gains power at peak torque and at peak horsepower. However, for racing applications, the most significant benefits to sequential injection are good idle quality with large injectors (for the reasons in my quote above), improved fuel rail pressure balance (pulses), individual cylinder fuel correction (adjustments), and Injector End Angle tuning (LINK). FYI: Sequential Injection always requires a cam sync sensor/unit.

Another benefit of sequential EFI, is the capability to phase the injector timing, during the intake stroke for best efficiency. Also, since sequential EFI only injects once per cycle, the injector dead time is not doubled (like it is with non-sequential injection, which results in decreased fuel flow and sometimes requires a larger injector to compensate).
Read "Injection Timing", "Injector Dead Time" & "Individual Cylinder Trim":
http://www.motec.com/filedownload.ph...pdf?docid=2490 (Motec Glossary, page 70.)
http://www.tracklab.biz/html/motec_terms.html (← Or read the same document here.)

An individual runner intake manifold (multiple throttle bodies) is the one application where sequential injection has the least amount of benefits (power-wise), because the individual runner design gets it's own (isolated) air & fuel supply for each cylinder.

Holley's Untimed Sequential injection strategy still injects once per revolution, but without a cam sync sensor. It still injects fuel in accordance to the engine's firing order (programmed into EFI software), however, without a cam sync sensor, it can't identify #1 cylinder. Holley's Untimed Sequential injection (like full sequential) also has the benefit of good idle quality with large injectors and improved fuel rail pressure balance (pulses); even though it doesn't inject in sequence with each cylinder's intake valve opening.

Injector Dead Times - What You Don't Know Can Hurt You

The dead time compensation is added to the base pulse width to linearize the output of the injector. By linearizing the output of the injector, the fuel flow responds accurately to the simple multiplication performed by the
ECU.

Here is how it works. Let's consider a situation where we have a dead time compensation value of zero entered into our ECU. If our base pulse width is 5.000 ms, and our known injector dead time is .920 ms, the effective fuel flow of the injector is equivalent to a 4.080 ms pulse width.
(5.000 - .920 = 4.080)

Now let's consider what happens when the manifold pressure doubles, and the ECU attempts to double the fuel flow of the injector by doubling the pulse width. If we take the base pulse width of 5.000 ms times 2, we get 10.000 ms. 10.000 ms minus our known injector dead time of .920 ms equals 9.080 ms, and so our effective fuel flow is equivalent to a 9.080 ms pulse width.

To quantify the results of doubling the pulse width, we divide 4.080 into 9.080 and we get 2.225 which shows that by doubling the pulse width, we have increased the fuel flow by 222.5%. The result is a 12.5% error!
(2.25/2.00=1.125, or 12.5%)

Now let's consider the same injector and the same request for double the fuel flow with the correct dead time compensation applied.
With the correct value of .920 ms entered as our dead time compensation, our ECU will add .920 ms to our base pulse width of 5.000 ms for an actual pulse width of 5.920 ms. The resulting fuel flow is now equivalent to a 5.000 ms pulse width.
(5.920 - .920 = 5.000)

To double the fuel flow, our ECU will multiply the base pulse width of 5.000 ms by 2, and then add the dead time compensation value of .920 for an actual pulse width of 10.920 ms. The resulting fuel flow is now equivalent to a 10.000 ms pulse width.
(10.920 - .920 = 10.000)

To check our results, we divide 5.000 into 10.000, and we get 2.00 which is exactly what we were looking for.

Now let's apply the same math to a more realistic situation and see what
we get. We'll consider a light throttle cruise condition with the following parameters:
Base Pulse Width = 2.000 ms
Actual Known Injector Dead Time = .920ms
Incorrect Injector Dead Time (entered into the customer's ECU) .600ms.
Static Injector Flow = 460cc/min RPM = 2500
Lambda = .9500

The base pulse width of 2.000 ms represents the tuned value during our dyno session and includes the air temp, barometric pressure, manifold pressure and engine temp compensations before the dead time compensation is added.

With our incorrect dead time compensation value of .600 ms added to our base pulse width of 2.000 ms, our actual pulse width is 2.600 milliseconds. Based on our dynamic flow test results, we see that our actual fuel flow per injector is 32.200 cc/min.

Once our customer is in traffic, we may find that the air density decreases by 6% due to high air intake temperatures. Our ECU will make a 6% correction to the base pulse width, and arrive at 1.887ms.
(2.000/1.06=1.887)

It then adds the incorrect dead time compensation value of .600 ms for an actual pulse width of 2.487milliseconds, and an actual fuel flow of 30.034 cc/min.

If we divide our corrected fuel flow of 30.034 cc/min by our original fuel flow of 32.200, and then take the reciprocal, we get 1.072 which means that our fuel flow was actually reduced by 7.2% instead of the 6% that the ECU asked for!
((1/(30.034/ 32.200))=1.072, or 7.2%)

To find our error, we divide 1.072 (7.2%) by 1.06 (6%) and we get 1.011
If we multiply this by our target Lambda value of .950, we get .9607
Rounding this, we get a resulting Lambda of .961 and this may be lean enough to cause a slight surge, or lean misfire.
In this case, a slight leaning of the mixture may be acceptable. Maybe we tuned a bit on the rich side, or maybe the engine just doesn't mind running a bit leaner.

What if the same "over correction" occurs while the motor is under boost? Did we give a large enough safety margin to account for this? If so, could we tune for higher horsepower if we knew that the ECU could properly account for changes in atmospheric conditions?
In the real world, we all apply some safety margin to our maps, but what happens if our customer finds that his E.T.s are inconsistent because of errors introduced through improper dead time compensation?
What if our customer is running a total loss electrical system and his battery voltage changes during a pass? The errors described above could easily be amplified if our dead time/battery compensation curve is incorrect!

And what about closed loop control? How effectively will our closed loop control work if it is constantly having to hunt for the correct air fuel ratio because it either over corrects, or under corrects at its first attempt to adjust the mixture?

The short story is that all the compensations in your ECU are based on the assumption that you have entered correct injector dead times. The dead time compensation linearizes the response of the injector, and if it is incorrect, fuel flow will never accurately follow the commands from your ECU.

