i have an 1985 firebird 4bbl 305 im replacing the cat with a straight pipe but i don't know how big the stock pipe is is it 2 1/4 or 2 1/2

 

2 1/4 all around... did the same exact thing a while back... sounded better in my application, didn't trigger SES light either.

Good luck,
Brandon

 

LG4 (base 4bbl 305) had 2.25" I think. The L69 (305 4bbl HO) had 2.75" pipes.

 

I just finished doing the exact same thing. Removed the old cat, y pipe, and manifolds. I upgraded to manifolds and y pipe off of a '91 iroc, the y pipe outlet was 3". I had to get a pipe made that went from 3" - 2 1/4" at the rear pipe. Sounds really nice this way and I noticed as well that no ses/eng light has come on.. very pleasing ). I'm trying to figure out what kind of HP you get from such a mod. As well as removing the A/C compressor and lines.

 

i actually have a similar question, i bought an 86 trans am 305 engine and has tpi, whoever owned it before me tore this car apart, i am trying to figure out what the stock exhaust was on it because i believe i am not getting the proper back pressure for it. it has a single exhaust pipe and where the two pipe come into one it is 3" the entire way back with a "super turbo" exhaust pipe at the end. like it said i an just trying to play mad scientist and put this car back together the way it was meant to be!! so any help would be apreiciated

 

when i took off my cat i get a eng light after driving for like 30 mins...

 

Backpressure is the enemy of power... you don't want it. A 305 TPI will do just fine with a single 3" exhaust... in fact I would probably recommend that size.