New part on the Evo today – early enough (just one autocross) that I’m considering it a pre-season mod. I replaced my 100 cell cat for a resonated test pipe. From the pics, it’s clear this will be a huge improvement in flow. Additionally, cars that run high ZDDP oil eat cats anyway. The pic shows the cat when cold, obviously, and these cells expand when hot. Regardless, that’s a pretty high blockage, so I am hoping the test pipe lets the exhaust flow better. Yes, it will smell more and it’s not great for the environment, but given this car is not really street-driven anymore, it makes sense to just run a test pipe. This is basically a resonator with flanges on each end, so I am hoping it wont add too much noise – or maybe even quiet things down more.
I’ll hold on to the cat in case I move to a state that will require emissions again, but this is my planned setup moving forward. Weight difference was marginal; 5lbs 11 oz for the resonated test pipe vs 6lbs even for the cat. This is likely due to the additional material for the resonator. A straight test pipe would be lighter, but I still prefer the car to be as quiet as reasonably possible.