Common Misconceptions:
1. You can tune around incorrect dead times -
Yes you can, and as long as the engine never leaves your temperature and pressure controlled dyno room, it will run just as it did when you originally tuned it.
2. The injector dead time can be determined with an oscilloscope by measuring the amount of time it takes for the pintle to move to the fully open position -
This is incorrect. The amount of time required for the pintle to move to the fully open position will always be greater than the actual dead time of the injector. The injector dead time is related to the difference between the actual dynamic flow, and the theoretical dynamic flow.
3. I can determine injector dead times with my flow bench which uses high viscosity, low volatility mineral spirits as a test fluid -
This is only correct if you intend to run your engine on mineral spirits.
4. The injector dead time/battery compensation table only exists to account for low battery voltage -
This is incorrect. Even at alternator voltage the injector does not respond instantly and so we need to account for that with our dead time compensation values.

Paul Yaw
InjectorDynamics.com

Atomization

High atomizing injectors are usually used in Throttle Body applications only, and have a rather wide spray pattern. A wide, finely atomized pattern is wonderful for emissions and economy but can cause problems in higher performance engines. At low RPM, with a low air flow rate, the slow moving finely atomized fuel has enough time to get past the valve and create a close to stoichiometric mixture. (Air/Fuel mixture of 14.70 - Chemically ideal) As RPM increase this mass can't keep up, with valve open time, and many of the fuel droplets impinge the port wall and condense. Atomized fuel can only travel at "port air speed" and in large quantities it can actually displace air in the port. With a highly atomized mix in the port, at intake valve opening, the lighter droplets of fuel will be partly blown back up the port [intake port reversion]. This is caused by the residual exhaust pressure [overlap period] still residing in the combustion chamber. Some of this reverted mixture will adhere to port walls and condense. This puddling fuel may find its way home, on the next intake cycle, but it will cause cycle-to-cycle air/fuel ratio variances. The higher inertia of the more condensed fuel will carry it to its target. "The liquid film that wets the walls represents a capacitance that greatly reduces the transient response of the engine." (SAE 950506) This problem is compounded in Gang fire and Semi-gang fire systems, but is not as troublesome in sequential fire systems. Gang fire systems fire all injectors, every rotation, at the same time, discharging half of the required fuel at each event. Semi-gang fire systems fire groups of injectors in the same fashion, half-and-half, each rotation. Sequential systems fire each injector at a predetermined time and discharge all required fuel in one event, prior to intake valve opening. In either of the Gang fire systems there is no timing-of-event technology in operation, and as you can see it's a rather simple system.

At 8000 RPM the intake valve is opening and closing at 66 times a sec. and is only open for an average of 9 Mil/Sec. At this cyclic rate the transient time to complete the delivery of fuel from injector to valve, is critical. This is why Indy car injectors are very precisely targeted and timed to provide a solid stream of fuel with non-existent atomization, LBDS - Laser Beam Delivery System. In these engines the injectors can discharge fuel, at a "just prior to valve open position" and get it all down the hole. As the fuel impinges the hot intake valve it virtually vaporizes and mixes quite well with the incoming air forming a very homogeneous charge. This is one of the most extreme situations but it's a real interesting one. As an added benefit, the latent heat of fuel vaporized in the chamber also provides charge cooling that makes the mixture denser. A denser, heavier mixture (cold and thick) will produce more power then a thin (hot and light) charge. This is why Turbo intercoolers are so effective. Injector timing, phase angle, is altered by the ECU according to RPM in these systems and can control the delivery impact time precisely. In a Steady State Pressure Fuel System, the injector pulse is always moving at the same speed, regardless of engine speed changes. The velocity of discharged fuel is relevant to the area of the discharge port and the net operating pressure. Pressure changes activated by boost, at a 1:1 ratio, only compensate for port pressure and don't change the static pressure, flow rate or velocity. RPM adjusted fuel timing is utilized for this reason, it advances the injector timing based on engine speed, and maintains perfect impingement timing at all speeds.

It's a known fact that you can't burn fuel until it's atomized. It's also known that you can't burn fuel without air. The most important, of all known facts is that you can't burn anything, if it's not in the combustion chamber. The secret is to provide 'adequately atomized' fuel with as much air as possible. 'Adequately atomized' is the secret phrase of the day. Fuel does not have to be completely atomized at the injector tip (SMD of 10um - 20um) but it does have to get past the valve to do us any good. The more condensed the fuel delivery is the faster it will travel, (regulated by discharge area and pressure) and the more accurately it can be targeted. Recent (S.A.E.) "Injector Atomizing and Targeting" studies have provided us with one of the most prominent advances in High Performance Engine Management. These test programs have concluded that "accurate impingement onto the center of the valve head is vital for good vaporization" and "the targeting orientation of the injected fuel spray is a critical parameter in fuel evaporation" also that "fuel injected directly onto the intake valve yields a significantly better engine response" (SAE950506) What all this means is, different engine designs require a different type of injector to operate efficiently and that 100% atomization is not always required or desired. In racing situations we usually have to do the best we can with what we have or what's available. The goal, of course, is to do the best in all cases and in all situations. The best injector for your engine is the one that will yield an optimal fuel-air mixture and provide the required power output consistent with smooth and reliable operation. This is our goal, and all things considered, we feel that we provide an excellent service in this very specialized field.

Russ Collins - RC Engineering
Torrance, CA
310-320-2277
http://www.rceng.com/technical.aspx

EFI Nozzle Location

Question: I'm going to be putting electronic fuel injection onto a Buick 455 street engine. There are few choices of intake manifolds for this engine, so a custom one with EFI bungs will be necessary. I see that most aftermarket companies that make intakes with bungs design them so the injectors sit vertically, placing them perpendicular to the airflow path. If you look at most factory EFI setups, the injector sprays toward the valve, or parallel to the airflow stream. Would it be worth my while to attempt to spray the injector toward the valve, or should I keep it perpendicular to the airflow? Are there any gains to be had in angling the injector?

Answer: High-end experts like turbocharging guru Ken Duttweiler and EFI University's Ben Strader have spent countless hours on the dyno and in the lab playing with fuel-injector location and angle. There are several observable trends from this research, but in the end, any individual engine may deviate from theory or previously observed trends, so the following should be taken as only a baseline recommendation, as your outcome may vary.

Basically, there are three factors that have to be juggled: idle quality (which would also include emissions in an OEM smog-legal application), proper fuel/air atomization, and the physical constraints of the engine and intake-manifold configuration. These factors combine to determine injector location and angle within the intake manifold's inlet runner.

In a perfect world, nozzle location should be as parallel to the airflow stream as possible. The nozzle angle in relation to the airflow stream is termed the "intercept angle." According to Strader, the intercept angle should "not be more than 45 degrees, although it can be less." Maintaining the proper intercept angle generally helps low-speed driveability and may also improve performance throughout the engine's operating band. The lower the inlet airspeed at idle, the more critical it is to maintain the ideal intercept angle. Idle vacuum correlates well with inlet airspeed-if you have 14-18 inches Hg of vacuum at idle as read on a vacuum gauge, maintaining the proper intercept angle is not as much of an issue in terms of driveability, although there still could be some emissions ramifications.

