ptuomov

Can you please post the whole power/torque curves?

Cheburator

No, that engine made another 50-odd on top due to the cams and higher compression and more aggressive engine mapping due to lighter internals, and flywheel.

The stock GTS engine belongs to another rennlister/competitor who went down the route of BMW M5 ITBs and I supplied adapters and know-how. He designed/made his own CF dual plenum airbox

Strosek Ultra

These guys from the UK asked me if I was interested in working with them developing a Direct-to-Head Throttle Body System for the Porsche 928. I had to decline but think this may be of interest for another party like Carl or anybody else. Do make contact with at AT Power (Ben@atpowerthrottles.com) for further details.
Åke

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Carl Fausett

Adam,

Would you prefer we start an independent intake manifold thread so as not to detract from your Devore build? Or leave the discussion here?

Carl

Catorce

I think we should leave it here for two reasons:

1. It is germane to the build of the car, and it will directly influence the outcome of the build
2. Bob Devore was a serial fiddler and tinkerer and I think he would have enjoyed the heck out of this process more than actually driving the car.

hans14914

AT-Power makes some really nice hardware. I know someone over there, and I have models of the adapter plate if someone wanted to go that direction.

I could also fairly quickly model some ITBs and have them machined here, but they would be the more conventional shaft and butterfly configuration.

With Shartuner Alpha and various other aftermarket options, its actually easier to design and build an ITB intake than a traditional long runner manifold for the 928.

Strosek Ultra

The AT-Power 928 hardware look like this. Any seriously modified engine should have a separate throttle valve for each intake port. Fabricating the runners between the ITBs and the cylinder heads is not particularly difficult.
Åke

AO

FIFY...

Carl Fausett

You must keep in mind that, although sexy to look at, ITB's can have problems.

If the runners are all short, the power output is going to be very peaky and very much at the highest RPM ranges. Even when designing for the 2nd harmonic for the Helmholtz effect, a runner of 12" to 14" in length is not uncommon. A really short ITB (necessary if you plan to use the stock 928 hood) will be relegated to the 3rd or 4th harmonic (and harmonic's grow weaker with every reversion) and will produce what power it does way up top in the rpm range where very few drivers and cars can use it.

The second is an effective air filter system. Absolutely necessary, and can be a PITA with ITB's. You can choose to run sans air filters, but have $$ for frequent rebuilds handy when the guy in front of you throws sand and dirt up...

I usually see a big airbox built around the ITB's in an effort to provide air filtering, but wonder 1) what it might be like to work on that engine around that monstrosity, and 2) whether the roof of the box is too close the the air bells such that it may be effecting optimum flow, and 3) whether the roof of the air box (often plexiglas or similar) isn't sucked down during WOT.

Carl Fausett

Actually, we already have done the hard work and have runners to the head ready for purchase.
You may cap our runners with ITB's, or plenums, or whatever.

https://928motorsports.com/parts/intake_runners.php

I thought as Strosek has, that the runners were the biggest impediment to a guy making his own manifold. Have had these for sale for probably 6 years now... sold only one set.

So, with that experience in mind, why I am developing a complete after-market bolt-on manifold solution instead. Runners (and just flanges by Hans) don't seem to sell. Yet the interest in an intake manifold replacement still remains high.

ptuomov

My opinions below:

Fitting long runners to ITB's like Ake's isn't really much more difficult than fitting them to any flange. It's the same problem and the solutions are the same also.

A short-runner intake manifold or ITB system produces a less peaky torque curve than a long-runner intake manifold or ITB system. I don't think there's any question about that. That's why modern turbo cars often use short runner intakes, they intentionally want to remove the torque peaks since the engine is knock limited anyway. If anyone is confused about this, take a look at Simard's 7L ITB motor's torque curve -- it's almost perfectly flat because of the short runners. Louis Ott's ITB motor also has short runners and has the same flat torque curve. The reason to run long intake runners is to get the peaky torque curve peak at the rpm you want.

