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Originally Posted by jpnovak View Post
First, let me clear up a few misconceptions.

ITBs do improve throttle response in NA and boosted applications. For NA this is done by providing equal intake tract lengths and plenum volumes. Equalizing the distribution across each cylinder builds power and provides balance to the air-flow. In boosted applications the throttle response comes from the location of the thottle plate relative to the intake valve. Remember the throttle acts as a pressure regulator of sorts. If the throttle is far from the intake valve there is a time lag to pressureize the volume between the throttle and valve. For a common plenum this can be substantial. For ITBs it can be greatly reduced adding to throttle response.

The addition of a common plenum style tract to ITBs is a secret of tuning. Porsche has done wonderful engineering to optimize the resonance tuning. Think V-ram at this point. You can take advantage of the Helmholtz Resonator Calculations for intake tuning and then the throttle bodies help throttle response. It is not hard to cut the lower section off a Carrera manifold and mount them to ITBs. In fact, it is even easier to do this using a 3.6 manifold. To quote Steve Weiner, "truly impressive gains" can be had.

Some high HP motors do suffer from low speed drivability. Some of this is due to tuning but usually it is due to equipment limitations. Large HP numbers require large flow injectors. These injectors have a finite limited open/close time usually around 1mS. They just flow too much fuel during that time to run clean at low rpm. The solution is stacked injectors (duel fuel rails) or low impedance injectors that control the transient response better for faster open/close response.

Placing injectors at the top of the intake tract behind the butterfly can help higher rpm and WOT response. The fuel simply has more time to atomize before being sucked into the combustion chamber.

I absolutely 100% disagree that mid range throttle response suffers with ITBs. It is greatly improved save the tuning on the car. The issue is transient response on throttle tip-in. When one changes the throttle position there is a momentary lean condition and a huge rush of air reaches the combustion chamber. Unless someone monitors this and compensates with extra fuel (not too much) the engine will have a lean stumble. This happens on a 100mS time scale and can easily be overcome with enrichment parameters on an EFI system. CIS has no hope here.

The cost and complexity of these systems is due to making sure every component is properly matched for volume, volumetric flow, flow velocity (cross sectional area of heads and intake) as well as turbo and intercooler setup and finally fuel delivery (EFI, ECU, timing). You can't just have one piece of the puzzle. However, when you have all the pieces of the puzzle you get staggering performance. PWR is a testament of that.

Yes, I saw the PWR 996 car (2.1l built on a 76 targa?) during one of its inaugural races at TWS. Yeah, it was fast.
Jamie, thanks (as always) for that concise summation of the 'physics' of ITB's. Much clearer to me now.

If i am understanding correctly, ITB's in a turbo application are extremely difficult to tune correctly, and if not done so, will result in major driveability issues (large injector issues nonwithstanding), whereas single plenum intakes are very simple for the DIY'er by comparison, and for all intents and purposes, will not really be the limiting factor in producing more power (as evidenced by Gabe's motor)

That being the case, is the advantage of ITB's mainly just in the obvious improved throttle response, or are ultimately large HP gains in a turbo'd flat-6 only to be found with ITB's and improve flow vs single plenum intakes?

Andy, what were the design advantages of the 962 style tanks over the ones you were previously using?
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Old 11-18-2009, 06:29 AM
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