3" cat back
 

I know a 2 1/2" cat back will bolt on to a stock 90 GT, will a 3" cat back bolt on as well? If not, what will I need to do?

Also, will a 3" cat back on a stock motor make the car lose some power? I know a 2 1/2" will build some power but I'm thinkin on a stock engine 3" might not provide enough backpressure.

Mufflers
 

I've heard that if you have mufflers that is enough back pressure for a stock 302 efi engine. That is why a lot of people put MAC Off Road X Pipe. Of course if your car is OBD 2 you need some kind of Cats or the engine light will come on.

Yes, you probably will lose power going from 2 1/2" to 3". What you have is perfectly fine. Just another note: If you have an offroad pipe (no cats) your check engine light will not come on. ~Brian

You won't lose any power. The whole backpressure thing is really kind of a myth. All you need is to have something extending beyond the headers to improve scavenging. Not to mention if you have a 2 1/2" H-pipe, there will be backpressure present anyways.

i was also wondering..will a 2 1/2 cat back bolt up to a 3 inch prochamber?