Is it possible to over rev an engine with PDK?
06-27-2014, 03:37 PM
#31
Race Director
Sorry but I have a scenario to discuss 
To help illustrate:
Car on flat ground. Car speeds up through 1st gear and reaches redline at 40mph. Engine cuts throttle so car can't gain any more speed in 1st gear.
Car on steep downslope. Car speeds up through 1st gear and reaches redline at 40mph. Throttle is cut, but momentum is in play and gravity takes the car past 40mph. Car is now traveling faster than 1st gear is designed for. Over rev?
PDK scenario:
During autocross, lightly braking on a downslope, I was shifting from 2nd gear to 1st gear, with a heel toe for a higher rev match going into 1st. Brake is completely off as the car releases 2nd gear, where I'm already rolling onto the throttle after the initial kick to get the revs up. As soon as the car engaged 1st, the rears locked up momentarily and put the car in a slide. It felt like shift lock, which I didn't think was possible with dummy-proof PDK. This was with PSM on.
Maybe PDK sensed it and locked the transmission to prevent engine damage, but I remember hearing the rpms sounding unusually high, so I'm inclined to think it was mechanical shift lock from over rev. What actually happened, I'm not sure.
To help illustrate:
Car on flat ground. Car speeds up through 1st gear and reaches redline at 40mph. Engine cuts throttle so car can't gain any more speed in 1st gear.
Car on steep downslope. Car speeds up through 1st gear and reaches redline at 40mph. Throttle is cut, but momentum is in play and gravity takes the car past 40mph. Car is now traveling faster than 1st gear is designed for. Over rev?
PDK scenario:
During autocross, lightly braking on a downslope, I was shifting from 2nd gear to 1st gear, with a heel toe for a higher rev match going into 1st. Brake is completely off as the car releases 2nd gear, where I'm already rolling onto the throttle after the initial kick to get the revs up. As soon as the car engaged 1st, the rears locked up momentarily and put the car in a slide. It felt like shift lock, which I didn't think was possible with dummy-proof PDK. This was with PSM on.
Maybe PDK sensed it and locked the transmission to prevent engine damage, but I remember hearing the rpms sounding unusually high, so I'm inclined to think it was mechanical shift lock from over rev. What actually happened, I'm not sure.
And say in 6th gear while the cars I've owned will increase speed even with the throttle closed going down a not too steep a grade -- like the one on the eastern side of I-580 going down to the Pleasanton/Dublin CA area -- it would take a heck of a long grade to get the car going fast enough to cause an over rev assuming gravity could overcome air resistance. However in gears other than 1st and 6th I would not be surprised if an over rev could occur.
My thinking is to be safe as part of my PPI of a car even if it was equipped with a Tip or PDK transmission I'd get a reading of the over rev counters and engine run times at which the last (if any) event occurred and total run time just on general principle and look them over for something that would have me reject the car.
For the auto-x thing my Boxster does not disable or cut the throttle when I apply the gas pedal and the brake. I use this "technique" to drag the brakes to dry the brakes after running through water or after leaving the car wash.
However, my Turbo does cut the throttle when I apply the brakes as long as I have the gas pedal depressed some. Thus I can't use the same technique as I use with the Boxster to dry the Turbo brakes.
06-27-2014, 04:02 PM
#32
Addict
Rennlist Member
Rennlist Member
I've been crunching the numbers for the past week, and have concluded you have a greater chance of being struck head on by a GM car that has shut itself down then you have causing any damage to the engine in a PDK equipped car.
06-27-2014, 05:38 PM
#33
That is my understanding although I'm pretty sure it would take a heck of a steep grade to over rev engine in 1st gear.
I agree. The gearing is so low in 1st gear that absent fuel being supplied to the engine, the car will slow down much less accelerate when going down even very steep hills. Now if you lose control of your car when going down a mountain road and ride down the mountain side, then overrevs will occur, but that would be the least of your worries.
I agree. The gearing is so low in 1st gear that absent fuel being supplied to the engine, the car will slow down much less accelerate when going down even very steep hills. Now if you lose control of your car when going down a mountain road and ride down the mountain side, then overrevs will occur, but that would be the least of your worries.
