needforspeed

Just a question really:

If I accelerate suddenly down to about 3/4 way the transmission drops a gear - I think this is the gearbox switching modes rather than a true kickdown as I haven't floored the pedal past the resistance point.

Usually though, this dropping of a gear follows a momentary delay and actually feels SLOWER than pulling away with less revs.

Driving progressively (more gradual increase in revs) actually feels faster cause it avoids this sort of unnecessary cog swapping.

Does anyone else find this to be the case and is it particularly true of the diesels given their high levels of torque ?

PS - I've also noticed that when acelerating from 50-70mph with about 1/2 throttle (thus avoiding the downshift) the vehicles speed increases faster than the revs.... I've never seen that before in ANY car.

PPS - Also given the gearboxes preference for downshifting at 3/4 throttle - why have a kickdown?

colejl

This is due to the slipping in the gearbox before the lock-up occurs. This is also the main reason why automatics burn more fuel because they often are turning fast than they need too!

Because it's probably cheaper to use the existing parts than to not bother...

converted2bmw

I might be wrong, however due to the way the power is spread across the rev range, I was under the impression that you inherently get more increase in speed per rpm on a diesel, than in a petrol, which may be why you experience this. Just a thought....

needforspeed

Yeah - I think this is purely a diesel thing and is the result of a low revving engine with MASSIVE torque. It's just really strange to see the MPH increasing and the RPM almost standing still at 3000

0700700

quite normal. When you really press on the gas, the transmission has to find an optimum gear not only for brief, but for continuous acceleration. Because of the limited delivery of power in a diesel engine (think about the 5000rpm redline), the gearbox has to do a lot more work swapping gears. IN petrol bmws you will not find such behaviour.

kickdown is quite the same, but basically tells the gearbox to downshift IMMEDIATELY, its more for last minute overtaking and such

Also of interest is that the x5 here with the same diesel engine as the e60 does not exhibit the same type of variation in the behaviour of the gearbox.

needforspeed

I don't think thats right - the point I was making was that speed increases faster tham revs (in fact rev counter almost stands still)

Diesel engines have a wide band of high torque, but don't rev highly. Because of this, you would need to swap cogs LESS often in a diesel than in a powerful free-reving petrol car.

Thinking about this, it sort of answers my original question about progressive driving. Because, in a diesel you get limited advantage from downshifting in many circumstances so avoiding the 'unecessary' downshift should make you a lot faster.

0700700

ok.. got it ! its cause the turbo is working at full boost


What i wanted to say is that if you changed driving styles a lot you would have to swap
cogs in a diesel far more... if you are in an e55 amg you can be in 5th gear at 50mph and still
overtake with no problem

KAF

Considered acceleration is definately better, IMO, than flooring it in a diesel.

Gets much better results as you rightly observed.

colejl

Not strange at all - it's how automatics work! The gearbox slips the incoming motion until the lock-up happens...

pedrollo

Agree about the torque converter locking up. I had a Carlton when torque converter lock-up was a new thing, you noticed it most at constant speed when the engine revs would drop a little (I think you had to be doing over 30 mph, and it only did it in top gear).

The thing that has stood out for me is how slow the gearchanges are in this box, it seems to have to think about everything before doing it. I'm sure this gives rise the much reported 'slam', and also the slow kickdown response, which I have also noticed. You have to think about overtaking far more than I would have expected.

Much of the action in an auto box is done by oil pressure, controlled electronically via electric valves. The problem with diesels is that they are working at lower revs. At low engine speeds (like pulling away) the oil pressure is not building quickly enough to do the necessary shifting and consequently we have the 0.7 second delay that ZF admit to.

As a recent e39 530d driver, I'm sure the old box was more responsive than this.