Automatic diesel cars and Over-rev - Why?

[R12HCO]

Drove a friends 05 plate Mondeo today with the 2.0 TDCI engine and Auto box. I was suprised at how nice it drove and expected it to feel cheap for some reason which it did not. Anyway, Only thing I didnt like was how the box over-revs the engine and it sits on the redline for a little while before changing gear. This does nothing for the diesel engine due to its nature, and you can feel it tail off after 3800 revs where you would think it would change gear then.

I haven't driven many autos, but all the ones have driven are diesel and all seem to do it. This includes a 2006 Mercedes c220 CDI coupe, A 2006 nissan navara and a 2005 BMW X5.

Can anyone explain why?

[NateWM]

Slow boxes.

Unoptimised solenoids, not made for quick shifting etc. However I have rebuilt Autos reknowned for there slow shifting and put in updated, modified parts, which has improved the shift times considerably.

I done the same when giving my GS (3.0 Petrol) a full rebuild, putting in modified solenoids when rebuilding the valve body. Shift times are drastically reduced now, although obviously not upto DSG standards. Only a DSG can shift as fast as a DSG!

[deeps]

Were you liftng your foot off the pedal slightly just before the red line? I've always found that drags out the change slightly as the gearbox works out what to do. No worse feeling than hovering in no mans land at high rpm!

[R12HCO]

I had a go at backing off the throttle to see if that help (also tried to do it using the tiptronic) and it would go up a gear then soon as you press on again, it would kick down again into the gear you have just came out of.

[R12HCO]

When driving normal or just accelerating at say 70% it was no problem, shifting when needed. Just when pressing on 100% it held the gear when not needed and you can feel the rate of acceleration tail off and soon as it changes up you get a huge surge of power.

[deeps]

Yeah, I'd like to be able to dial in my own rev limit when driving an auto on full throttle, with all the gadgets they have these days that's one that would actually be useful. Instead of that I drive mine in manual mode on the flappy paddles so I can change up at say 5000 rpm on full throttle instead of waiting for the auto to redline.

[HustleRussell]

I very much doubt the engine is hitting the rev limiter (Because this is what 'red line' means).

[StottyZr]

R12HCO said:

When driving normal or just accelerating at say 70% it was no problem, shifting when needed. Just when pressing on 100% it held the gear when not needed and you can feel the rate of acceleration tail off and soon as it changes up you get a huge surge of power.

It may feel like the power is tailing off but the box may be smarter than you think. It may realise at 4800rpm it still has more power than it will do at 3200rpm in the next gear? Even though the HP has dropped off from 4500rpm (where you may have changed gear) it could still be the optimum changing point.

When I had a 123d Auto recently I noticed it overrevved right to 5100+ in first and second, then it would upshift at 4700 from 4th to 5th. I expected this is because when shifting from 4th to 5th the rev drop is much less so it will still be in the power.

Just a guess

[davepoth]

HustleRussell said:

I very much doubt the engine is hitting the rev limiter (Because this is what 'red line' means).

No it isn't. The "Red Line" is a "do not exceed" mark on the rev counter, above which engine damage may occur if the engine speed stays at that level for any length of time. The rev limiter is usually set quite a bit higher than the red line in a petrol engine in any case.

In a diesel engine it's quite difficult to get to an over-rev condition by accelerating in gear because of the way the governor operates, combined with the way diesel power drops off at higher revs anyway due to things like preignition and slower flame front movement.

It sounds like the gearbox could have been configured wrongly so it might be worth asking Ford whether the gearbox mapping is correct (if a petrol map is in use it would likely do what it's doing now).