Dyno result for the risky business Porsche

[ptuomov]

Proof of life:

[ptuomov]

Engine bay mostly buttoned up:

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[vannik]

Looks really great!

Do you have a blow-off valve on each side?

[ptuomov]

Looks really great!

Do you have a blow-off valve on each side?

Yes, every version of John’s kit has had two recirculating bypass valves. It’s a blow thru MAF system for many reasons and the recirculating happens upstream of the MAF.

[ptuomov]

Nikasil break-in:

[ptuomov]

Six hour shakedown on the dyno starting tomorrow, says John Kuhn. Here’s a shot:

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[vannik]

Looks really great!

Do you have a blow-off valve on each side?

Yes, every version of John’s kit has had two recirculating bypass valves. It’s a blow thru MAF system for many reasons and the recirculating happens upstream of the MAF.

How does he prevent the bypass valve and the wastegate from interfering with each other? Seems to me it can lead to an unstable situation and possible overspeed of the turbo.

[ptuomov]

Looks really great!

Do you have a blow-off valve on each side?

Yes, every version of John’s kit has had two recirculating bypass valves. It’s a blow thru MAF system for many reasons and the recirculating happens upstream of the MAF.

How does he prevent the bypass valve and the wastegate from interfering with each other? Seems to me it can lead to an unstable situation and possible overspeed of the turbo.

Can you elaborate on the problem scenario that you have in mind?

Throttle closed, the intake manifold sees a vacuum. That signal closes the wastegate. This helps maintain the turbo speed even though the exhaust flow is minimal. The throttle closed and intake manifold vacuum opens the cold-side bypass valve. The compressor circulates air with atmospheric conditions in the inlet, including thru the intercooler, but without having to push against high pressure, therefore doing work and slowing down but not too fast. The turbo slows down but doesn’t stop. What’s the concern?

Pre-compressor throttle would probably maintain the turbo speed better, but with that arrangement you could potentially over speed the turbo because the compressor would be operating in vacuum. We don’t have that.

Or is the concern about running two sides and twin turbos? The two turbos are tuned to run at almost exactly the same speed at all operating conditions, so the system behaves just the same as half of it would.

[vannik]

I misunderstood that the BPV was used for boost control on top of the wastegate but you make it clear that it is to stop compressor surge during closed throttle running. Thanks.

[ptuomov]

Dyno shakedowns continue

Here is one dyno shakedown graph. This is with the new short block and valve springs set up a little tighter. At 0.8 bar (12 psi) and 6200 rpm, valve spring setup change doesn’t enter the equation, so for practical purposes the only change is the short block.

The new short block seems to be running very well so far:

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[ptuomov]

Dyno shakedowns and initial mappings continue.

John is taking quick high acceleration runs to map the fuel and ignition to be approximately right enough to be safe for slower dyno acceleration rates. Recall that we are now in a territory that hasn’t been previously mapped, this run terminated at 7200 rpm. And there’s still about a thousand more rpms to map. The torque curve at low rpms reflects the fast acceleration rates on the dyno.

This is all done with boost level of about 12-12.5 psi which makes enough power to be relevant/informative but low enough not to stress the engine too badly.

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[englertracing]

we don't want pictures, we want video

[ptuomov]

we don't want pictures, we want video

Not only will I deprive you of video but downgrade the thread to text for this reply...

As many people hot-rodding 1987-1995 Porsche 928 S4/GT/GTS engines already know at this point, the stock S4/GT/GTS active resonance intake manifold doesn’t really want to make power above 6000 rpm. It's great in the 2000-6000 rpm range though.

I just recently saw some data where someone had dynoed otherwise sensibly tuned normally aspirated 928 GT 5.0l engine (11.5 compression, E85, Colt 2 cams, headers, exhaust) and picked up literally 100+ hp in the 6000-7000 rpm range by swapping the S4 intake to 48mm AT Power individual throttle bodies with trumpets and no other changes (except ECU calibration, of course).

So even with valve events timed appropriately for higher rpms with bigger cams, the S4 intake manifold reduces the volumetric efficiency of the engine pretty dramatically above 6000 rpm. Furthermore, the penalty gets bigger the higher the rpms.

Now, since we’ve got turbos, we can literally force feed the engine whatever medicine we think it needs thru the reluctant intake manifold. The question is how much do we need to ramp up the boost at high rpms, starting from 6000, to keep the torque curve about flat?

The engine produces about 19.7 lbf-ft per psia at 6000 rpm, 17.3 lbf-ft per psia at 7000 rpm, and estimated/interpolated 14.3 lbf-ft per psia at 8000 rpm. From drivability and traction perspective, we want flat torque curves and will employ low torque curves at low gears and high torque curves at high gears. This is how much I think we need to ramp up the boost:

1) To produce a flat torque curve at 500 lbf-ft, the system needs to generate 10.7 psi boost at 6000 rpm and 20.3 psi boost at 8000 rpm.
2) To produce a flat torque curve at 600 lbf-ft, the system needs to generate 15.7 psi boost at 6000 rpm and 27.3 psi boost at 8000 rpm.
3) To produce a flat torque curve at 700 lbf-ft, the system needs to generate 20.8 psi boost at 6000 rpm and 34.3 psi boost at 8000 rpm.

The "bad" news is that with the stock intake manifold, the existing data says that the boost ramp up after 6000 rpm needs to be very steep. Much steeper than I expected before running the numbers, we're talking about numbers like 5 psi extra at 7000 and 10 psi extra at 8000. Which instinctively sound like high numbers, I guess substracting the 14.7 psi when going from psia to psig has a large psychological effect!

The good news is that torque/absolute pressure is almost perfectly linear in rpm up there so a linear correction will work really well and that’s much easier to calibrate than a full rpm-specific boost profile.

[englertracing]

sheet metal intake manifold
or call the fellas that make the itb kit, and ask them how much cheaper for one without throttle shafts, that you can bolt a pair of plenums on top of

[ptuomov]

sheet metal intake manifold
or call the fellas that make the itb kit, and ask them how much cheaper for one without throttle shafts, that you can bolt a pair of plenums on top of

Why? I’d much rather have the turbos spool up at low rpms with the stock intake manifold and then ramp up the boost at high rpms than have the turbos not spool up at low rpms with a short runner intake. And there’s an additional thermal bonus at high rpms when running pump gas.

Here’s how I believe the ramping up of boost at high rpms is going to work in my favor on pump gas. My street car is well intercooled, so it will pull out a ton of heat from the charge, especially when the boost is ramped up at high rpms. Then, importantly, the cooled charge will lose about 25% of the absolute pressure at 8000 rpm because of the intake manifold Helmholtz resonator being out of tune. This will cool the gas that actually makes it in the cylinder, and that's what matter. Even though the compressor outputs horribly hot charge, the cylinder actually gets not too hot, less dense charge after Intercooling and anti-tuned intake. Then one can give the engine more ignition advance on pump gas (compared to engine with a manifold tuned to increase the charge density like in NA engine).

The gain comes from the ability to run very hot charge thru the intercooler and have the inter cooler take out more heat from the whole intake system. Porsche is using this strategy in the four cylinder turbo models now in 2020 also after having switched to this strategy in the 2008 Carrera Turbo.

See what I'm saying?

Only way this "backfires" if the exhaust pressure gets too high and messes up the gas exchange.