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Thread: C950 MPFI - startup backfire through intake and AFR question.

  1. #1

    Default C950 MPFI - startup backfire through intake and AFR question.

    I'm still learning the Commander 950 software and tuning in general, but learning fast .

    Recently I switched to a larger air intake (round 2.5" tall instead of 2" tall), and adjusted my A/F ratio to compensate for the additional air. Truck runs pretty good, however, every now and then I get a small intake backfire and it struggles a bit when starting after warmup. Seems to happen more after the truck goes into closed loop.

    Doing some reading it would seem I'm running lean when starting. My understanding is that I need to enrich the startup tables, however I was told it should be the last thing I do when tuning.

    I don't want to damage the engine running too lean, however, I don't want to screw things up changing things I shouldn't. How do I tell if my A/F is good? Do I need to enrich the startup tables to compensate for the additional air? Anything else I should do or check to make sure I'm running a good A/F ratio?

    I have the latest Holley Commander software, and I have a wideband O2 sensor. Thanks!

  2. #2
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    Quote Originally Posted by landshark99 View Post
    Recently I switched to a larger air intake (round 2.5" tall instead of 2" tall), and adjusted my A/F ratio to compensate for the additional air.
    You don't adjust the air/fuel "ratio" to compensate for increased intake airflow.
    You increase the amount of fuel (cell values) to "maintain" the established air/fuel ratio.
    Perhaps you were just using the wrong terminology.

    My understanding is that I need to enrich the startup tables, however I was told it should be the last thing I do when tuning.
    Correct. The Main Fuel Map must be well tuned before the Startup Enrichment parameters can be optimized.
    This is because the Startup Enrichment parameters are modifiers of the Main Fuel Map (except for the Cranking Pulse Width).
    Look at the "Idle Fuel vs. Temperature" graphical representation - figure 8, page 29 of the C950 manual.

    How do I tell if my A/F is good? Do I need to enrich the startup tables to compensate for the additional air?
    Before starting the engine, turn the key to run and wait for the WBO2 sensor to heat & display AFR.
    This allows you to view the AFR during the starting & warmup period; even though it's still in Open Loop.

    Remember, tune the Fuel Map when the engine is fully warmed up, then never adjust it to
    compensate for colder engine temperatures
    ...that's what the temperature modifiers are for.
    http://www.chevytalk.org/fusionbb/sh...?post/2236964/ (C950 Fuel Map Tuning Procedure)
    http://forums.holley.com/showthread....2773#post12773 (Additional Fuel Map Tuning Tip)

    ...and I have a wideband O2 sensor.
    Which WBO2 sensor kit do you have? Holley's 534-197?
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

  3. #3

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    Yes, I have the Holley WBO2 sensor.

    I ended up retarding the timing at idle a bit (12° now) and that seemed to help the backfire somewhat. I always wait until the engine is fully warmed up before I start tuning any A/F ratios. I have started leaning out the Fuel Map a bit more as it seemed really rich especially at cruising speeds. Once I have the Fuel Map where I like it I will play with the Startup Enrichment. I just didn't want to start changing multiple things at once, and from my reading, seems smart.

  4. #4
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    Quote Originally Posted by landshark99 View Post
    I ended up retarding the timing at idle a bit (12° now) and that seemed to help the backfire somewhat.
    I would also ensure the ignition timing is synchronized with the ECU. At idle (turn & lock down distributor) and at higher RPM (adjust Propagation Delay).

    TIP: There is an easier way to synchronize the ignition timing, however you must have your Spark Map saved/copied somewhere. You can simulate locked-out timing advance, by entering the same value in every cell of the Spark Map. I like to use 30°. You can use the "Cell Action" function to automatically enter 30° in the entire map, so you don't have to do it manually. Also, temporarily uncheck the "Enable Idle Spark Control" box in the "Spark" drop down menu, so the idle timing doesn't rapidly fluctuate. This is not real time editable and requires a restart. Enable it after you're finished with the ignition timing. This is easier, since you won't need to look at the laptop computer while synchronizing the idle timing and the Propagation Delay.

    Without disconnecting anything and using a timing light, start the warm engine and turn the distributor to synchronize the engine's initial timing with the idle timing on the laptop computer. When the initial timing (idle) is synchronized, lock down the distributor's adjustment bolt and never touch it again (for this purpose). After synchronizing the initial timing (distributor hold-down bolt tight), ALL timing adjustments/tuning is done on the laptop computer.

    Then use the "Ignition Propagation Delay" function to synchronize the timing advance at the higher RPM (3000-4000 RPM). It's all 30°, so you can momentarily rev the engine, and watch for the timing to drift away from 30° to synchronize the Propagation Delay. Adjust the Propagation Delay until the ECU maintains itself at 30°. If at 3000 RPM , the actual timing you see with a timing light is less than what's on the Timing Table (software), increase the Propagation Delay until it matches. If at 3000 RPM, the timing light shows more than the Timing Table (software), decrease the Propagation Delay until it matches. You can adjust it while the engine is running. It is real time adjustable (click enter after each numerical change). Some applications use a setting well above the default value of 248 usec. Believe your timing light above all else!

    Think of the "Ignition Reference" as just a software setting; no adjustment or tuning required. With a GM HEI or Ford TFI, simply set it to 10 (it's not your idle timing) and forget about it. Conversely, the Ignition Propagation Delay needs to be adjusted to synchronize the timing at higher RPM.
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS TW 11R 205 heads, 11.8:1 comp, TFS R-Series intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 160A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/2800 RPM converter, M4602G aluminum driveshaft, FRPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 tires.

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