Since introduction of our modified chips we have stated based on our first dyno tests that the standard exhaust system is not capable of handling the higher power levels properly. Yes, you can enjoy the higher power/torque also with a standard exhaust system, but you don't get out all the power that the engine is delivering.
During the past two years or so I have read/received many reports from users who have modified also the exhaust system to cope with the higher powers.
Here's a short summary of those reports:
First of all, in theory one 3" exhaust pipe can deliver more power than two 2.5" pipes. There have been quite a lot of debates which one is better in practice.
* Majority of those who installed one 3" pipe complaint high
noise levels or resonances at certain RPM levels
* Both versions allow a lot of power. No one has yet reported of any problems even at 1.5 bar boost levels.
* Oviously BMW designed the standard exhaust system quite to the 252 HP limit, as many reports show that power was increased considerably already at standard boost with a 2*2.5". Our estimation of 50 HP power loss with standard exhaust system at 1.1 bar/400 HP seems to be on the lower side.
* Power gain with e.g. a 2*2.5" exhaust is not linear over the RPM/boost range. The standard exhaust system generates 0.3 bar back boost at 1.1 bar turbo boost. 1.1 - 0.3 = 0.8 bar effective boost ( approx. 70 HP loss). At higher boost levels the back boost would be also higher resulting in even higher power losses.
So, as a summary my personal recommendation is 2*2.5" exhaust system ( of course with free flow mufflers).
Please send your comments on this subject.