So much for injector angle-what about injector placement? Should it be closer to the valve (downstream, near the cylinder head) or closer to the air meter (upstream, toward the top of the inlet runner)? It depends on the engine and application. A stocker is primarily concerned with idle quality, low emissions, fuel mileage, and engine-compartment packaging constraints. On a stocker, fuel-injector capacity (rated in lb/hr) is low (compared with a race engine), while inlet-runner velocity and low-speed vacuum are high. The small-capacity nozzle develops a good spray pattern that disperses uniformly within the incoming air stream. With good atomization, the nozzle can be located downstream, close to the valve. Small injectors don't have a lot of fuel to waste, so targeting the spray toward the back side of the valve ensures that the available fuel is used most efficiently. On the other hand, in theory, high-idle vacuum generated by mild stock engines permits placing the injector farther upstream without significant low-speed driveability degradation. In the end, OEM style downstream injector placement simplifies system packaging and makes it easier to mount the fuel rails.

Everything changes with really large injectors (over 96 lb/hr). High-capacity injectors generate a relatively poor spray pattern with a large fuel-droplet size. As Duttweiler puts it: "You're practically just spraying raw liquid. If you put a big injector too close to the valve, there's not enough time for the fuel to mix with the air." Large injectors would most likely be used in large-displacement or high-RPM engines with lumpy cams. High RPM translates into less time between injector firing pulses, lumpy cams generate poor vacuum, and the typically large-volume inlet runners needed to feed all those cubes generally mean lower air velocity downstairs. Obviously, all this adversely affects proper fuel atomization. Moving the injector farther away from the valve allows more time for the air/fuel to atomize properly and remain in suspension when air velocity comes up at high RPM. This should improve peak power but-because of poor low-RPM velocity-at the expense of idle quality (there's no free lunch).

Looking at some real-world examples, Strader reports that on a 1,000hp engine, the injectors were originally located 7 inches back from the valves. Doubling this distance to 14 inches was worth 50 hp on top, a 5 percent gain-but "it wouldn't idle below 1,600 RPM." For an even more extreme example, consider the injector placement on today's 15,000-RPM Formula I engines. The injectors, wiring harness, and fuel-distribution rails are located topside, even inside the manifold plenum area, so they can maintain the proper intercept angle.

In the real world, mass-produced aftermarket cast-aluminum manifolds have the bosses added as an afterthought to a preexisting design. The placement is more for convenience than for best engineering practice-the available packaging architecture (including fuel-rail mounting and clearance) to a large extent dictates the nozzle location. A decent compromise for a hot-rod engine is to locate the nozzle about 1-2 inches upstream from the manifold flange to give atomization a chance, positioning the fuel rail at the best angle you can get away with and still package the harness and fuel rails. As Duttweiler puts it: "If you aim the injector more toward the valve, the fuel rail usually hits the plenum" on a converted classic V-8 carburetor-style intake. Note that at the OEM level, the trend on today's new-tech V-8 engine designs is to make them wider than a similar-displacement, old-school, classic engine. The included valve angle in some of the new late-models is nearly straight up and down in relation to the bore. That means the runners are also near vertical, which in turn allows mounting the injectors more vertically to provide room for the fuel rails and wiring harness while still maintaining a good intercept angle to the runner.

Duttweiler Performance
Saticoy, CA
805-647-5732

EFI University
Murrieta, CA
866-316-7744 or 909-972-6865

http://www.hotrod.com/pitstop/hrdp_0...ctor_location/

[Danny Cabral]

This three part article is written by Ben Strader of EFI University. (I contacted EFI University for this complete document and permission to post it.)

Air/Fuel Ratio Management For Racers, A Three Part Series

When it comes to racing, there is never any shortage of hard work and chores to be done before the next event. Often, race teams are required to travel long distances during the week, prep the car, show up on the weekend ready to run, and then do it all over again the next week. This doesn’t leave much time for experimentation and trying out new concepts. That means most of the time, when racers find something that works okay they tend not to change it, even though there might be a better way. They simply can’t afford to risk missing an event or losing a race.
Often times a discussion arises about the best Air to Fuel ratio to use for various tracks and atmospheric conditions. I want to try and address a few of these questions in this series.
Here is a list of some common questions asked by racers and tuners:
1) What Air to Fuel Ratio gives the best power?
2) Does the Air to Fuel Ratio that produces the best power change as the altitude my car operates at changes?
3) Does the Air to Fuel Ratio that produces the best power change as the intake air temperature my car operates at changes?
To find answers to these questions I have spent years on the dynamometer testing various engine combinations, talking with other knowledgeable tuners, reading various publications on the subject, and even wrote a book about tuning Electronic Fuel Injected engines, but I found the most convincing answers to these questions in a document written in 1922 by Stanwood Sparrow of the “Bureau of Standards” for the “National Advisory Committee for Aeronautics” (NACA), called NACA Report #189 “Relation of Fuel-Air Ratios to Engine Performance”.
In this report, a government agency set out to answer these and many other questions about the effect of Air Fuel Ratios on engine performance over a wide range of parameters, and the evidence proves out many of the answers I am about to present to you for the above questions in this three part series.