A word about terminology: Helmholtz tuning in intake manifold design terms of art is limited to manifolds where the plenum acts as as the resonating chamber. Given how pulses work, this means three or four equally spaced pulses per plenum. Flappy closed, the stock S4 intake manifold is considered a Helmholtz resonator intake manifold, flappy open it's not considered one. If you just run all eight pipes to a single plenum, it doesn't act like a Helmholtz resonator and you don't get the characteristic tuning peaks. For ITB systems or single plane intake manifolds that connect all runners to one plenum, the plenum volume doesn't really matter from pulse tuning perspective as long as it's not too small and as long as it distributes equally. That's because it doesn't act as a Helmholtz resonating chamber anymore when you connect eight runners to it.

The reason for running ITBs is that they allow you to run more camshaft overlap. For example, Ake's planned cams would run terribly at cruise with a plenum manifold fed from a single throttle. If you don't run big overlap cams, then you can do about as well with a single throttle manifold as with ITBs. And you shouldn't even think about running big overlap cams with exhaust manifolds, in a cross-plane V8 you need headers to run high overlap cams. So the system where ITBs shine has high overlap cams and long-tube headers.

Strosek Ultra

Carl, I can see one reason why your runners do not sell well - the price. The runners do look nice indeed but I prefer them made in aluminum in case alterations need to be performed.
Åke

Strosek Ultra

This is what I said about tuned intake tract length on Jerry Feather´s thread about fabricating S4 intake manifolds. Post #78. https://rennlist.com/forums/928-foru...earance-6.html

"I just took the intake manifold off our BMW Alpina B10 4,6L V8 car. The tuned length of the runners including the length of the intake ports is almost exactly 19 inches tuned for 4800 rpm right in the middle between peak torque and peak power rpm. Think this is the tuned length one should aim for when making an intake manifold for a fairly stock 928 32V engine. The BMW runners are forming almost a perfect circle and are opposing each other 3.5 inches apart inside the plenum. The manifold volume is appr. 5650cc or 345 cu inches.
For street performance engines I have always been in the favour of tuning the intake length at an rpm in the middle between peak torque and peak power rpm.
Tuning the intake length for peak power rpm (14-15 inches) will improve the top end at the expense of less mid range torque."

From my avatar picture you can see it is not possible (or at least difficult) to keep a good ITB intake under the hood.
Åke

Carl Fausett

I do not have a degree in fluid dynamics, but this statement does not match my understanding. My understanding is the percussive wave is created by the valve closing against the onrush of the air, the wave reverses direction there and travels back to the bell-mouth, which reverses it again and sends it back to ward the valve.

This is irrespective of whether there is a plenum or it is open to atmosphere.

At least that's what I understood.

Picture from those glorious Can-Am days... and how they ran inlet runners of different lengths to generate a wider power band (more useful in road racing).

ptuomov

That's extra wrong, the part about "percussive wave is created by the valve closing against the onrush of the air". Don't feel too bad, I used to make the same mental mistake! ;-)

The high pressure wave generated in the intake port by the engine is not caused by the valve closing. It's caused by the piston decelerating and eventually reversing while the valve is open.

Furthermore, it's not the case that the valve closing generates any sort of meaningful high-pressure wave in the port. That's because at peak torque rpm, the air is already flowing out of the cylinder when the valve closes. At peak power rpm, the air is very close to not flowing in either direction when the valve closes, so the valve closing event is the proverbial "non-event".

If the air would still be rushing into the cylinder when you're closing the intake valve, why wouldn't you close the valve later? You see?

On terminology: What you're describing is a simple pipe resonator. The Helmholtz resonator needs a resonating chamber. What complicates matters is that half a century ago some people considered the cylinder as the resonating Helmholtz chamber during the IVO-IVC period. This turned out not to be a terribly useful way to think about it, because it's really not an acoustic wave setup with intake valve open, it's a finite wave setup. It doesn't really matter what it's called, but using accepted terminology makes communications easier.