GLI

I'm a die-heard "self-shifter" and admit to an over rev situation on previous models a few times. My error and fortunately no foul as the computer did it's job and protected the car.
I have a question regarding how the auto trans handles such situations. Will the computer not allow over revs even when shifted with the paddles? How about an unintended paddle downshift causing a serious over rev?

NSC5

The computer will protect against over rev both by limiting engine speed in a gear and denying a shift to a gear which would result in either excess engine speed or insufficient speed/torque capability for the requested gear. If you try to shift up or down into an inappropriate gear you will get a "shift denied" message.

The only case where you might be able to exceed the safe engine RPM would be to select a low gear on a steep grade and not use any braking. If the incline is sufficient I suspect the controller would allow the engine to go to excessive RPM rather than force an upshift when in manual mode. This is one experiment I will not be conducting.

rrsperry

If you have a A8, you are not a self shifter. Just saying.

Bill Dearborn

If the A8 is in manual mode why isn't the driver a self shifter? The driver can choose the gear and how long they want to be in it other than the protections to prevent engine or car damage that are impossible to implement with a manual transmission. It only takes one missed downshift from 5th to 2nd instead of 5th to 4th with the M7 to put a car violently into a wall on the track let alone blow the engine.

Bill

GLI

To clarify, it's an M7!

rrsperry

You most definitely can mechanicaly over rev the engine with the M7. No computer can save you from being ham fisted, that that's just engineering.

Otoh, an automatic trans, (not really shifting), simply will not let you hurt the engine by downshifting. It simply will not shift until it's ok to do so. And most of the time it shifts when it want to, not necessarily when the driver asks it to.

Glenn Kline

You cannot over rev while accelerating as the computer will cut fuel at redline. If you are accelerating and at 95 mph in third and try to go to fourth and accidentally put it in second, prepare for your valves to float with an awful smacking sound of pistons when you let the clutch out. There is no way the computer can cut off the manual connection between wheel speed and the engine RPMs in a M7 when in gear and not on the clutch.

Note, the auto rev match will not over-rev your engine. If you go from third to second in the same scenario but don't let off the clutch, there won't be any damage. I think it rev matches up to 5500 RPM.

NSC5

And to add to what Glenn said if you are paying attention when you attempt to accidentally shift into a lower gear you will hear the synchros screaming and that is your clue NOT to let off the clutch.

If we continue to try to idiot proof cars I expect some manufacturer will add a lockout solenoid to prevent the clutch pedal from returning if an incorrect lower gear is selected during a shift. Of course this brings up the interesting safety/liability concern for the rare case where someone would prefer to purposely "blow" the engine in an attempt to slow the car but I don't think there are many cases where a car speeding out of control due to a loss of brakes will be better off spinning out of control due to suddenly locked rear wheels from a highly oversped and now seized engine

shaimerej

Bill Dearborn

It does not apply the rear brakes. That would make a bad situation worse. Downshifting to the next lower gear at a speed higher than the max speed achievable in that lower gear can spin the engine to much higher rpms than it could achieve under power. For example accidentally shifting to 3rd gear from 4th gear at 150 mph when trying to shift to 5th. The max speed the rev limiter will permit in 3rd gear is 123 mph at 6500 rpms. However, unless the engine fails in some way and locks up it and the rear tires will continue to turn to some degree. That means the car will slow to the point the rear tires regain grip. Applying the brakes would ensure the rear tires are in complete lockup and delay the point where they would regain grip thus lateral control of the car's direction.

Bill