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Thread: Cold start & warm up - rear cylinders excessively lean.

  1. #11

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    Danny, I say the driveability with the 1:1 is bad because it’s very abrupt response with little foot movement. This vehicle is driven by my 17 year old daughter and she had the rear end break loose on her. So, from my perspective, it’s a driveability issue.

    You’re comment that it’s not a fuel distribution issue must be beyond my simple mind. All cylinders have good compression, all cylinders have strong spark, yet the rear cylinders aren’t seeing fuel or firing until the engine gets heat in it. It then runs absolutely perfect.

    On cold start, all 8 cylinders were checked with a thermal gun at the header tube exit from the exhaust ports. Front cylinders are rising above 200°F quickly, while the rear cylinders are stuck at 58°F. No oil being sucked in or burned, no coolant either. Rear plugs are dry as a bone. So, what would YOU call it?

  2. #12
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    Quote Originally Posted by Flatlander View Post
    You’re comment that it’s not a fuel distribution issue must be beyond my simple mind. All cylinders have good compression, all cylinders have strong spark, yet the rear cylinders aren’t seeing fuel or firing until the engine gets heat in it. It then runs absolutely perfect.
    I stated, "IF the engine ran fine with a carburetor (and I'm sure it did from Ford), then it's not a distribution issue."

    On cold start, all 8 cylinders were checked with a thermal gun at the header tube exit from the exhaust ports. Front cylinders are rising above 200°F quickly, while the rear cylinders are stuck at 58°F. No oil being sucked in or burned, no coolant either. Rear plugs are dry as a bone. So, what would YOU call it?
    So why does this fuel distribution problem only occur when cold? Maybe X-Tau Fuel (wall wetting - LINK) does dictate a slightly enriched AFR only at those Coolant Temperature °F columns.
    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, 232°-244° duration/.623" lift/114° LSA camshaft, 11.8:1 comp, TFS R-Series FTI ported intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 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/3000 RPM converter, FPP aluminum driveshaft, FPP 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 MT tires.

  3. #13

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    The Sniper has distribution issues when run in progressive mode. I've collected data on this on the dyno with engines that have EGT in each pipe. On the same intake manifold a carb (which has progressive linkage) will run just fine while a Sniper in progressive mode will not run properly. The last engine we tested this on had cold cylinders at the rear of the engine, which is exactly what the OP is saying.

    I do not have a good explanation for this. My assumption before the test was the same as Danny, that is, if a progressive carb works fine then a progressive Sniper will work fine. But that was not the case. The engine would barely run with the Sniper in progressive. It ran just fine when we changed the linkage to 1:1. There's some sort of distribution issue caused by the design of the Sniper that I do not understand.

    If the OP insists on running the Sniper in progressive then he'll need to modify the intake manifold and/or use a spacer. Personally I'd switch to 1:1 and use a throttle extension, but to each their own. I run 1:1 on my car with 600+ ft-lbs of torque and it drives just fine. I do need to be careful in how hard I get on the gas since it will take off hard if I push on it.

  4. #14
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    I agree with Andyf. I've always stated, "You don't want to use a progressive secondary throttle linkage unless you have to." However, you may want the throttle lever extension (LINK) for easing into light throttle.

  5. #15

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    I poked around at Holley a little bit to see if anyone could explain why the Sniper didn't run very well in progressive, but I didn't get a solid engineering (fact based) answer. I was just told it wasn't designed to run that way. My guess is that the design of the Sniper throttle bores makes it act differently than a carb. They look kind of the same, but I don't think they act the same. There's no venturi in a Sniper and there isn't a booster assembly, so I think the fluid dynamic behavior is different. Most of the time a Sniper works okay on either dual-plane or single-plane intakes, but I've had enough "weird" distribution problems on the dyno that I know better than to make blanket statements. There's something going on inside the Sniper that makes it act differently than a carb on some intake manifolds.

