I will be getting the Hooker super comp. shorties for my 89 l03 formula and the hooker catback for the TPI cars. My question is what size high flow cat will i be needing. 3INCH IN/OUT?

 

yep, 3"

 

Better bet would be to get a high-flow, direct-fit cat for single-cat TPI applications. No modifications required - will mount to both the y-pipe that comes with the headers (assuming you're talking about p/n 2055), and the cat-back.

 

I thought i would need a tpi cat if i didn't want to do and modifications. I just wasn't sure on the size of the hooker y pipe.
Thanks

 

So that would be 2.75 in and 3 out right?

 

i believe its 3 in 3 out