PART I
What Air to Fuel Ratio gives the best power?
Many folks have tried to shed light on this subject based on single-case observations made in sloppily controlled test environments which show results of all sorts, and yet other, seemingly more knowledgeable sources, (such as the companies trying to sell Air-Fuel ratio meters) are constantly trying to convince us that while they cannot (for reasons of liability) tell us what the magic number is, we cannot possibly hope to achieve maximum engine performance without the help of one of their whiz-bang doo-hickeys!
Well, according to NACA report 189, a wide variety of engines were tested across a large range of Air/Fuel ratios and what they found was basically the following:
“In adjusting the carburetor to obtain maximum power, The following method was employed. First, the mixture was altered until approximately maximum power (for the chosen set of conditions) was obtained. As will be shown later, values of power within 1 percent of maximum are obtained over a wide range of fuel-air ratios. Hence, little difficulty was experienced in finding an Air/Fuel ratio to give approximately maximum power.”
The report goes on to state later that, “From the results to date it is concluded that ordinarily maximum power (at least in so far as aviation engines are concerned) is obtained with gasoline-air ratios of between 0.07 and 0.08 pounds of fuel per pound of air (12.5 to 14.5 pounds of air per pound of fuel).”
What all this means is that basically, if simply making lots of power is your only goal, nearly any Air/Fuel ratio can get you pretty close to the mark.
This corresponds quite closely to what my years’ of engine testing have show as well, and in fact, this is what we have been teaching at EFI University for almost four years, but what I find surprising is the number of supposedly “expert” tuners out there who are still arguing against this point, and pretending that what they do is a special brand of “Magic”.
My experience has been that typically the best engine tuners in the business are the first ones to say: “Ask me anything you like, I have nothing to hide.” Recently, I spoke with Shane Tecklenburg of FAST Motorsports in Huntington Beach, California, who is widely regarded to be one of the finest engine tuners in the USA, and he had this to say: “Nothing I do is black magic. Everything is based on simple laws of physics that anyone can learn with a little effort, so there is no reason for me not to answer a racer’s question about engine tuning, even if he is a competitor.” I have also spoken to a number of other well known tuners who have had quite the opposite attitude and tried to make it seem as if they knew some special trick or held the golden nugget of knowledge that, if shared with others would seriously jeopardize their standing. Most of the time, when I find a tuner with this attitude, it means they don’t actually know the answers and fear they might reveal this ugly fact if they say too much.
The simple fact is, ten years ago, before the age of $300 wide bands for everyone, nobody even knew what their Air Fuel Ratios were. The rule of thumb was to change the jets one size for every one-thousand feet of elevation, and that was just the way it was. We looked down the tailpipes and at our spark plugs for various color patterns, and even that wasn’t an exact science.
Most racers would have been horrified if they actually saw what the A/F ratios were doing in their engines during a run, but because the engine still performed well, no one cared. What has changed the industry so dramatically in recent years is the advent of the low-cost wide-band Air/Fuel ratio meters. Suddenly, everyone could afford access to this tool to gain priceless insight into their engine’s performance, and then “numbers game” began.
It is not uncommon to go to the racetrack these days and find any number of racers with their laptops plugged into their cars trying to get that last tenth of an A/F point in line. I’ve heard guys say “yesterday she was running a 12.8 A/F ratio, and today it seems to be running about 12.7 and that’s just too rich!” I wonder if either their dyno, or their E.T.’s would support that. If what the NACA report says is true, then I suppose it begs the question, “What is to be gained by agonizing over minute changes in A/F ratios”? Isn’t there some other chassis or tire component that would be better served by spending this time tweaking them instead? What good is ultimate power if it can’t reach the ground?
I’m not suggesting that we abandon this great new technology and throw away our wide-bands just yet. I simply want to help folks get back to the reality of what it is we are trying to accomplish: Getting the maximum performance from the engine… not getting bogged down in the data. Let’s all take a step back, close our eyes, take a deep breath and remember, the only numbers that really matter are not the ones on the wide-band, but the ones that say the letters “E.T.” next to them! Good luck out there folks!

Coming up in Part II:
Does the Air to Fuel Ratio that produces the best power change as the altitude that my car operates at changes? Tune in next time to find out!

Hello again everyone! It’s time for another installment of our three part series called “Air/Fuel Ratio Management For Racers”! In our last issue, we discussed ways to evaluate the correct ratio of air and fuel for your engine when trying to make maximum power. We used a paper written in the 1920’s that contains evidence, which still holds true today! The thing is, making max power is one of the easier tasks involved in mapping an EFI equipped engine. The hard part is getting the consistency, and reliability out of the engine that will win races!

So, once you’ve tuned your engine on a dyno at one location, what happens if you go racing somewhere else? Will the altitude and air density changes dramatically affect the engine, and if so, should you be thinking about using a different air/fuel ratio to maintain the best power? Let’s take a look!

PART II
When I find the right Air Fuel Ratio for maximum power, will that number change when I race at tracks of various altitudes?
When we first start thinking about a solution to this particular problem, we must begin with a strong understanding of what happens to the engine when we change altitudes.

First, and foremost, it is extremely important for us to recognize that as we gain altitude, the air gets both thinner and colder. We say that the “density” of the air is less.
Density of the air can be described as the “weight per unit of volume”. What I mean is this: If we have two empty one-gallon milk jugs sitting on a table and we fill one all the way to the top with goose feathers and one all the way to the top with sand, it should be obvious to most folks that the gallon of sand will weigh more, even though both containers have the same unit of volume: one gallon.
So, the air we breathe, much like the air our engines draw in, gets less dense as we go up in altitude, but the volume of air inside the engine does not change. We say that the “Volumetric Efficiency” of our engine stays the same!
So, if the engine’s Volumetric efficiency is the same at any altitude, then why does the engine make less power when we go up?
Because the quantity of air we are concerned about when supplying fuel to the engine is called the “Mass” of air. The “Mass” is determined by multiplying the volume of air you have by what it weighs.

So, M = V x D, where:
M = Mass in Lbs/ minute
V = Volume of air in CFM
D = Density in Lbs/ Cubic Ft

So, if our engine is flowing 650 CFM for example, then at sea level where one cubic foot of air weighs about .076 pounds we would have a Mass of about 49.4 Lbs of air per minute.
Ex: 650 * .076 = 49.4

However, if we take the engine up to the top of a mountain where one cubic foot of air might only weigh .064 pounds, we would only net 42.18 Lbs of air per minute!
Ex: 650 * .064 = 42.18

That’s about a 15% reduction in power at the SAME volumetric efficiency!
So, the question becomes then, since I’m making 15% less power, should I change the A/F ratio to be 15% leaner?
The answer: NO! Leave it the same.

We are talking about a “ratio” of air to fuel, so since the air is 15% less, we will want the fuel to be 15% less as well, which would still net the same Air to Fuel Ratio!
This means we can build some automatic compensation tables into our ECU settings to detect a change in altitude or barometric pressure and add or subtract fueling as necessary. Wouldn’t that make life so much easier?

If we reference our previous document, “NACA Report #189 “Relation of Fuel-Air Ratios to Engine Performance” to try and solidify the above statements we’ll see what our old buddy Stanwood Sparrow has to say.
On page 111 of the report, we find following:
These tests were done at various air pressures from sea level all the way to 30,000 feet in elevation, and the results show that it is desirable to maintain the same ratio of air to fuel at all altitudes.

Of course, what we are talking about here is only the A/F ratio that produces maximum power. This is not to say that better fuel economy might not be possible by a reduction in A/F ratio due to the fact that since we are making less overall power, we may not need as much fuel for cylinder cooling, and could find some savings there. Then again… we are talking about RACING, not grocery shopping, so lighten up a little and get out there and have fun!

