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  1. #1

    Default LT1 Lean/Hot Cylinders

    Just curious if anyone knew if certain cylinders run hotter/leaner than others?

    I assume 7 and 8 are prone to running hottest?
    Correlation does not imply causation.

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  2. #2
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    Default

    I'll have to pull out my copy of the 1992 SAE technical paper on the LT1 engine design, and the impact of reverse flow cooling on the distribution of temperature from cylinder-to-cylinder, and vertically for each cylinder. I'll post a .pdf of any charts. As I recall, they are graphs with no actual numbers, but a trend line to show relative temperatures. I have an EGT sensor on #7 primary with the understanding that runs the hottest, but I'll see if the tech paper verifies that.

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    You can get a bit of an idea of how it works out by looking at the fuel trim by cylinder at idle and off idle in the programing. One of the procedures I have seen for setting those numbers, if you don't have a dyno with 8 wide band O2 sensors, is to measure the temperature of the exhaust manifold as close to the port as you can and then set the trim to get them the same. I will try to get a print out of our stock program and put it up.

    IIRC, the center cylinders had the largest compensations on our stock everything program in the 96 RMW. My guess would be that, like many engines, the mixture, compression variations by cylinder, quench differences per cylinder would probably be the largest variables, with the actual cooling coming in later when you look at it from the combustion temps. If you were looking at actual head temp profiles, then the cooling would be more involved, and head temp can be important for detonation and cylinder fill sometimes.

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    Here is the screenshot of the two by cylinder fuel trims you can set. Idle is probably for about 580rpm or so, and the off idle I have heard a couple of different highest rpm covered, but probably in the 1600 range. These settings would be very intake manifold dependent, I think, and above these settings for rpm the manifold would have been designed to get the flow even. The baffles in the manifold that I can see through the throttle body opening look to be for higher rpm flows. As the tables show, the mixture is getting closer to even at the higher rpm settings.

    LT1 fuel trim by cylinder.JPG

  5. #5

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    I figured 7 and 8 were problem children, so will pulling those plugs to check after pulls and perhaps giving em a lil bit more fuel with a degree or two less timing than overall.
    Correlation does not imply causation.

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    Quote Originally Posted by booster View Post
    Here is the screenshot of the two by cylinder fuel trims you can set. Idle is probably for about 580rpm or so, and the off idle I have heard a couple of different highest rpm covered, but probably in the 1600 range. These settings would be very intake manifold dependent, I think, and above these settings for rpm the manifold would have been designed to get the flow even. The baffles in the manifold that I can see through the throttle body opening look to be for higher rpm flows. As the tables show, the mixture is getting closer to even at the higher rpm settings.

    LT1 fuel trim by cylinder.JPG
    If you call 1.00 the nominal center of range based on whatever, 1 and 2 are much further off at a idle than 7 and 8 are. It also appears all but 3 and 6 moved closer to nominal at higher flow and are actually right on for nominal for four of them. If these numbers are correct for your engine (they may or may not be) you wouldn't want to add or subtract fuel and I would have to look at the program to see if you can trim the timing by cylinder at all. These numbers are for a stock 1996 iron head Buick with stock manifold, cam and pistons and EGR hooked up. If you have aluminum heads, bigger cam, did EGR delete they would certainly be different. The only way to really tell, AFAIK, is to put it on 8 wide band O2 sensors or check the temps on the exhaust as best you can with an infrared thermometer to look for cylinder imbalance. Reading plugs can help, but it covers a much wider range of rpms than this so could be way off, too. In these aiflows, the engine would be running almost all the time on closed loop, so really all that has to accurate to is staying within the range of closed loop fuel trim.

    Unless you have absolutely symmetrical intake airflow passages, head ports, and combustion chambers/pistons it is nearly impossible to get even airflow to the cylinders over a large rpm range. I am sure the high end EFI systems used in high end systems would be programmable for fuel and timing by cylinder, and would be tuned on the dyno based on mixture and exhaust temps.

    If you are talking about pulls on the dyno you may be able to get the O2 readings if they have manifolds or headers that are the same as yours. If you are talking about pulls driving or on the track you will only see an average of mixture weighted toward the later part of the pull. If they are full throttle runs, you would want to adjust the main fuel map, but you can't do individual cylinders, I am pretty sure.

  7. #7

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    I have a holley EFI, so I can add fueling/timing per cylinder once I am into boost.

    Guess I can't cheat here, and I'll be pulling all the plugs after a WOT pull lol
    Correlation does not imply causation.

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    Quote Originally Posted by shownomercy View Post
    I have a holley EFI, so I can add fueling/timing per cylinder once I am into boost.

    Guess I can't cheat here, and I'll be pulling all the plugs after a WOT pull lol
    Does the Holley have a wide band O2 option? Can you add fuel per cylinder over the whole rpm range? If not you may want to invest in a wide band monitor that lets you log data. Especially when you are doing wide open throttle only, you can learn a lot that way, but not at cylinder level. For boosted applications, a wide band controlled EFI system is priceless IMO. I had one one the last rendition of system on my 1970 Challenger twin turbo I built. Super sweet to just put in 12 to 1 in the fuel map, let it adjust the mixture during a log test, then look at the correction it did make that adjustment to the injector duty cycle map so the system would need to adjust less so no delay. If I had an assistant with me, there was a laptop key stroke you could do, even, and it would correct that spot on duty cycle map to get the right mixture there. Do a dozen spots on the map and then extrapolate the parts in between, then run a log to confirm.

  9. #9

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    Yup, wideband and will do either closed loop fueling fix, and/or "learning" function.

    I can add fuel over the whole range, or only when in boost.

    End of the day though, looks like I will be pulling all 8 plugs and verifying which is hottest.
    Correlation does not imply causation.

    Una salus victis nullam sperare salutem

  10. #10
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    Which Holley model do you have? I downloaded some software, but it won't show the table unless you have a pcm read or file, it appears. It would be interesting to see what you are dealing with.

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