  6. #16

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    I agree Andy. Sure, there are injectors involved and injectors atomize, but they’re not injecting into a runner. One look into the bores at idle shows you the difference. A Holley 4150 is a hard act to follow, and it has a completely separate circuit for idle. You don’t see a bunch of fuel hitting the backside of the throttle blade and momentarily pooling. That’s exactly what you see on the Sniper. I think the extra inch of “bore” directing the flow at the front of the manifolds gives the the forward cylinders an advantage in the particular manifold I have, when the walls are cold. Zero fuel introduced from the rear bores isn’t helping the situation. I think I may look into a plate to keep the IAC whine down and mount a deep open spacer below that. Perhaps removal of the plenum divider would help as well, but a PI manifold probably shouldn’t be cut up.
    Last edited by Flatlander; 02-01-2020 at 08:54 PM.

  7. #17
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    Quote Originally Posted by Flatlander View Post
    Zero fuel introduced from the rear bores isn’t helping the situation.
    Exactly, a 1:1 secondary throttle linkage operates all four injectors even at idle. With the "Progressive Throttle Linkage" enabled, the "Primary To Secondary TPS Switch" value (under Fuel Injector Information) is used to start the ramp-in of the secondary fuel injectors as the rear throttle plates open. Then use the AE Correction vs TPS table in post #88 of this thread. Click on the LINK (& PDF instructions) in post #1 of this forum thread for more information. However, you may want the throttle lever extension (LINK) for easing into light throttle.
    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, 232°-244° duration/.623" lift/114° LSA camshaft, 11.8:1 comp, TFS R-Series FTI ported intake, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 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/3000 RPM converter, FPP aluminum driveshaft, FPP 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 MT tires.

  8. #18

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    I think we're over analyzing this a little bit. This is an air pump, air and fuel will flow in the path of least resistance and in this case the most vacuum.

    With a progressive linkage you still have all the injectors firing with only 2 throttle blades opening. This has to be fine tuned for driving conditions.

    Now in this case of the rear cylinders not getting fuel, but the front ones are with essentially the same runner length makes complete sense with the progressive linkage. Which 2 throttle blades are the first to open? The front, case closed on that.

    NOW in your scenario if the Sniper is setting with all 4 blades closed at idle and your rear cylinders are "lean" then there's an air flow issue.

    With my question regarding how you were monitoring this now answered there is still room for error. Is it lean OR rich? If we are not meeting stoichiometric efficiency then we won't get combustion. You could be too rich or too lean. But it's obvious they're not firing so we need to get the right air and fuel mixture in there.

  9. #19

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    Cow Dude: Its not an airflow issue. As stated all cylinders have great compression. This was verified, as I thought one or two cylinders may have had tight lash and holding a couple exhaust valves open until the aluminum heads expanded. That is not the case. Lash is perfect and compression verified.

    Ignition is perfect, threw new plugs in for good measure. Verified spark on all 8 visually and with a timing light.

    The rear cylinders are NOT rich. Also shared was the fact that the rear plugs are bone dry and light when removed after shutdown during the first 2 minutes of warm up. If it were rich (which it isn’t), how would you explain a perfect running engine 3 minutes later, at temp?

    The issue is fuel distribution at low temps. The fuel is condensing or pooling before getting to the rear of the plenum or end of the rear runners. Plenty of bends and volume in this manifold.

  10. #20

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    Quote Originally Posted by Danny Cabral View Post
    Exactly, a 1:1 secondary throttle linkage operates all four injectors even at idle. With the "Progressive Throttle Linkage" enabled, the "Primary To Secondary TPS Switch" value (under Fuel Injector Information) is used to start the ramp-in of the secondary fuel injectors as the rear throttle plates open.
    Danny, is there any way to add an idle flow of fuel at the rear injectors? For example, can I reset the rear TPS offset somehow so it thinks it’s at, say 2%, when it’s at zero? I’m not asking for a purest response...I have no interest in going back to the 1:1 linkage. I'll change the intake to a single-plane before I do that.

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