What cat should I go with using Hedman 17478 y pipe? Seems most tpi cats I’ve seen are 3” but the y pipe is 2.5”?

 

DO NOT spend A SINGLE DIME on any 2½" exhaust parts, past the Y-pipe.

If the Y-pipe is 2½", throw it in the trash, and get a 3" one. No doubt Hedman offers one. Then get a 3" cat and a 3" cat-back.

Otherwise you are retaining, or INSTALLING, the LG4/L03 BOTTLENECK, a guaranteed restriction that will invalidate ALL other efforts to upgrade the engine.

 

Most if not all of the HO exhausts are 2.75". What causes confusion is that the stock intermediate pipe necks down to 2.5" where it enters the muffler. So, some of the aftermarket stuff rounds up to 3", some rounds down to 2.5". The low output exhausts, and 6cyl exhausts are all 2.25". So for some aftermarket exhaust parts, they average everything out to 2.5" and use one part fits all.

So, measure what you have first to be sure. Usually the ball/flare flanges are flexible enough to go up or down .25-.50". There's usually .25" just going from inside diameter to outside diameter. Most auto parts stores will have bushings, reducers, and adapters to go up or down in size. In fact, it looks like the Hedman Y-pipes come with a transition to go up/down a size.