Right so i'm after a decat pipe for a ZT conv. It's got all the running gear from a FRS so obviously i can pick a decat pipe up off the shelf.

My question is.. i have seen some that are 2.75" which flow better.. a) will this even fit to the flexi? and b) will the sudden step down in size cause pressure and give a little blow past the gasket..

This is probably stupid shit, but i'd rather buy once not twice.

Cheers Lads!

 

What is the bore size of the rest of your system?

 

If the flange is made to bolt in the cats place then it will fit. I don't think pressure will cause a problem as cats are very restrictive so in theory that would build pressure as the gas try's to escape through it. And that would more than likely cause a blow with hot gas trying to escape where it can, but that doesn't cause a blow. So you'll be ok I reckon . Just make sure the de cat pipe is the same diameter as your exhaust.
I will say that make sure you fit flanges on straights and not on bends. I had a full stainless made that had de-cat and it had a joint on the down pipe bend that was forever blowing gaskets it soon got sorted after an argument

 

It's a 2.5" (standard size)
But i think SBT83 cleared up everything here! 2.5" decat it is! Good that you got your decat sorted, scream loud enough and you will always get what you want!