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Stock pipes/headers are 1.75". I don't know about the cat. converter. But I think if you go bigger than a 2" pipe diameter, you start to loose your low-end.

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Yes, cat's are directional. The O2 sensor is on the inlet side, wouldn't work right being on the outlet, least not on our Soobs.

 

As far as exhaust size goes, it's been batted back and forth on here many times. Consensis is to use 1-7/8" ID pipe from the heads to the cat, then some go 2" ID others 2-1/4" ID back to muffler, then maybe 2-1/2" tailpipe.

 

If that cat is 2" ID inlet, then 1-7/8" ID pipe should fit inside it as it's basically a 2" OD pipe. Note that I said "should fit".

 

I think you'll be alright with the 2-1/2" outlet, as it's mostly the pipes from the head ports that are the critical ones for size. Would probally be of good benifit to make both header pipes the same length leading to the cat. You'd get a better balance of exhaust flow that way. Think it was Phizinza that did an equal length header pipe on his, looked good if it didn't improve anything else.

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