I am the original owner of a 1989 GTA with a 5 liter TPI engine with a 5 speed manual transmission. A year ago I installed a new 5 liter long block from GM and had a Comp Cam and Heddman Hedders installed. The catalytic converters are gone with a 3" exhaust pipe emptying into a Walker super turbo muffler. A Bosch mass airfow sensor and K&N airfilter have been installed. The stock twin 48 mm throttle body is still in place and the plenem has not been ported and the two humps have not been ground down. My questions are: 1) with such a set up would I benefit from a twin 52 mm or 58 mm throttle body and which one both BBK and Holley make these; and 2) would I necessarily need to upgrade my stock 19lb/hr fuel injectors with a bigger throttle body. Once I make this throttle body decision I will complete the engine with a modified chip. I have received mixed opinions on whether a larger throttle body is required for my engine set up. I would appreciate some opinions and people's experience on this subject. Thanks.

 

Double check with the TPI guys but your 48mm throttle body is more than enough. In fact, I don't think it becomes a restriction until you are making 400+hp. The 58mm is only for HUGE inch small blocks making monster power levels. There are 396 LT1's out there making 600 hp and they aren't even using the 58mm TB. Your next mod should be doing something about the stock TPI intake/runners/base. Like I said though, double check with the TPI guys.

 

I agree with the above post. You will be just fine with the 48mm throttle body and a larger one will not add 1 HP to your car. Put your money into other areas such as the intake system. Sounds like you have upgraded your exhaust. Speaking of the exhaust make sure you do not still have the factory "Y" pipe. A good aftermarket "Y" pipe is good for at least 10 HP. Allen

 

DO NOT GET A 52MM THROTTLE BODY!!!! Is that clear enough? I had a freind running 10's with his stock throttle body. At that point in time he should have gone larger but sold the car before that. You are better off doing something else other than that. I would say port out the plenum and lower intake if you want to feed more air into the motor.

 

Thanks everyone. I have made up my mind I will stick with the stock twin 48 mm throttle body and save myself several hundred dollars.

 

FWIW, the GM crate Ram Jet 502 makes 500 HP using the same 48mm TB you've got (Vader pointed that out a little while back).

Of course, there are a few other differences betwwn a Ram Jet and TPI, but...