iroc_cragg

Hey

How big is the stock exhaust system on a 1985 IROC Z. Is it 3" or 2.5"

Thanks

Mark A Shields

It's not 3". I would most likely say 2.5".

Corbi_85-IROC

I'm pretty sure its 2.25. I can't be sure though cause its been a while since I've had the stock size.

DiecastZ28

The LG4 is 2 1/4" I doubt a 350 is the same.

Jono

out of the cat its an oval, 2.5 inches high and I believe 3 inches wide, its not mandrel pressed so it changes size all the way through. I'm not exactly sure of the y-pipe though, i have it sitting at the side of my house, I'll go check it later today when I get home from work if its not raining

1991 RS 305

I also have a 85 TPI 305 IROC, in Silver. Mines the 4 bolt flange into and out of the cat, 3 inch total area, but oval shaped. This funnels down to 2.75 inch mandrel bent pipe to STOCK muffler. As it enters the muffler, its reduces to 2 1/2 inch. Trust me, I have researched this system well and I am this weekend changing it to a Dynomax Ultraflow welded #17227, 3 inch inlet and dual 2 1/2 inch outlets. I have studied this topic well, and was going to go to have a shop near me bend a 3 inch pipe from cat to muffler, but he could NOT mandrel bend it. Therefore, the stock 2.75 inch mandrel pipe outflows a crimped 3 inch. Can't wait to get it done this weekend and hear/feel the difference.