There are always a lot of variables in engine tuning that we could continue to converse about until the cows come home, but the only thing left to answer in our three part series is whether or not we would need a different A/F ratio when the intake air temperatures get hotter or colder.

Tune in next time for an in depth description of how air temperatures affect air density, as well as how they affect the way our engine runs! See you there.

Hello again everyone! It’s time for the final installment of our three part series called “Air/Fuel Ratio Management For Racers”! In our last issue, we discussed how the engine reacts if we tune it on a dyno or at a track in one location and then take the engine to a totally different altitude or location. We found a government study put out in the 1920’s shows that the same air to fuel ratio would be required of the engine at any reasonable altitude. The engine made less power overall due to the lack of air density, but the ratio of air to fuel did not need to be changed because of this. Knowing this, the only thing a racer needs to do is make sure to maintain the same air fuel ratio at the track that they found to work when they were on the dyno!

In this final article of the series, we wanted to ask the question: “What happens to my engine at various inlet air temperatures, and how does this affect my choice of air fuel ratios?” Let’s take one last look at our favorite document, “NACA report 189” to see if we can find the answer!

PART III
When I find the right Air Fuel Ratio for maximum power, will that number change when I race at tracks with different temperatures?.

If you’ll remember back to the last article, we used a mathematical formula to calculate the mass of air that went something like this:
Mass = V*D
Where:
V = the CFM of air the engine was breathing,
and
D = the density, (or weight) of one cubic foot of air.

There are primarily two things that affect the density of air. One is the air pressure, and the other is the air temperature.
We can use the following formula to determine how much one cubic foot of air weighs:
Density = 2.7 P/T
Where:
P = PSI (absolute)
And
T = Temperature in degrees Rankine (Degrees F + 460)

If we use the standard temperatures and pressures at sea level, we will find that one cubic foot of air weighs around .076 Lbs.
Ex: 2.7 [14.7/(60 + 460)] = .076

Now, if we simply plug in different values for various altitudes or temperatures, we can find out how much change in air density we have and then add or subtract fuel from the engine accordingly to maintain the same air to fuel ratio.
Take a look:
Let's say we are up in the mountains, and the barometric pressure is down to around 12 psi absolute, (which is around 24.4 inches of mercury, or about 82 kPa), and the outside temperatures are about 40 degrees F.

Using the above formula, we see that:
D = 2.7 P/T
D = 2.7 [12/(40 + 460)]
D = .0648 Lbs per cubic foot
So, .0648 / .076 = .85 or about 85% of the original air density at sea level!

That means in order to keep the same air to fuel ratios, we would need to subtract about 15% of the fuel we were previously giving the engine!
We can very easily program a table into the engine computer to automatically measure the intake air temperatures, and then add or subtract fuel to maintain a constant air fuel ratio at all temperatures.

The question is though, do we need a different air fuel ratio when the air gets very hot, or very cold?
Well, to find the answer, we must once again visit “NACA Report 189”.
On pages 111 and 112 we see this following statements, (which are paraphrased here):
“An analysis of a large number of tests covering an inlet temperature range of –20 C to +40 C has shown maximum power to be obtained with approximately the same air fuel ratios at each temperature.”

This would indicate that one would always want the same air fuel ratio, regardless of the inlet temperatures. However, the report goes on to state the following:
“The volatility of the fuel is in reality the determining factor in this question. A constant fuel air ratio is desirable only so long as a change in air temperatures does not appreciably change the relative quality of the mixtures supplied to the various cylinders or the amount of fuel that has been vaporized at the time the compression stroke is completed.”

Essentially, what they are saying is that if the intake temperatures are so hot or cold that they cause the fuel to be ignited prematurely, causing detonation, or cause the fuel to remain in a more liquefied, unvaporized state, which would make it not ignite so easily then the need for a richer or leaner air fuel ratio might exist.

Overall, what we learned from this is that if the fuel being used is fairly stable, and the temperatures encountered while racing are not extreme, then a constant air fuel ratio is desirable across a wide range of air temperatures. If however, the temperatures your engine will see are extreme, then there is a possibility that a change in air fuel ratios might be warranted.

However, most ECU manufacturers have understood this for some time, and nearly all give you one or more tables to create a method for adding or subtracting fuel as the inlet temperatures increase or decrease.

Hopefully, this series of articles has given you a small amount of insight into understanding the engine’s requirements when it comes to selecting, and maintaining a given air fuel ratio. Only thorough testing and some trial and error will tell you what is exactly right for your engine, but perhaps with a better understanding of the factors involved we can shorten the time spent tuning, and increase the time spent racing and enjoying your vehicles! See you at the track!

[Danny Cabral]

Air Fuel Ratio:
In addition to the ignition timing, the other aspect of vehicle tuning that is most commonly addressed is the fuel delivery. The amount of fuel being sent into the combustion chambers is commonly measured as an "air / fuel ratio" which is just like it sounds - a number representing the ratio of the amount of air to the amount of fuel being burned in the engine. An internal combustion engine mixes fuel with oxygen in the air and then ignites that mixture with a spark plug. From a strictly scientific point of view, the optimum mixture of air and common gasoline is around 14.6 parts of air to every one part of fuel for an air/fuel ratio of 14.6:1. At this ratio and under the right conditions, all of the gasoline and all of the oxygen can burn leaving nothing except for the combustion products. This is called the "stoichiometric" ratio. It is just like the ratio of 2:1 for hydrogen and oxygen, as when they react (or burn) in that ratio and under the right conditions, everything is used up and only water (or H2O) is left. Fortunately for us, the oxygen in the air is never completely used up when gasoline is burned. The main reason for this is the fact that air is only 20% oxygen, and the remaining 80% is comprised of things that will interfere with a perfect reaction. If one were to mix gasoline with pure oxygen, the stoichiometric ratio would be approximately 3:1 and the reaction would be entirely more dramatic with a much greater chance of a "complete burn."

Higher ratios, or mixtures that contain more air than what is desired are considered "lean." Lower ratios, or mixtures that contain more gasoline that what is desired are considered "rich." The two terms are used much like "retarded" and "advanced" when describing ignition timing. Historically, the terms were used to describe deviations from stoichiometric, much like "retarded" and "advanced" were used to describe deviations from TDC, but they can also be used to describe deviations from the desired point and changes to the current state. "Leaning" the mixture means adding air (or reducing fuel) and "enriching" the mixture means adding fuel (or reducing air.)

