I'm gonna be doing full exhaust over my spring break and I'm wondering if the power gains from going to dual cat headers would be worth it? I plan on starting to build a 400+ hp 355 by the end of the summer so I'll want to use the exhaust I'm going to get on that motor also. Would it be worth the extra effort and expense to get dual cats now in preperation for the motor? Whether single or dual cats, I'll be getting SLP 1 3/4" headers. So what do you guys think? What diameter are the pipes in the single cat setup and dual cat setup? Thanks.

 

Factory dual cats are 2-1/4" OD, single 3". If you figure in wall thickness, the flow area of the duals is slightly more than the single. That is ignoring boundry effects on flow, and any flow restrictions induced by y's, bends, etc.

If you went with dual 2-1/2", it's no contest - duals win.

 

GM makes new N10 dual catalytic converter exhaust available, freeing up 13% more power from some LB9 and L98 equipped Trans Am's.

from http://pontiac.sk/history.php

so factory dual cats helped out a bit..

 

Dual cats are a waste of money compared to 3" high flow setup.

 

If I were you I'd just run no cats, unless you need them of course. If you really think you will be making 400+ hp, then you'd probably benefit more from dual exhaust. I'd recommend it. With dual exhaust you'd obviously be running dual cats too.