I noticed this on both GTS's that we have had in the shop. I felt it when the car was completley stock and with the bypass pipes installed. We are looking into this and trying to see if we can do something with the ECU tuning to get rid of the flat spot. I don't think you notice it as much with the stock cats. If you unplug the valves on the rear exhaust it takes care of that flat spot because the valves will stay open all the time.

 

isn't there a circuit or plug you can remove/unplug instead?
my old 335i had the same type of valve for the exhaust, and in the trunk all i did was unplug the vacuum motor. i'd rather not pull off the hoses unless i have too

 

The electric portion of the switching is done internally of the DME. On the 996/997 you could unplug it electrically and accomplish the same thing. I found another way of doing it today by disconnecting the vacuum hose in the engine bay that feeds the sport exhaust. You will need a vacuum line cap and a screw. Unplug the vacuum line, put the cap on the engine side, and the screw in the muffler side. Worked great and sounded very nice for a stock GTS. No pic but its the T-line at the rear of the engine bay, beside the intake manifold on the passenger side. The one I tried it on had a white stripe near the end of the vacuum line.

 

Since the trap is vacuum operated; the manifold vacuum changes with throttle position and RPM's, so it can partially close at given moments.

 

Reviving this dead thread. Has anyone found a dealer that can reprogram to have the PSE button always on? My dealer could not figure it out.

 

DUDE you're the ****in man. Just did this and since I had caps laying around already it took all of 2 mins and WOW this sounds perfect now!