Quote:
Originally Posted by Remonster
I agree with you that the ZF 8AT feels a little too soft and sluggish in the M5 but the previous F10 M5's DCT also could not handle full power upshifts (and neither could M2C, M3, or M4). This is the reason for the fart sound these cars make during a full boost upshift, S63 V8 and S55 I6 use different methods of cutting torque while keeping the turbos spinning so the cars can get through a shift without sending the full amount of torque through the DCT and also without turbo lag on the other side of the shift.
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Nope, disagree.
The DCT is more than capable of handling an all out power shift. On a DCT WOT upshift, there is a surge in forward acceleration due to the DCT recuperating the inertia from the engine rpm drop. This can mostly be felt in S3 with DSC off. With the 8AT in the M5, there is a clear reduction in acceleration with each upshift, even in S3 AWD DSC off. The engine power needs to be cut for the clutches in the 8AT to clamp properly.
This particularity is nicely displayed in the Tiff Needell video where he tests all gens of the M5. In the F10, you can see the passenger's heads bob rearward with each upshift from the surge in acceleration. In the F90, you see the passenger's heads bob forward with each upshift from the drop in acceleration.
Now, the fart sound is for another reason altogether. It is the wastegates opening to avoid a pressure spike in the intake plenum. The S55 goes without an intake blow off valve and relies on the exhaust wastegates to control boost.