Which part of the X's exhaust is the most restrictive? For instance, replacing just the axle-back would give you how much hp? Just Midpipe? Just Downpipe/O2 pipe? Anyone done any clear experimenting on this (keeping everything stock except for the piece being tested)?

 

I'm pretty sure it's the 02 housing section. Secondly I'd say the test pipe if for no other reason because it helped spool up tremendously

 

I think it's a toss up between the muffler and cat.

I measured the factory system at 9.5 psi backpressure in the down pipe, like 8 inches in front of the cat.

Nobody has done pressure measurements of each component, and I suspect I'm the only one who has done the whole system. However, there are lots of dyno graphs from shops changing just one part vs the other.

 

The stock muffler for sure. It's really badly designed with respect to flow.

 

^^+1 believe it or not The stock muffler is usually the most restrictive part in an exhaust system

 

pretty much everyone got it. O2 housing, cat, and muffler.

 

Check out the stock muffler and how many turns and chambers the exhaust has to go through.