Since there is always some oxygen left in the exhaust gas stream of a running engine, we have an easy way of measuring the air/fuel ratio. An "oxygen sensor" can be used to measure the percentage of oxygen left in the gas stream, and a computer or other electronic device can be used to back-calculate the air/fuel ratio that will result in that particular oxygen percentage. "Narrow-band" oxygen sensors respond with a voltage output that is sent to the computer that is between 0 and 1 volt. "Wide-band" oxygen sensors send a 0 to 5 volt signal which allows for a much higher resolution and are therefore much better for tuning. "Lambda" is a commonly used term that is used in place of the air/fuel ratio number, as many devices use or report lambda values. A lambda of 1.0 is equal to the stoichiometric ratio (14.6:1 for air/gasoline) and is adjusted accordingly - a lambda of 0.82 is equal to 12:1 air/fuel ratio.

The "best" air/fuel ratio for a particular vehicle is a matter of great debate and I will do my best to avoid that debate in this article. Simply put, there are a number of factors that one must consider in determining the best ratio, including power, safety, and fuel economy. Fuel economy is the easiest to understand, as a lower air/fuel ratio means more fuel and obviously lower fuel economy. As far as safety is concerned, richer is considered safer (to a point) as the extra fuel helps things run cooler. The lower temperatures help reduce the chance of autoignition and can literally keep engine components from melting. The safest air/fuel ratios are continuously being debated, but it is widely accepted that 13:1 is a good ratio for normally aspirated engines and 12:1 is good for forced induction engines. Many choose to go even richer, even 11.5:1. Autoignition (or "detonation" or "knocking") is considered a critical concern with rotary engines, and many tuners choose to go even richer than that. One must also keep in mind that these "safe" ratios are considered safe because they have been tried with many thousands of vehicles over many years by dyno operators that use the same equipment that most people are likely to encounter. Therefore, a safety margin that takes into account the accuracy of that equipment is inherently factored in. If it were common for turbocharged cars to blow up at 12:1 as measured on commonly used equipment, then the "safe" air/fuel ratio would have been lowered.

As far as power is concerned, I'll say only this: Every vehicle is different. If one wants to find the best air/fuel ratio for generating power, one should put the vehicle on a dyno and test it. Many believe that a particular ratio will result in the most power under any circumstances, and that belief is just too narrow-minded. There are far too many factors involved to make such blanket statements.

Regardless of the actual ideal air/fuel ratio number, almost everyone wishes to see a nice, flat air/fuel graph. This means that the ratio stays constant throughout the RPM range. A perfectly flat air/fuel graph is certainly not necessary for optimum engine performance or safety, but it is a nice thing to show off when tuning a vehicle. The smoother the air/fuel curve, the better the driveability will be and the smoother the power output will be. All good tuners realize that a little variation with the graph is perfectly acceptable, especially when one considers the factors involved. One must consider the accuracy of the oxygen sensor, where it is placed in the exhaust stream, the velocity of the exhaust stream at different points in the RPM band, the tools that the tuner has at his disposal to make changes, etc. Another important factor is that most air/fuel ratios are measured via a tailpipe sniffer. This method has proven to be an excellent way of measuring the ratio, but it is not perfect at low RPM. At low RPM, an engine may not be producing enough gas to displace all of the atmospheric air in the tailpipe, and this will produce a false lean reading because of the extra oxygen - as one can see in this chart. This phenomenon is going to be more pronounced in small-bore engines with large diameter exhaust piping. Two important things must be considered when one is tuning with a tailpipe sniffer because of this phenomenon. One, a flat line across the entire RPM band will mean that the actual air/fuel ratio is too rich at low RPM. Two, a real-world driver is almost never at wide-open-throttle at such a low RPM, so the air/fuel curve at that point is something that the driver will never see. One can also see from the chart that the catalytic converter has no significant effect on the air/fuel ratio in this particular vehicle.

The flatness of the air/fuel graph when one is done tuning is mainly going to depend not on the competency of the tuner but on the type of fuel management system being used and its resolution, and the patience of the customer and/or his willingness to pay for dyno time. One must also ask - is a perfectly flat air/fuel curve best? Many assume that a flat line at 12:1 or 13:1 "across the board" is best, but why is that? How could it be possible that the exact same air/fuel ratio be optimum for every RPM & load? This idea has been largely ignored in automotive enthusiast circles, as "good" tuners with adequate engine management equipment produce air/fuel curves that are flat "across the board" at the desired ratio. Thankfully, this notion has been challenged recently, and experienced racers and tuners have begun to realize that air/fuel curves should not necessarily be flat. Turbos can spool up faster if the ratio is a little lean during that time, and rich ratios are more needed in the higher RPM range where more heat is being produced. Keep in mind that wideband oxygen sensors have only been in widespread use since the late '90s, and chassis dyno testing has only become truly popular in recent years. All of us are still learning. Few people have been able to perform true scientific experiments, and therefore few people truly have the knowledge to make blanket statements concerning what is best for a particular vehicle or group of vehicles.

When performing dyno testing and tuning, one must ask oneself "what am I trying to achieve?" If maximum power is the goal, then just look at the power curve first and make adjustments accordingly. The fuel curve is only used as an aid. Many NA race car owners tune in this manner, and by the time they are done the air/fuel ratio is sometimes between 14:1 and 15:1. This is usually not considered "safe" by anyone, but most race car teams accept the fact that they usually change the engine at least once during a typical season. Most street car owners are willing to sacrifice the 3-5 HP that they might get by running so lean and instead opt for an air/fuel ratio that will help their engine last for many years.

Ignition Timing:
Making changes to the ignition timing is one of the easiest ways to increase the power and efficiency of a four-stroke internal combustion engine and it has therefore become one of the first things an engine tuner will address. Ignition timing is a term that defines when the spark plug fires in relation to the piston's position within the cylinder. Without all of the knowledge that I'm about to give you, one would naturally assume that the spark plug should fire and ignite the air/fuel mixture when the piston is at the top of the cylinder and the air/fuel mixture is compressed as much as possible. At this point (referred to as Top Dead Center or TDC) the igniting air/fuel mixture will rapidly expand and push the piston back down, powering the engine. Unfortunately for all of us trying to tune our engines, there is one thing that prevents us from doing something so simple - it takes some time for the flame front to ignite the air/fuel mixture once the spark plug fires, therefore the spark plug must ignite a short time before TDC to achieve the desired result. It only takes a matter of milliseconds to burn the mixture in a cylinder under any circumstances, but when pistons are flying up and down at the rates they do in an engine, those milliseconds become critical. If the spark plug fires when the piston is at TDC, the piston may be well on its way down the cylinder by the time the air/fuel mixture is completely burned. To make matters even more difficult, there are a number of factors that can greatly affect the speed at which the mixture burns, including cylinder shape, mixture strength (lean or rich), type of fuel, compression ratio, how much air/fuel is in the cylinder, pressure, temperature, and humidity. Since cylinder shape, compression ratio, and the type of fuel are going to remain constant for an engine while it is being tuned, this article will concentrate on the remaining factors. Compression ratio is something that can be changed between tuning sessions, so one should know that an increase in compression ratio can be treated as a general increase in the cylinder pressure, which will be discussed in detail. The type of fuel used also greatly affects ignition timing, but it will only be briefly discussed here as a full discussion would double the size of this article. To learn more, see the link at the bottom.

