I got an extra exhaust (cat section) from the PO when I bought the car. One exhaust has the cat intact, one has had the cat punched-out. I'm currently running the car with the punched-out cat, as I didn't want a cat during my wideband O2 sensor tuning sessions.

How does my punched-out cat exhaust compare to a testpipe? I suppose I might be getting some strange flow characteristics as the exhaust opens into a big empty chamber, then squeezes back into the exhaust section. Thoughts, ideas? Do you think I'm losing any ponies with this set-up versus running a testpipe?

 

I think the main problem is the double walled piping before the cat collapsing...if yours isn't collapsed then I don't think its much of a big deal...

What you can do though is stick a piece of 2.5" or so piping in the cat..and walla...no more big open area...

 

Yeah, I am thinking the big open chamber would allow for a big standing wave to set up and mess up exhaust flow.

 

Most testpipes are 3 inces too, versus the 2.5 inch factory pipe. Plus it eliminates that (very possibly collapsed and choking your engine) double-wall.