question for you mod gurus...just by swapping these in, will the engine sound louder? is there any increase in HP? engine is stock aside from an EVO Vflow system and PSE.

 

I don't think so. They are less restrictive than stock headers, but the cats are where the real gains are from.

 

i have them, sound is not louder but different. Definitely a better tone. Only way i can describe it is that you can more or less hear the sequentianl exhaust pulses which sounds really nice. Like a big sport bike motor.

 

I have them on mine, it sounds a bit more throatier... For HP gains, not so much... The acceleration has increased just a tiny bit, due to the less restrictive exhaust. The throttle is a little more responsive than before.

 

I too also have the X-51 headers with stock exhaust system. Sound is slightly throatier from 4K onward. There's a bit more response in the midrange from 3500-4500 rpm. Feels peppier in that range, courtesy of the butt-dyno .

 

I debated doing the x51 headers for a while. If you can get a good deal on a used set, go for it!

 

You'll notice a slightly different tone from doing just the headers, but as a few people have said, to unlock a little more hp and sound, you'll need to replace the more restrictive parts of the system, i.e. cats.

High-flow cats, exhaust, and an EVOMS tune will give you some nice gains across the board.

 

AWE cats seem to be the popular choice among the posts i have read, is there any other cats recommended as well?

 

Absolutely, cats are the only part of my exhaust that made a difference. Headers, mufflers, and intake only change the sound.

 

All headers and mufflers make a little difference, but not huge... The difference is not that obvious... 5 HP will not make your car go from 0-60 .2 seconds less... But it will improve the responsivenes...

 

I used to subscribe to that same sentiment until I found the AH pipes. I've dyno tested them 10 ways till sunday on the 996 and 997, noticeable gains both times on the chart and on the butt. I don't know the physics behind it, I just know they made a (surprisingly) noticeable diff on both of my pcars. - I'm glad because I did not want to mess with changing cats or headers.

Typically though and from my own experience, what you are saying is the norm- I only change mufflers for the sound factor.