Originally Posted by Ilia@IND
One area I would suggest checking is the vacuum line running to the rear mufflers. If the vacuum lines are not plugged during the exhaust installation your M5's performance can be affected significantly. It is possible that these were left hanging and open during the Eisenmann install, and then re-attached to the OE BMW valves when the OE exhaust system was re-installed.
Installers frequently omit installing the vacuum line- so often, in fact, that we have actually moved to attaching warning labels on each exhaust system we ship:
"All Eisenmann systems not using the OE vacuum control system must plug and close the vacuum lines at the rear of the car. Failure to do so will result in reduced power and ECU trouble codes."
This. I know, it happened to me. I drove away and could not believe that the exhaust did this. Just as its been mentioned that exhaust will not create huge gains; I could not see it causing as much of a power loss as I felt. I drove back and my installer forgot to plug the lines. Boom, instant power return. Ill bet that is what happened here. It was like driving a non boosted turbo car.