Louie928

Well, "benefit" is a nebulous term. Whether there is a benefit depends on the use of the car. As an example, suppose a stock S4 automatic trans driven on the street in normal everyday use. That takes a quite wide range of smooth operation. The stock manifold is going to be the best option. It gives the torque bump at about 3000 rpm, then another torque peak about 4300 rpm with a dip in between. It's a pretty good compromise. Since the transmission isn't totally flexible in gear changing, that 3000 rpm torque bump can come in handy. Best of all, the stock intake comes with the car.

Some shortcomings with the stock S4 style intake are that the runners aren't the same length and some of the runner openings are too close to the side plates for optimum flow. You can space out the side plates to help with the unequal flow problem, but the unequal length is not fixable. These physical properties is what causes some cylinders to run rich/leaner than others and for some cylinders to begin knock retard before others. Could a stock style intake be made better than the stock one? Probably could be but at large cost.

With the ITB intake, you'll lose the lower 3000 rpm torque peak. If you have a 5 spd tranny, you can downshift to get into the higher torque at higher rpm and not really lose anything, but gain a lot. The ITB throttles located close to the valves, keeps reversion flow from affecting the other cylinders when at part throttle. That gives smoother idle and part throttle operation. The unrestricted flow capacity of the ITBs gives the opportunity to achieve more overall volumetric efficiency than with the standard type manifold. ITBs may help a little on an S4 engine in the 4000 to 6000 rpm area. I think a GT cam engine would benefit more. When I get rid of a few other projects, I plan to try a ITB intake on a GT motor and see what results could be had.

Here is a scan of a page in a book I have, "Design Techniques for Engine Manifolds". It's an SAE book authored by DE Winterbone and RJ Pearson. They have done a lot of work on the dynamics of engine breathing. This illustration shows two results on the same engine. It is a 4 cylinder engine, but the principle is the same whether 4, 6 or 8 cyl. One result is using a double tuned manifold similar to what we have in the 928. Notice the solid line power curve which bears striking resemblance to that of the S4. Notice the dashed line power curve as would be found with a ITB intake. The primary pipe length is the same on both; 306mm or about 12". On the 928, there is about 4" length between the intake valve head and the intake mating surface on the head. That would mean an 8" runner length above the head to stay close to these curve shapes. Other things such as cams and exhaust affect it too.

With the ITB, you lose the lower torque peak and would require more shifting to keep in the fat torque area.

https://rennlist.com/forums/attachme...1&d=1221596864

Louie928

I suppose by "enclose in a manifold" you mean "enclosed in a large plenum", then it should work fine.
I'll do some engine simulation work comparing an S4 motor with stock manifold and ITB, and show the results.

BPG_Austin

Thank you Louis, that was the information I was hoping to understand. Much appreciated.

ben

dprantl

Forced induction nullifies many manifold design drawbacks. It's not that expensive either (especially when considering price/hp) and with proper engine tuning can produce a daily driver rivaling a factory stock car. These days, more and more cars are being either supercharged or turbocharged by car companies stock from the factories. IMO this is the way of the future.

Dan
'91 928GT S/C 475hp/460lb.ft

Louie928

Very true. Just about any form of forced induction is economical power and the "pump" takes care of what may otherwise be difficult intake design deficiencies.

123quattro

Well, mostly yes, but not completely. A better intake design will make more power than a bad one. The difference is just much less when comparing the same two intakes on a boosted engine vs. a max effort NA engine.

V8 Man

Hey Sterling M800 or M48 ?

BC

With the engine analyzer software, changing the length of the intake runners on the S4 engine produced (for me) up to a 150hp difference at 5500 rpm.

But this in extreme circumstances (and high boost).

123quattro

I wouldn't trust that number. As a trend yes, but as an absolute no.

Lizard928

V8 man, check out Sterling's website, link in his signature. he lists what he is using there.

As well for the ITBs on a bone stock engine with retaining the LH/EZK I would imagine you would see probably between 20-50 HP gains, depending on exhaust work, cams etc.

V8 Man

Thanx Lizard, good tip

Louie928

I mocked up an S4 engine in Engine Analyzer Pro last night. Stock exhaust and intake. The tq and hp curves are pretty close to S4 after some manipulation of the unknown specs, such as flow coefficients, to get to what we know the engine does make. Then I changed intake to ITB with 50mm throttles with 8" runner length. I didn't try below 3500 because EAP can't do the Helmholtz resonance/double tuned stock S4 intake to show the lower torque peak. This isn't to be taken as absolute numbers for the ITB, but only an indication of the change for comparison. Even with the maybe nice ITB gain, your power increase to $$ ratio would favor supercharging. However, I plan to try one on a GT just to see.
https://rennlist.com/forums/attachme...1&d=1221667132

Louie928

That will be the plan. All the gain is above 4500 rpm. If I get too greedy, there is always the oiling problem at high rpm to keep a lid on it.

Peter F

Thanks Louie for a lot of interesting and detailed information as always
Can't wait to see what numbers the GT will produce with this setup.

/Peter

BC

The exact number, no, of course - but the system parameters favor the shorter runner, and they also show very little loss down low as the compromise.