Steve Weiner-Rennsport Systems

Thats right,...one checks these things at valve adjustment time,...

scarceller

Steve,

I started this thread because on several 3.2L cars (some stock and some with SSIs) I've seen the intake air flow hit a wall at about 5800RPMs. My personal car does this with the stock AFM as well as my newly developed MAF system. While doing the MAF setup is where I really saw this clearly because the MAF was flow benched and it supports 22,000g/min and the car never pulls more than about 13,000g/min at WOT. Attached at the top of this thread is a graph of my intake air vs rpm from my data logger. It shows the Air Flow dropping off after 5800RPMs.

I'm also working on another 3.2L all stock except exhaust is 74 style with SSIs. It shows same behavior.

I'm simply looking to see if others have also observed this and if anyone has possible reason(s) for why? It could be that this is simply how these 3.2Ls are?

Thanks.

scarceller

Phil, I suspect you may very well be correct that the issue is Valve timing not optimal at hi-rpm 5800 and above. I really do think the issue is exhaust back pressure or something like this not allowing the exhaust stroke to fully empty the spent gas. My AFR numbers certainly support this theory. So the question still stands: is there something about the way these engines where built that cause them to have this issue? What concerns me is that I see many folks raise the rev limit on these cars above the stock 6520rpm but with the air flow numbers I'm seeing I see very little value in raising the rev limit because the air flow simply is suffering terribly at these higher RPMs. If intake air flow does not change or changes very little as RPMs go up then no point in continuing to push the motor. Power is directly related to the amount of air being ingested so if the air being ingested simply hits a wall at a certain RPM then pushing the motor beyond this given RPM has very little return.

Could be you are correct and that different more aggressive cams are needed. Looking for the experts to chime in here. I'm simply reporting the data I'm seeing for others to comment on.

whalebird

The spent gas in the cylinder is what the overlap in the cam is intended to reduce. Combined with SSIs, your motor should flow adequately thru it's intended torque curve. I don't think this "wall" exists in a stock motor, at least not to the degree you are witnessing.
If your cams are advanced, you could be seeing their limits to early. I don't recall exactly what a 3.2 gets for all-in ignition advance, but 32* may be a bit short.
Also, are you running an O2 sensor on both banks? Can you sample mixture at the outlets?

rusnak

Below that rpm, the motor is burning more air than the sum of the cylinder volumes per cycle. It was said that this engine had one of the highest volumetric efficiency for street Porsches since the mfi motors with plastic trumpet volume stacks.

Amber Gramps

The benefit I get from a higher rev limit isn't some increased horsepower number, but rather a longer run time it the higher RPM range. The further out the limiter, the more time I can be at the higher revs without hitting the limiter. Up in the hills it isn't about numbers on a chart, it's about having fun in second and third gear. Bumping the rev limiter to north of 7,250 allows you to play around at 6,250 all day long without hitting the limiter every other second.

Anyway, keep up the good work. Love what you are doing.

scarceller

Rusnak,

Very interesting post!

I ran some numbers from my data:
- At 5500RPMs I'm flowing about 11,250 grams per minute of air (taken directly from MAF voltage at 5500RPMs at WOT)
- 1 liter of air = 1.168g at ambient temp 68F and pressure 100Kpa
so I convert the g/min to l/min like this:
11,250 / 1.168 = 9,631 Liters/min

Then calculate L/rev like this:
9,631 / 5500RPMs = 1.75 l/rev

A 4 stroke engine has 2 revs per stroke so:
1.75 l/rev * 2 = 3.5L/stroke

That's a calculated VE of 1.09! this engine at 5500RPMs does indeed seem to ingest more air than the 3.2L displacement! This is actually very incredible! Almost a 10% gain in VE! My MAF may be a tad off calibration but it's not 10% off. If the math above is correct it certainly implies this motor is at a VE > 1.0 at 5500RPMs

I ran these calcs at several different RPMs and below 5000RPMs the VE drops below 1.0 but from 5000-6000 RPMs it looks like the VE is above 1.0 it then starts to drop below 1.0 again after 6000RPMs

Steve Weiner-Rennsport Systems

I don't have my data at hand as its been a long time ago, but back in my IMSA days, I did a LOT of different kinds of airflow research and noted positive pressure in the engine bay at speed, using various rear spoilers (not with the stock decklid). We didn't have any downforce options available to us at that time.

You are likely looking at limitations imposed by the camshaft profile since these common-plenum, single-throttle intakes are quite intolerant of cams that have lobe centers narrower than 113 degrees and much duration at all.

In short, the intake system, camshaft profile, and exhaust system are all very well designed and optimized for their particular mission.

dshepp806

One helluva' interesting thread! Thanks to all contributors..

Although you won't see me raising my rev limiter,..I'm a wus,......as well, probably one of the owners of those broken pieces, should I be able to rev her to 7500...no way,..nadda......checkbook's not THAT healthy...good where I'm at!!!!!!!

Doyle

scarceller

Doyle,

I run my limiter at 6800RPMs without any issues and I rev into the 6700RPM range often during testing of WOT. The 3.2L can certainly push past the stock 6520 limit but I won't push my 3.2L much past the 6700RPM or so.

scarceller

Can someone explain what would happen to air flow at hi-rpms if ignition timing is to conservative? The stock 3.2L 89 chip has ign timing set at 24* in the WOT IGN map above 6000RPMs but what most don't know is that the chip has a 2nd ign map that bumps ign timing another 2* at RPMs above 6000RPMs. This means that at above 6000RPMs the stock 89 chip runs 26* ign.

My dyno tuning has shown these cars can tolerate 30-32* ign at WOT on 93 octane fuel. I never really pushed past the 32* above 6000RPMs but I'm wondering if these 3.2L single pug motors don't need even more ign at these hi-rpms?

Does anyone have ign timing numbers higher than 32* above 6000RPMs?

dshepp806

I recall conversation with our beloved Steve Wong, back when I was looking into his programming. He took me through the discussion quite nicely, but I recall wanting to establish a 6500 limit MAX. He stated "no problem".....(VERY COOL guy to deal with, BTW). I don't want to frick with the Porsche gods too much.......cash isn't as abundant as before!

I don't run my girl in that zone at any frequent intervals for any lengthy durations, anywaze,..she has her "moments", however,..and she does just fine!!!!!!!!!!!!!

Again,..this thread is very interesting...and teaches me what I "don't" know.......BIG THANKS!

Doyle

Edgy01

Don't forget to factor air temperature into all of these.

rusnak

There was a good article around the time that the chip tuners were getting going. Andial was selling a chip and sport muffler package. I remember too much advance raised cylinder pressure and obviously detonation. Too conservative? I would think the stock setting is fairly conservative.

scarceller

If ignition is retarded to much would this not cause part of the fuel burn to still be burning when the exhaust valve opens? If this happens I think you would possibly have a condition where the burning mixture is still expanding into the exhaust headers and muffler. This could increase pressure in the exhaust system and result in the cyl not being able to expel all the exhaust gas.

I fully understand that you can't run optimal ignition advance at WOT because of detonation but if you where required to dial back ignition you could be in the situation I explain above.

This is why I ask the question about what happens to exhaust pressures if ign is retarded?

Thoughts?
Thanks.