What size pipe for cat delete?

[cyaford]

I'm cutting the cats off of my non-street legal off road use only '97 GTS.
What size pipe are you guys using for cat deletes? I've been told 2.5" ID and 2.5" OD by different people. I hate to run back and forth to AutoZone. I have the stock manifolds with cats(2.5"), and what looks like an older 3" Corsa cat-back exhaust(if that even makes a difference). I plan on clamping the pipes down to see if I can take the sound, and also to make sure I don't get the sickly smell that some owners get. If everything sounds/smells good after 100 miles, then I'll have them welded. Any thoughts or suggestions would be appreciated. Thanks!

 

[1999GTSsilverwblue]

I did a comparison of 3 inch ID Cats vs no cats and found around 10 RWHP difference with 3" Corsa Cat Backs . I have 791 RWHP with 3" high Flow Random Tech Cats . It wasn't worth the smell hassle for 10 RWHP.

Corsa Exhaust can be identifird by their unique 4" od/ 3" ID tips. I visited Corsa and they do indeed increase flow about 110% over stock and eliminate that resonance/drone at 55-70 mph .

My Venom will make people jump when Istart it and it is LOUD without being irrating when cruising .WOT is great.

There are 2.5 to 3.0 tapered adapters to connect to your headers.

That is my .02 cents.

John

 

[cyaford]

I should've clarified the main reason I'm doing this is for the heat issues, not any hp gains. My side sills have heat blisters. I recently took them off to go get estimates on a paint job. The heat shields were toasted around the cats, but looked brand new everywhere else. I thought I'd go ahead and cut the cats while I have the sills off. That way I wouldn't have to worry about future heat blisters. If I'm with the group that gets the bad smell, I'll go with the high flow cats.

 

[EllowViper]

Just did mine yesterday. I have the Corsa Cat-Backs which accept the 2.5 OD pipe perfectly. Now the "tricky" part. The Corsa accepts the 2.5 OD yet to get a good slip-fit on the section of pipe you just cut to get the cats off, you need 2.5 ID. Here's what I did. Went to a muffler shop and had them cut a 2.5 OD pipe 24" long. On the end that will go where you cut the cat off, have them stretch the opening JUST A LITTLE BIGGER...NOT MUCH...JUST A LITTLE. Not enough for the 2.5 OD to fit back in..THAT'S TOO MUCH...just swedge it a little. The AUTOZONE 2.5 ID/2.5 OD adapters were still too loose on the stock pipe...I think the stock pipe must be somewhere just under 2.5 OD. Maybe since it's just after the bend it gets reduced in the process. At any rate, a real simple thing to do but the fit on the stock section of exhaust can be a real PITA. If you are welding, probably no big deal...

 

[EllowViper]

Just got back from a short cruise w/o cats for the 1st time. WAY TOO LOUD for my taste..plus the OEM drone is back. Looks like a set of high-flow cats are back in the equation. Too bad. I was really looking forward to a deeper/racier sound...but now it sounds amateurish vice "tuned".