My friend bobby ran many different turbo combinations... form 18gs to gt35rs over the years and a straight pipe always did better especially at higher boost levels...
just my .02 cents...
most of us with big HP run straight pipes for record numbers... including me...

 

If you do not build your exhaust flanges with a lip in a certain way, you have sound waves that exit the port, bounce back up the port and go up the valves and valve train. It is very real and the loss in power is evident. Matching ports, flanges and gaskets is a big no no. People have this issue and don't even realize what the issue is.

As stated above, A. Graham Bell does a great job of explaining in one of his books. I enjoyed the first book so much I have read all three. I had recommended Bell to Zippy on this forum a long time ago. Ask Mike what he thinks about it.

On another point, the cam can have the same issues. When we tested and designed our cams and Porsche stock cams we were not able to find one Porsche cam that did not have harmonic distortion. We designed ours with the objective of completely eliminating this problem and we were successful.

 

Heavychevy,
What range rpm that you feel? Stock turbo or mod? I have diff exp. My previous and many friends' turbo cars, Supra, Mitsubishi and STI engines: stock turbos & cat remove=better resp, little more tq all rpm range. Big turbos= all effect above with more. (I think with big turbo, cat becomes more restrict.) Latest I did with my 996tt. It improves as the other turbo cars do.

 

ttboost,
Last month I did with Cargraphics exhaust. Old model. Good quality and hard to drill and gut out. PITA. So it could be done and you could remove/grind around material housing area and leave it smooth.

 

One thing that's very important here is to not just run the same software with a full exhaust and then straight pipes. That's a very big change in terms of hardware and flow etc.... you need to re-tune for something like that drastic...

+1 on the books by bell

 

thats excellent, this is the area where people forget all the time.

+1 sharhster

 

You can really feel it throughout, but more so on the 3500-5k rpm's. There was not that punch there that used to be. Of course I did not re-tune for the straight pipes, so that may in fact be the problem. I was looking into something like a cross exhaust for that reason, but if that is the case, I'll stick with the SP's.

I completely agree with removing cats, done that, infact my TT hasn't had cats since I got it, they were gutted, then I moved to straight pipes. My Corvette also burned the cats out through the exhaust so it was catless and improved. But just switching from my catless full exhaust to straight pipes with no tune did the opposite of improve.