dose it effect it?

 

You'd see some gains if you were to switch from a stock one to a nice high-flow.

 

I got a solid 2 tenths out of my car with just a full exhaust (edelbrock headers, flowmaster american thunder) by taking off the converter when it was still stock tpi. I even had a random technology cat on there. If you got a single cat car, it definitely costs some power.

 

I just recently put on a carsound (magnaflow) cat on my single cat car. You could see through it, somewhat like a bullet muffler. It can't hurt flow that much.

 

High flow cats do not restrict power like they used to (up to certain HP levels). There are plenty of dyno and flow data sheets on the web to prove this. You will always see your biggest gains with a complte system that incorperates larger pipe size (over stock) and mandrel bends.