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      04-12-2008, 11:17 AM   #74

Drives: 2008 E92 M3 M-DCT
Join Date: Dec 2006
Location: Mission Viejo, CA

iTrader: (0)

Originally Posted by MysticBlue View Post
8. What is the actual shift time of M-DCT in S6 mode (in milliseconds)?
They don't give them a time, however it's so instantaneous that they had to build some feel into it so it didn't feel like a CVT transmission. "It just goes, it's so fast"
That's probably about the best answer you're gonna get. With two input shafts and two clutches, during a shift, one clutch is engaging before the other one is disengaged. To quote a "shift time" you would have to define what is the beginning and what is the end. In the world of double input shaft and double clutch, that is a grey area and tough to define. By some definitions, it could be said the shift time is zero. If you define it by the time it takes to go from clutch fully engaged to clutch fully engaged, then the number will be smaller than the SMG or any conventional manual, which is great, but that doesn't give the DCT the full credit it deserves for the fact that no matter how small the number is for an SMG, there is some amount of time where there is zero torque being applied. With the DCT that never happens... there is never a point where there is zero torque. So just comparing some definition of shift time in milliseconds, even though the DCT will definitely compare favorably, doesn't give it the full credit it deserves.
It might feel as though there is "never a point where there is zero torque", but there must be a very small amount of time that both gear ratios are NOT connected to the engine. If you are beginning to disengage the clutch for one gear ratio while the next gear ratio is being engaged there is only one acceptable option...they cannot be connected to the engine at the same time.

If two gear ratios are connected at the same time (shift overlap) then the clutches must slip which causes unnecessary wear and heat. If both ratios are actually connected (engaged) at the same time then the output shaft (drive shaft) will be locked and cause the drive wheels to lock (like having an automatic trans. car in Park). If the vehicle is moving when the driveshaft locks the tires will chirp (skid) or the weakest power train link will break. Manual transmissions vehicles use a shift interlock to prevent two gear ratios from being selected at the same time. Automatic transmission vehicles have a torque converter that allows slippage.

Bottom line: You must have some time when both gears are not engaged (connected) to the engine. BMW likely designed in some “zero torque time” to prevent M-DCT owners from have early clutch failures. The wet clutch design with plenty of cooling should help also.