First I've heard of them.
They came off a 91 Formula.
Are they considered rare?
How would they compare with headers on a <5K RPM torque motor?

I posted in the Exhaust forum too.
Need a quick reply.

 

Any L69 or TPI manifolds from any year are as high-flow as you can get for one of these cars. There's nothing special about 91 Firebird ones. And they're still a long way from what headers will do for you. And, if you change the manifolds, you'll still have to deal with a Y-pipe since the LG4 / LU5 / L03 Y-pipe won't bolt to the larger manifold outlet; and the larger Y-pipe won't bolt to the small cat you have, so you'd have to change that; and the larger cat won't bolt the the I-pipe you have, so you'd have to change that; etc. In other words, you'd run exactly as much maze to do that as you would to put headers on, except there'd be a whole lot less cheese at the end.

Just get a set of application-specific headers FOR SOME OTHER CAR, specifically a L69 or L98; and the cat & cat-back to match.

 

I just went and checked the "high flow" manifolds aginst what my parts car (87 GTA) came with, and yep,, same thing. Glad I didn't bid on them..
My parts car came with a complete stock exhaust. It looks much bigger than my stock CFI set-up. I was planning on using the whole GTA exhaust system thru the cat, then an aftermarket cat-back..
I really hate headers, but looks like I'll have to use them if I want to see the true potential of my heavy breathing TPI. A necessary evil I suppose. Thx.