"Be advised, upgrading the exhaust system on your boxster (i.e. headers and cats) is a serious undertaking that requires a systematic approach."
Very good advice. Shortly after I got my 2000 base boxster (about 3 years ago) I started playing with exhaust upgrades. Just to throw another variable into my exhaust tale, the first mod I did was a high-flow intake and chip. I picked up maybe 15 hp on the dyno. The chip was optimized for both higher flow intake and exhaust, which is why I started messing around with the exhaust. So my experience with exhaust system is with a modified ECU.
I first added headers, then high-flow cats, then muffler. I used quality parts (TTP) and I ran dynos after each of the upgrades. I picked up about 5 hp with the headers, very little maybe 1-2 with the high-flow cats. With the mufflers I lost hp and torque with two that I tried (Gemballa and a Dansk-type), maybe picked up a little with the Dansk high-flow (looks like stock) that I currently have on the car.
BTW, folks ask if you can feel the difference in performance with the butt dyno, sometimes I sure can. I was convinced when I put the Gemballa muffler on the car would be faster. It sure sounded faster, but it sure felt slower, and the dyno showed a loss of several hp and a stinky torque curve.
I also spent close to a year dinking around with chasing the CEL that came from the header cat delete and high-flow cats. Ended up having to replace all the oxygen sensors and one of the cats with all the messing around that was done. Car has been running great for the last year or more, so exhaust issues have settled down.
As far as I can tell (half a dozen or so dyno runs), in my car I may have picked up maybe 7 hp on a full exhaust (headers, high-flow cats, muffler) on my 2.7L. With the wronge muffler, only 3-4 hp. This was a very expensive learning experience. I actually had a lot of fun doing it, learned a lot, and the car sounds great to my ear, but would not do it again unless I was swimming in disposable income. Have fun,
Ed
