Hello,

I completed my build of a 357 for my 1991 GTA. I have AFR 180's with Accel high flow intake and runners. Cam is the Lingenfelter 74216. Static compression is about 10.5:1 and dynamic compression is about 8.34:1. TCI Streetfighter 700R4 with 2,600 stall.

I have a stock 48mm throttle body on the car now that is sticking. So, time to replace. I have been looking at Summit at their brand of throttle body as well as BBK.

So, my questions?

1. Based on my motor, will I gain anything by going to a larger throttle body?
2. If so, what size 52mm or 58mm?
3. Is the Summit throttle body a quality piece or should I go with BBK?

Thanks for the help!

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bump

 

I would recommend the 58mm throttle body. You will need to pull the plenum and open up the area behind the bores.

The throttle body will not cause you to lose low speed torque. The price for a 52 or 58 are very close so go with the bigger. Bigger is not always better but in this case it is.

I do not have experience with the Summit throttle bodies. A while back I was selling Professional Products throttle bodies and never had a complaint.

 

Put a vacuum gauge where you can see it while driving and run a line to your plenum. Get into 2nd gear (with automatic) and go WOT. Have someone watch the vacuum gauge. When it reads zero at WOT, which usually happens at low RPM, that means you do not have a restriction from the plenum forward.

As the RPM comes up you may see the vacuum start building. If you reach your top RPM with only 1/2" vacuum or less, then you probably will not gain from swapping to a larger TB. Around 1" and you could gain between 1% and 2% power at the top of your gears. Above 1" vacuum, or vacuum at your shift recovery RPM, and you should seriously consider addressing the restriction, whatever it may be.

A swap to a larger TB on EFI usually requires some tuning work to make up for the increased manifold pressure at given throttle positions, especially just off idle.