OK put my car on the lift tonight to check out the space. Originally I thought I would buy the Dyno Don headers that were made for a 91 Formula and run the twin pipes from the headers to the 2 cats....then was thinking about continuing to run twin pipes all the way back and over the axle and split into two straight tail pipes.

To my dismay there is not any room to go over the axle for twin 2.5" pipes as the stock pipe is routed between the crossbrace (or whatever it is...equalizer?) and is actually ovalled (compressed) for clearance.

So now I am considering going with a Y pipe from the headers into a single 3" Cat and running a new 3" stick all the way back over the axle in place of the smaller stock mid-cat back back pipe.

I also noticed and was really amazed that the stock twin pipes leading to the cats were actually bent purposely leading to fit into the twin cats from the factory....do twin cats and pipes that are so small really perform better then a single 3" cat and pipe?

Either way there is not room to run twin pipes all the way back over the axle if I was to come off of the stock cats with twin 2.25" pipes.

So I am thinking it would be better to merge them into one single 3" cat and pipe since the twin ultimately ends up in a 2.5" stick anyway over the axle into the muffler.

Just wondering what others have done with respect to going to a single from factory dual cats...and increasing the diameter of the tubing.

 

GM rated the dual cat system at 10 more horsepower than the single cat system. But that was using two 2.25" cats over a single 2.75" cat. I dont know if a 3" cat would be better than two 2.25" cats. Don's dual cat y-pipe is 2.5" diameter, so you would use dual 2.5" cats.

Its my opinion that dual cats is better than a single cat. You have 4 cylinders flowing thru 1 cat each, instead of all 8 cylinders flowing thru a single cat. Its the same principle why dual exhaust is so popular compared to a single exhaust.

There is room to run dual 2.5" pipes under a thirdgen. Check out the dual exhaust sticky at the top of the Exhaust forum for pics. However, there is barely enough room and its mostly for sound and looks. A single 3" cat-back or even a 3.5" cat-back is just as good and much easier and less expensive to run.

I've never seen any dyno comparisons between a single cat over a dual cat, though. I've also never seen any results from a single exhaust to dual exhaust on a thirdgen.

 

Thanks for re-directing me to that link. I had looked at before but forgot about Willie's setup that follows the stock locations. I will be copying Willie's setup as I like it the most. I was not interested in having a system that dumped before the rear axle...although I can understand why people choose to do it.

I wished Willie would post up his Dyno numbers or track times.