Here is a quote from the isupage on exhuast basics:
"Again, the entire system will only flow as well as the tightest restriction, and regardless of the flow capacity of the header and exhaust system, a two inch catalytic converter will limit exhaust flow to what will pass through a two inch diameter hole going into and out of the catalytic converter."
"Installing a performance muffler onto OEM or OEM replacement exhaust pipe will not improve engine performance noticably and will at best simply increase exhaust noise. The problem is that the small OEM size pipe and its squeeze bent construction restrict exhaust flow to the point that a freer flowing muffler will make no measurable reduction in back pressure. The muffler itself is only part of the problem, the pipe itself is most of the problem."
