i have a 1989 GTA with a L-98 TPI i have all the bolt ons and ported plenum ETC. the ? is i still have the cast iron manifolds and 2 3/4 catback.im puting on 1 5/8" headers and 3" y and 3" flowmaster american thunder catback next week how much H.P. will i gain? will i really feel the diffrence?

 

yeah you should really feel the difference since u are moded.
i am going to guess about 25 hp its hard to say but exhaust in your case will really help out.

I gained about .3 in the quarter mile, with slp 1 3/4 headers, no cats, and slp catback.

 

You could easily see up to 25 at the wheels with your L98. I just read an article where an auto 99 LS1 car made 34hp more at the wheels with longtubes, cats, and cat back over stock. That is another sotry with hat motor but you will feel and hear a seriuous difference with your set-up. It will make you

 

Howdy

I have the same car that you do with dual cats. I also did some intake mods etc. Then I put on the SLP Tri-Y 1 5/8" headers. That was the single biggest seat of the pants improvement I made to my car. Like the others said I would think 25HP easily. Allen