Yeah, race gas seems like a hassle for a real driver car. The extra power on some of these turbo cars is so tempting though...

 

Anytime you have a less restrictive exhaust on a turbo car you should always yield more power in my opinion. Especially with larger turbos. That's why you'll make the most out of catless straight pipes.

 

True BUT only if your other exhaust piping,muffler piping,can handle the extra flow of catless..I have been there done that with a Cargraphics exhaust.
You will gain quicker spool and low/mid end power time for sure BUT top end improvements depend above comment...

 

I'm not quite sure that makes sense. Have you ever ran catless on any of your exhausts? I thought you always had 100 cell on all your applications? I would be very surprised if you were to take a stock 996tt exhaust and gut the cats on it that you would yield more power all the way across the powerband compared to stock form. Now of course you would gain more power on say a 3" exhaust than the stocker as the muffler/piping would flow better. The faster you can expel exhaust gasses out of a TURBO car you should see gains all across the powerband. You didn't gain power all across when you removed the cats on your Cargraphics? I'm surprised if that's true.

 

Well,I spent aplot of money and time on the dyno,,so to come close to answering your suprise statement..
I didnot gain any top end HP/TQ going from a 200 cell cat to a 100 cell cat on my Cargraphics,just spooled faster and made a bit more in low to low mid RPM range..I never ran catless because Cargraphic told me it would not help over the 100 cell cats??
Went to a Milltek 100 system and picked up more HP/TQ,more boost and needed a extra fuel pump to feed my 72 injectors..Data logged on dyno, confirmed this by EPL doing the logging.
I measured the inner piping on both systems .The Cargraphics was significantly smaller the the Millteks..then I looked up flow rates of open piping with different ODs..That told me why there was a big difference. I am sharing my experience.
Also good cats,Like HJS have different HP ratings for the reason of matching up to the flow rating of the other piping in the systems being used.
I did threads on this when I encountered this..

 

On the GTR side, I've seen some big power come through 2.5" exhaust. It is much longer on a GTR, but all the gains are in the DP's and MId-pipes. The exhaust is really just for sound. We only saw 4whp on a 650whp car. It spooled only minutely faster too. The DP's make it spool 300rpm sooner though.
I would think the 911 would behave in a similar fashion, with 3"and larger only for those who want teh volume higher, and every last HP.

 

Very interesting stuff!

 

not an expert but from my experience not much was found from straight 2.25 muffler system to straight 3'' X pipe 6 resonator at 600whp level so i would say 2.5 id is good up to 650-700whp


the reason i switched for X Pipe is the more quiet and exotic sound you have!

 

I tried a catless Fabspeed 2.75'' X-pipe and I did a dyno test back to back which I did make a thread on in the summer.,,, couldn't stand it..
Put my 100 cell modified Milltek back on..

 

If you have external wastegates you can run smaller exhaust pipe diameter for same hp level as one with internal gates

 

Really? My SpeedTech 3" catless X-flow is glorious and doesn't produce hardly ANY drone. I wouldnt have it any other way.

 

Different type of mufflers and the X merger collector is different.

 

I believe that everyone agree that on our cars the best exhaust it the one that expels faster the engine gases. Now for a daily car, it is nonsense to run straight pipes. I tried it with race cats and couldn't slept the night after the install wainting for the openning of the exhaust shop to remove the pipes. The drone and cabin rattling was so bad at low rpm that it was even louder than the exhaust sound.

So what will have less back pressure and drone issues:

1. 100cells cats with full 3", X-cross and muffler
2. Decat full 2.5", X-cross and muffler

 

Good Question,
Only the dyno would know on each different build,
And it depends on how Large the 100 cell cats are.
IMO a good large cat on a 3'' should have more power potential.
IMO the drone would probably be similar..