Problem with AFR

[DaveW]

It will be a while until I have the opportunity to dyno again, so I won't be able to post anything about possible reversion in the rich RPM range, but I will certainly look for it next time.

In the mean time, assuming that the problem is one of reversion, what can I do to reduce that? Again, all my intake and exhaust specs concur with Pipemax.

[F-BIRD'88]

Is this a IR manifold/carb setup?

[Kevin Johnson]

If the engine is moving such that the fuel in the bowls migrates from one side to the other the height of the float is slightly reduced so more fuel is let into the bowl which would decrease the AFR.

weberidlecircuit5.jpg

40-46IDA3pic.jpg

weber+40+ida+exploded1251421017.jpg

[digger]

what is the actual exhaust configuration? Layout, lengths, diameters etc

[Kevin Johnson]

http://performancedevelopments.com/pors ... nefailure/ Harmonic issues.

[Geoff2]

Have you tried different emulsion tubes?

[Walter R. Malik]

Having large cylinders with Individual Runner intakes, (with no plenum what-so-ever), you will never make any consideranle power and get rid of the reversion. The best you can do is try to control it.

View it like a large motorcycle engine.

[DaveW]

http://performancedevelopments.com/pors ... nefailure/ Harmonic issues.

Thanks for the link.

[DaveW]

Having large cylinders with Individual Runner intakes, (with no plenum what-so-ever), you will never make any considerable power and get rid of the reversion. The best you can do is try to control it.

View it like a large motorcycle engine.

Walter,
I'm stuck with this configuration, which BTW other Porsche racers do quite well with. As others have said, I probably have excessive overlap, but I'm hoping I can help out the reversion issue with tuning, before changing out the cams. A friend speculated that by lengthening the collector, I might be able to lower the reversion out of the normal operating range (which is 4500-7500 RPM). Does that sound reasonable?

Dave

[Walter R. Malik]

Having large cylinders with Individual Runner intakes, (with no plenum what-so-ever), you will never make any considerable power and get rid of the reversion. The best you can do is try to control it.

View it like a large motorcycle engine.

Walter,
I'm stuck with this configuration, which BTW other Porsche racers do quite well with. As others have said, I probably have excessive overlap, but I'm hoping I can help out the reversion issue with tuning, before changing out the cams. A friend speculated that by lengthening the collector, I might be able to lower the reversion out of the normal operating range (which is 4500-7500 RPM). Does that sound reasonable?

Dave

Intake tract volume & length will do more for tuning reversion pulses than anything done on the exhaust side.

[Kevin Johnson]

viewtopic.php?t=43020 A possibility.

https://www.youtube.com/watch?v=AyeUsED3Lzo

[DaveW]

Having large cylinders with Individual Runner intakes, (with no plenum what-so-ever), you will never make any considerable power and get rid of the reversion. The best you can do is try to control it.

View it like a large motorcycle engine.

Walter,
I'm stuck with this configuration, which BTW other Porsche racers do quite well with. As others have said, I probably have excessive overlap, but I'm hoping I can help out the reversion issue with tuning, before changing out the cams. A friend speculated that by lengthening the collector, I might be able to lower the reversion out of the normal operating range (which is 4500-7500 RPM). Does that sound reasonable?

Dave

Intake tract volume & length will do more for tuning reversion pulses than anything done on the exhaust side.

Walter,
My intake tract length (according to Pipemax) is very close to the 3rd harmonic. The cross sectional area is within a good range as well. So I don't think I can make much improvement there (especially since I don't have any room to go to the 2nd harmonic).

I have been reading about anti-reversion cones (either incorporated into the headers themselves, or a slip-in piece). What do you think about that? Would they diminish the wave tuning effect?

Dave

[DaveW]

viewtopic.php?t=43020 A possibility.

https://www.youtube.com/watch?v=AyeUsED3Lzo

Yikes! That can't be good (unless your emulsion tubes are set up to work with that).

[Kevin Johnson]

You would have to experiment with a float but it seems plausible that bubbles would decrease lift and this would richen the AFR.

https://www.youtube.com/watch?v=VPmTgsWFtSA

There might be unsung reasons that the move was made to Einspritzung...

Power/dynamic imbalance in opposing banks would be enough despite the 911 crank differing from a true boxer.

[MadBill]

Hmmm...I would have thought metering foam instead of 100% gasoline would cause a lean shift Kevin.