i have an 03 Coupe and my left cat is bad.... So i want to go aftermarket, why wont 350Z cats fit the G35 coupes? Because i was looking at the Ultimate Racing Cats but they say 350z only... Sorry if this is a dumb question

 

Careful of aftermarket cats they are designed to just pass one emission test [how why they are high flow- big air space in honeycomb and thin coating of catalyst].
Unlike oem which are built to last 100-150,000 miles at the expense of a 1% higher restriction..tighter honeycomb, more reactive area, thick catalyst coat.

 

Any Cat that fit's a "Z" will fit a "G". There are good after market hi-flows out there. I have a set of Crawfords. I don't believe anyone on these forums has failed an emissions test with them. They are far less restrictive than the stock cats, one chamber vs two, etc. BTW, it is unlawful to replace any cat on a late model car, unless it has failed, which yours has, or has less than (not sure of the milage number) 50K miles on them. In any event, both your cats are covered by the manufacturer's emissions warranty. Hi Flows are not designed to pass just one emissions test, not sure what that even means. However, there is no emissions warranty with any of them, and they all say "for offroad use only." It's your call, I chose Crawfords, and have had them for 2.5 years now, with no issues.

Lou

 

Measure the ACTUAL emissions components out of so called high flow cats you will see that they are not equal to oem cats after a year or two of use........in HC reduction and CO to CO2 conversion.

Off road use is an admission of not meeting FEDERAL specs.