The units most commonly used for ignition timing are degrees BTDC (Before TDC), with 0° BTDC meaning that the piston is at TDC when the spark plug fires. If the ignition timing is at 10° BTDC, then the spark plug fires when the crankshaft is rotated 10° before the piston is at TDC. If there is any number of positive degrees BTDC and the spark plug fires before the piston has reached TDC, then the timing is considered to be "advanced." If the degrees are negative and the spark plug fires after the piston has reached TDC, then the timing is considered to be "retarded." These two terms are also commonly used when making changes to the timing, as increasing the degrees BTDC is referred to as "advancing" the timing, and decreasing the degrees BTDC is referred to as "retarding" the timing. Engines usually have marks on the balancer or pulley that is attached to the end of the crankshaft and a mark(s) on the engine block. In order to determine an engine's timing, a timing light is used. A typical timing light has an inductive pickup that clamps over the number one spark plug wire. When current passes through the wire and the spark plug fires, a signal is sent to the timing light, and the light flashes at the same time as the number one spark plug. The flashing light will appear to freeze the motion of the crankshaft, and the timing can be read with the marks. Normally, 0° BTDC is when a mark on the balancer lines up with a mark on the block. Some engines have many marks on the balancer or the block indicating degree increments, but others have only one. When there is only one mark on each, an "advance" timing light must be used. This kind of timing light has a dial on it marked in degrees. One operates it by turning the dial until the two marks line up and then reading the degrees from the dial.

Now we get to the good stuff. In order to make the most power, the spark plug must fire at the right time so that the air/fuel mixture is completely burned at about the time when the piston reaches TDC so that the expanding gases can shove the piston back down. If the ignition happens too late, the expanding gases are shoving against something that has already moved away on its own and full power is not realized. If the ignition happens too early, the expanding gases are shoving against a piston that is moving towards them, and they will actually slow the piston down. If this condition occurs when one is starting the engine, one may experience "kick-back," as the engine doesn't yet have enough momentum and the starter motor isn't strong enough to overcome the gases trying to push the pistons backwards. Once the engine is running, this overly-advanced condition may become evident with a "knocking" or "pinging" sound. Parts of the air/fuel mixture will auto-ignite (ignite on their own, with no spark plug firing) if enough pressure and/or heat is applied. If the spark happens too early in the cycle, the air/fuel mixture parts that are sensitive to autoignition can react (ignite) from the pressure created by sandwiching them between the rising piston below them and the flame front and corresponding shock wave from the burning air/fuel above them. This autoignition can create quite a shock wave of its own that is commonly heard as the "knock." Unfortunately, this shock wave can damage and eventually destroy the engine if it happens too often. This condition is more prone to occur in high heat and the higher pressures caused by higher compression engines and by forced induction. Higher octane gasoline produce mixtures with air that are less likely to auto-ignite, so their use will lessen the likelihood of knocking and allow the engine tuner to advance the timing further.

"Pre-ignition" is when the air/fuel mixture auto-ignites from excess heat and/or pressure before the spark plug has fired, and has little to do with the ignition timing. Unfortunately for the tuner and the customer, it can sound exactly the same as an autoignition from timing that is too advanced. Lowering the pressure and/or the heat in the combustion chamber will reduce the chances of pre-ignition. Lowering the pressure on a forced-induction engine is as simple as lowering the boost, but on a normally aspirated engine it may be as daunting as changing a head gasket to lower the compression ratio. Fortunately, pre-ignition may be caused by something as simple as a spark plug that is too "hot." This heat range rating on the spark plug refers to its thermal conductivity and its ability to dissipate heat. A "hotter" plug will retain more heat in its tip and may stay hot enough to ignite the air/fuel mixture at an unwanted time. Therefore, "colder" plugs are desired when the pressure in a combustion chamber is increased. Care must be taken when choosing a heat range for spark plugs, as plugs that are too "cold" will result in poor starting and driveability.

As an engine speeds up, the spark plug should fire earlier (timing should be advanced) for the simple reason that there is less time for the combustion to happen as the piston speeds increase. The engine tuner's job is to make that spark happen at just the right time throughout the RPM range. For those that want to get technical, it turns out that the spark should occur at a point so that the flame front travels through the mixture and burns it completely, and the heated, expanding combustion products reach a maximum pressure when the piston is about 15°-20° after TDC. With a dynamometer at his disposal, that job would be very easy if it weren't for all of the variables mentioned earlier. Fortunately, the effect those variables have is very well understood and they can be accounted for if the engine has the right management system.

Under normal circumstances, pressure has the most significant effect on the ignition timing. When the pressure of the air/fuel mixture increases, the flame front travels through it much faster as the front has less space to jump when traveling from molecule to molecule since the molecules are closer together. The same can be said if more of the mixture is in the cylinder or if the temperature is lower, as cold air is more dense than warm air. An increase in humidity will also act like an increase in pressure as the extra water molecules will help bridge the gaps between the air molecules, increasing the speed the flame front can travel. Since an increase in pressure results in a decreased combustion time, the ignition timing must be retarded as the pressure increases to avoid knocking. As far as what makes the pressure change - the cylinder pressure increases as the load on the engine increases and/or if forced induction (turbocharger, supercharger) is used. When an engine is idling or under light load and there is little pressure, the ignition timing may be advanced. Once the engine experiences a load or forced induction kicks in and the pressure builds, the timing must be retarded.

