Other than the obvious diameter of the pipes, is there a difference in the pipes? My car is a plain jane LO3 firebird, i have a dynomax cat back system and a y pipe off of a tpi car, the tpi wont bolt up, but the tbi pipe will. Do I also need the manifolds off of the car?, they are available too. I always thought the 305 manifolds were the same, this 91 bird has the same EXACT manifolds as my 85 TPI IROC had. Am I missing something here? Any help is appreciated.
Ben
Ben
come on...no one has done this before?
John in RI
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TBI and Carb LG4 exhaust manifolds and Y-Pipes are exactly the same style.
TPI and Carb L69 Exhaust manifolds and Y-Pipes are exactly the same style. ( TPI Cat is 3" and the L69 Cat is 4-bolt )
TPI/L69 manifolds require a TPI/L69 Y-Pipe and TBI/LG4 manifolds require a TBI/LG4 Y=pipe. Also pay attention to what way the Smog check valves are facing as they are forward in some set-ups and facing the firewall in others.
I have heard that the 350 TPI manifolds are bigger than the 305 TPI stuff - but I do not believe this to be true. ( I don't have any 350 manifolds for comparison ! ) The other difference in the exhaust system is the spacer on the passenger side between the manifold and the Y,..... the LG4/TBI spacer is of a smaller diameter than a TPI of L69 spacer. ( carb cars also have a vacuum operated flap inside this spacer,... TBI and TPI do not have it. )
I just finished re-building a TBI engine in a 91 RS. I installed 305TPI Exhaust manifolds and Y-Pipe to the engine rather than the stock TBI parts. The TBI cats back was still installed. Because the systems are not compatible I had to have a larger flange welded onto my TBI cat-back that matched the "ball of the TPI Cat converter.
In your case; If you want to use the TPI cat-back without modifications you will need to install the TPI Y=Pipe. In order to install the TPI Y-Pipe without mods,.......... You will need the TPI manifolds.
TPI and Carb L69 Exhaust manifolds and Y-Pipes are exactly the same style. ( TPI Cat is 3" and the L69 Cat is 4-bolt )
TPI/L69 manifolds require a TPI/L69 Y-Pipe and TBI/LG4 manifolds require a TBI/LG4 Y=pipe. Also pay attention to what way the Smog check valves are facing as they are forward in some set-ups and facing the firewall in others.
I have heard that the 350 TPI manifolds are bigger than the 305 TPI stuff - but I do not believe this to be true. ( I don't have any 350 manifolds for comparison ! ) The other difference in the exhaust system is the spacer on the passenger side between the manifold and the Y,..... the LG4/TBI spacer is of a smaller diameter than a TPI of L69 spacer. ( carb cars also have a vacuum operated flap inside this spacer,... TBI and TPI do not have it. )
I just finished re-building a TBI engine in a 91 RS. I installed 305TPI Exhaust manifolds and Y-Pipe to the engine rather than the stock TBI parts. The TBI cats back was still installed. Because the systems are not compatible I had to have a larger flange welded onto my TBI cat-back that matched the "ball of the TPI Cat converter.
In your case; If you want to use the TPI cat-back without modifications you will need to install the TPI Y=Pipe. In order to install the TPI Y-Pipe without mods,.......... You will need the TPI manifolds.
I greatly appreciate the info, that solves my dilemma. i will have to get the manifolds from the guy i got the pipe from then, i would love to make use of the larger exhaust for obvious reasons. Thnks again.
JamesC
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Apr 15, 2026
For more info on manifold size, check post #4 of the following link:
https://www.thirdgen.org/forums/exha...-question.html
JamesC
https://www.thirdgen.org/forums/exha...-question.html
JamesC