in my old setup I went from fabspeed straight pipes with cats to a custom exhaust with race mufflers and cats and I picked up 2mph. Both sets of cats were made by fabspeed and fabspeed claimed that the cats on the straight pipes flowed better even though they are higher cell count (200 vs 100). No other changes were made to the car.

 

This hurts my head - the turbo provides FAR more backpressure than an exhaust system will. Anyone logged knock sensors with straight pipes to see if that's the issue? this doesn't add up and it flies in the face of what I've found on other turbo cars. Actually measure the backpressure prior to the turbo and you'll see what I mean - it's usually fairly high especially on smaller turbos..

 

LOL. I agree. Just reporting my experience. In my case, it could be that the newer 200 cel cats which fabspeed claims flow better, indeed flow worse and are a bigger restriction. The older 100 cel cats may actually flow more and thus, even with more turns and inline resonators it still flows more than the straight pipe/cat system.

 

People argue this over and over again, but it's VERY VERY evident that low end torque is lost with straight pipes. Not to mention VividTx dyno to verify the butt dyno calibrations.

It's the truth. However they are just so light. Really Markski's exhaust is the perfect solution IMO, it's only ~2 lbs heavier than the straight pipes but also makes the power. Straight pipes WILL rob you of power at least with K16, K24 for sure. I don't know about the bigger turbos. This is a fact.

 

Good points.. I was wondering what happened to his trap speed on those AMS turbos, I was expecting 130's.. I may be the next "tester" for these.. That is if you plan to come over and help

 

There has to be somthing driving it though, if it is true. Has anyone actually logged af ratio or knock/timing advance during these comparisons? The striaght pipes should pick up power everywhere. The turbos should be spooling faster with the better flowing exhaust.

I am sure AMS will verify, but I have never had, or even seen, a turbo car lose power by dumping the exhaust.

 

What car are you running now? Do you have a 996TT and speak from experience with that car? Maybe it's inherent to the Pcars. Not sure.

 

...and they'll keep arguing it because its absolute rubbish on a turbo car

 

All I can say is that the only change I made was exhaust. Nothing else. and I gained 2 mph. My ET stayed within std deviation.

 

The P-car is still an engine built by men that must obey the same laws of physics. The backpressure the turbo presents to the engine FAR exceeds any exhaust system, the exhaust is just an additional cork. I'm NOT arguing that people haven't seen this - I'm trying to figure out WHY they see it because it makes NO sense. I suspect the knock sensor but that's obvious enough that I would think any number of tuners would have already seen it...

 

Maybe it's to do with the velocity of the gases as they exit the hot side... Let's say you had nothing attached to the flange on the exhaust side of the Turbo.. would that be a good or a bad thing? There would be a certain velocity on the stream of hot gas as it exited the turbo, but it would "spread" out in random directions very quickly and lose density and flow speed...

If you add a pipe now, what will that do to the exhaust gas? Will diameter affect the flow? Yes it will.. What would the perfect diameter be and would it change based on the length of the pipe? I suspect that a given combination of length plus diameter might be better than others.. I did some CFD (Computational Fluid Dynamics) at University and some very counter-intuitive things happen that would seem to be opposite to that which logic would dictate.. The same is true in Aerodynamics... strange things happen to air when you try to force it do do what you want it to do... giving a slight nudge and a hint at what you would like it to do is often a better solution than "making" it go somewhere (because then it doesn't).

I'm sure that someone out there is a Physics buff and can tell us what's at play here...

 

what you say is all correct, and my tuner agrees with you(Kevin at UMW) and he's not just a tuner, he's an engine builder, can machine heads, build turbos from scratch and is a mechanical engineer. However, I still gained 2 mph. And he is sure it is do to the cats that were on the straight pipes. They were severely restrictive compared to the old cats. The seller of the exhaust says that is not the case. who knows

 

I agree that no pipe at all would be a mess for all the reasons 80shilling stated. I'm also aware that air and gas do funny things and that it's often counter intuitive. But other cars pickup power doing this. Clogged catalytics in one setup could certainly have been the issue. A "straight pipe" with catalytics to me isn't quite a straight pipe either IMO, I'd also not use it as an example if you suspect there was something else at play. Honestly though I'm surprised more people aren't stepping forward with their experiences. If others had experiences that mirrored yours - especially since your engine builder and tuner found your results funky - then I'd really be worried.

What have others seen? Gains? Decreased torque? Anyone got back to back dyno sheets? IMHO - should be gains nearly everywhere in boost. For me it's pretty academic though since the noise with my "muffled" exhaust is already louder than i'd like in the cabin

 

I've always been under the impression that backpressure isn't what's important. Exhaust waves and velocity however, are.

Straight pipes may indeed have very low backpressure, but they may also slow velocity and cool the air making it more dense, which causes it to be more difficult to force out of the exhaust.

This might explain why people are seing reduced performance with straight-pipes, compared to standard exhaust systems.

 

Todd DID have better performance with the straight pipes, WITH cats then he did without cats. Maybe you need just enough exhaust piping for a scavenging effect?