Since measuring the pressure inside the actual cylinder would be highly impractical, engine management systems use the intake manifold pressure. Newer vehicles have what is known as a Manifold Absolute Pressure (MAP) gauge or sensor. The absolute pressure measurement has the normal atmospheric pressure (14.7 psi or 1 bar) factored out of it, so a full vacuum would read zero and normal, sea-level atmospheric pressure would read 14.7 psi on an absolute pressure gauge. In the engineering world, this differs from gauge pressure which really measures the difference between atmospheric pressure and the thing that one is measuring. For example, your fuel pressure would be read in gauge pressure, and the units would technically be "psig" as opposed to "psia." If the fuel pressure was 30 psi (psig), it would actually be 30 psi over atmospheric pressure. An absolute pressure measurement is used instead of gauge pressure so that the vehicle's management system may include the surrounding barometric pressure and know what the "real" pressure inside the manifold is.

Newer vehicles have sophisticated electronic management systems that control everything that happens within the engine and even some things that happen outside of it. These systems are commonly referred to as "Engine Control Units" or ECU's. For those with such a system, tuning the engine becomes a matter of electronics and computer software. The distributor has disappeared, and multiple coils, sometimes one for each spark plug, have taken its place. Through the use of sensors such as the MAP sensor, the Mass Air Flow (MAF) sensor, and the crankshaft position sensor, the ECU can monitor all of the variables within the engine along with the environmental conditions, and tell each spark plug exactly when to fire. If one has such a vehicle with an ECU that is not programmable and can not be changed with the addition of a chip, then one is going to be very limited when it comes to tuning. If one is fortunate enough to have a fully programmable ECU, then one may change the ignition timing and possibly many other things as well until one has gone absolutely insane. Many non-programmable ECUs can be reprogrammed, or "re-flashed" with new programs, and others can accept aftermarket add-on chips that change the programs. One may have to perform a little research to determine what type of ECU is in the vehicle.

Older vehicles with distributors can be much simpler to tune (sometimes.) Besides the timing light and a wrench, no fancy equipment or computer knowledge is necessary. Way back in the day before crank position sensors, ECU's, and individual coil-packs, the distributor was used to determine when each spark plug should fire. The distributor can be a very simple device, consisting of a rotor and a cap. Through the use of chains and gears, the rotor spins at the same speed as the crankshaft. Inside the cap are "points," which are small metal tabs. There is one point for each spark plug, and the rotor periodically touches the points as it spins. The rotor is connected through a high voltage coil to the battery, and the points are connected via spark plug wires to the spark plugs. As the rotor spins, it contacts the points, completing the electrical circuit and sending short-duration, high-voltage currents to each spark plug, hopefully in the correct order. Changing the ignition timing can be done by simply twisting the cap in relation to the rotor so that the points are touched a little sooner or a little later. Unfortunately, this changes only the "static" timing, which is the ignition timing that the engine will see throughout its entire RPM range if no other timing devices are used. If no other timing devices are used, then the timing can not change along with the pressure, RPM, and all of those other variables, and it will be impossible for the engine to operate at peak power and efficiency over the broad range that it must during every day driving. For racers that operate in a very narrow range, static-only timing may be sufficient.

Fortunately for everyone else, the automotive engineers way back in the day really knew what they were doing, and they came up with two devices that allow the engine to operate with greater power and efficiency over a very broad range. The two variables that have the greatest impact on timing, RPM and pressure, are taken into account with the "mechanical (or centrifugal) advance" and the "vacuum advance" devices. Using the same principal that keeps water in a bucket when one swings it around in a circle, the mechanical advance consists of weights on springs that move away from a spinning shaft. The weights are attached to some other movable parts and levers that will create the same twisting action between the rotor and the cap as changing the static timing by hand. As the engine RPM increases, the weights move further out, changing the rotor-cap relation further. As stated earlier, an RPM increase should advance the timing, so the mechanical advance device is used to advance the timing as the engine's RPM increases. The device used to factor in the pressure in the cylinder is the vacuum advance. It is a device that creates the same result as the mechanical advance, but it does so in response to a low-pressure situation in the intake manifold. Under low-load conditions the air rushing through the manifold creates a partial vacuum (negative readings on a psig gauge, readings below normal atmospheric pressure on a psia or MAP gauge) and the device advances the timing. As the load increases and the pressure increases, the vacuum advance will allow the rotor-cap relation to spring back, retarding the timing. When one wants to tune using these devices, one can simply adjust the static timing and allow the devices to perform their functions. If one wants to change the amount of timing that is advanced and the points at which the advances are made, one can replace the springs and/or weights within the mechanical advance and use an adjustable vacuum advance and/or change the location where it senses the vacuum in the manifold.

If you're looking for me to tell you where to set the timing - this articles is too long as it is. Every engine is different, and timing is going to vary from as little as 8° BTDC to over 40°. With a little research, one can find out a good starting point for the ignition timing. Fine tuning should be done on a dyno or under controlled conditions at the track.

To summarize:
1. Ignition timing is a way to describe when the spark plug fires in relation to the piston's position and is measured in degrees BTDC.
2. The ignition timing must take into account the fact that it takes time for the air/fuel mixture to burn.
3. The ignition timing should advance as the engine RPM increases.
4. The ignition timing should retard as pressure in the cylinder (as measured in the intake manifold) increases.
5. Every engine is different, and it's the engine tuner's job to take these factors into account (and a few others) when setting the ignition timing.

Gasoline has a significant effect on all of this stuff I just described. If you want to learn just about everything there is to know about gasoline in one place in a language that's easy to understand, please read: http://www.faqs.org/faqs/autos/gasoline-faq/part1/

More good articles by InnovateMotorsports.com:
http://www.innovatemotorsports.com/resources/myths.php (Ignition Timing)
http://www.innovatemotorsports.com/resources/rich.php (Air/Fuel Ratio)

Two similar articles by AutoSpeed.com:
http://www.autospeed.com/cms/A_109132/article.html (Ignition Timing)
http://www.autospeed.com/cms/A_1595/article.html (Air/Fuel Ratio)

http://www.bridgeanalyzers.com/wp-co...nce-Tuning.pdf (High Performance Tuning - Engine Exhaust Gas Levels)
http://www.streetrodstuff.com/Articles/Engine/Detonation/index.php (Detonation & Preignition - Allen W. Cline)
http://www.performancetechnician.com...ion-timing.pdf (Ignition Timing Distributor Recurve - Henry P. Olsen)
http://www.performancetechnician.com...age-Engine.pdf (Ignition Timing For Modern Gasoline - Henry P. Olsen)
http://www.firstfives.org/faq/timing...ed_engines.PDF (Ignition Timing For Modified Engines - Dave Andrews)
http://garage.grumpysperformance.com...83/#post-35064 (Centrifugal & Vacuum Advance - Steve Davis, Performance Distributors/Davis Unified Ignition)