yea i get the intake side of it but wut about the exhaust part, does it make that much of a difference if ur catted or not? dnt the maps on the AP call for catted pipes?
The biggest difference for having a cat or not having a cat is more or less a matter of boost control. MS3s are prone to boost creep, this is a mechanical issue unrelated to tuning. The problem is that the internal wastegate on these cars is not capable of venting sufficient exhaust gas when open if back-pressure is removed from the exhaust system. What happens is that even though the ECU has told the wastgegate to open and vent exhuast gasses, the turbo continues to make boost causing boost to "creep" past specified boost targets within the ECU.
Essentially what happens is this. In a catless exhaust system, when the wastegate opens at WOT, the pressure around the wastegate is higher than it is around the turbine so even though the wastegate is all the way open, exhaust gasses continue to pass through the turbine which keeps spinning the turbo and creating boost. So even if you turn your WDC (wastegate duty cycle) to zero, the turbo will continue to make boost.
You could take fuel out, like a fuel cut, but at very high-boost levels you would go instantly lean and cause detonation and grenade your engine. The only option in this instance is a mechanical fix either be enlarging (porting) the area around your wastegate OR be reintroducing a slight amount of backpressure behind the turbo in the form of a cat.
While some people resent the notion of keeping a cat in a high-performance car under the notion that a cat will kill performance, we have found this not to be true. Most "motorsport" cats flow rather well and really the loss in horsepower is often minimal. The other option is to install an external wastegate setup which is just like it sounds, a wastegate that is plumbed externally to the turbo.
These work very well, but of course there is a cost associated with this setup in extra parts and possibly fabrication depending on your desired setup and noise tolerance. Because this is a mechanical issue related to exhaust flow within certain physical parameters, a stand-alone boost controller or an upgraded EBCS will not help much or at all.
This is generally why we say that a high-flow cat in the system is better than no cat at all, emissions concerns aside. We have seen around 350whp and 350wtq on a car running a catted downpipe so really, I would just go with the catted version, COBB of course.
As far as doing a catback or a downpipe. If you have a stock downpipe and add a catback, you aren't really changing the VE of the engine at all, so it won't need a tune like a downpipe would. If you are looking for which part will make more power, a catback or a downpipe, do the downpipe. Then, add the catback or just a full turboback.
Travis
COBB Tuning