Originally Posted by Chris N
I interchange my test pipe and cat often, and the change in volume is ... non existent, zero, none, nothing. I do have a fujitsubo rm-01a, which is neither loud nor quiet (just right in my book, mellow yet agressive?). I noticed a) no change in tq/hp on the dyno between the two (only change between runs on same day) and b) no change in volume.
I've heard this before, but when I look inside a cat I can't believe that it wouldn't effect the flow. Of course I've never used a highflow cat or compared them on a dyno, so my opinion doesn't mean that much. The main reason that I didn't get a high flow cat was the cost. They are very expensive, and don't last very long. So I replaced mine with a small glasspack ($25 plus labor to weld on flanges). Luckily no emissions tests here in Tennessee. My stock cat was all f***ed up. I had been running very rich (and boosted) for 30K miles. I was already getting code 67 when I had it replaced.