Throttle body upgrade questions for NA engines

[Eiger]

Instead of hijacking Barks944's thread I thought I'd post this question here:

ORIGINAL: os951

Bigger TB can cause problems on lower rpm's since flap opens just as much as original lap does.

Could you explain why this is the case on a 2.5 normally aspirated engine Oli?

The standard TB is only 57mm from what I've found searching but the 928 is 75mm for eg and other BMW ones seem suitable too. Surely thats a good upgrade for better in flow of air into the intake manifold. Why will more air into the intake be a bad thing on a mappable setup like Barks' on a NA engine with a MAP sensor not AFM?

I've been told by a few engine bullders and read numerous comments on how the big problem with the na is the breathing and intake restrictions not the internals.

I'm not sure how big the opening on the na intake can be made but it's around 60mm as standard if I recall. Larger than the TB fitted as standard. It'd be interesting to know just how big that could be opened up and matched to a bigger TB.

Custom throttle bodies are not that expensive and if we can fit the oem throttle position sensor why not go larger is my thinking?

 

[os951]

Because oem style wire operated throttle body does not work progressively. Smaller and bigger TB's open just equally when you press accelerator pedal. Even if you have re-mapped the ECU or use standalone. Its pita to drive big TB cars at slow speeds. It helps if yo can build progressively work TB, or build a special TB flap, with restrictor. Audi have a wedge shaped restrictor installed in several engine type's TB's from factory.

 

[Lowtimer]

The underlying reason for this is to keep the gas speeds up at low revs. The bigger the throttle body, for a given throttle valve setting and RPM, the lower the gas speeds will be. A big throttle body is the same as an oversized carb in such respects- it only really works at high RPM, and leaves the thing worse off at lower RPM.

 

[Mike_Pollock]

Would having twin smaller TBs work? Maybe with differential opening?

The carb Lancia Betas had twin choke carbs and you could feel the 2nd choke coming in at 2500 - 3000 rpm

 

[Lowtimer]

Possibly, but the variable restrictor type as highlighted by Olli would seem to do the job more effectively and with less equipment cluttering up the engine bay.

 

[barks944]

If someone can figure out a suitable replacement TB, I'd be happy to do a review of the effects with different air intake/throttle body combinations. Initially I can report on the effects on manifold pressure, but I will be hitting the dyno soon as well .

On the topic of internals, I would say the 944 is pretty bomb proof within reason. The crank is immense and the rods and pistons are pretty solid.