Driving in too high gear is bad for the engine.....

[davek_964]

...apparently.

I've seen it quoted in threads, and it's something I "know" - that you should not labour the engine - i.e sitting in 6th gear at 1,500 rpm and then accelerating is a bad idea.

However - the auto-box on my Cayenne disagrees. It likes to get in high gears as much as possible, and it will often be accelerating from almost idle speed unless I manually change down or use kick-down.

So if labouring the engine is such a bad idea - how come auto-boxes get away with it?

[A911DOM]

Im sure someone will be along to give you a proper tech answer to this in a minute - but my guess is that with these very torquey large lumps, the ecu will stress the engine as little as possible and until you put more pressure on the pedal it wont kick down.

My old style Cayenne is the same, and having got used to it, if you want to just add a couple of extra MPH you can tickle the throttle and not get shot towards to the nearest horizon (or petrol station)

[BertBert]

But that's not the definition surely? Labouring is trying to accelerate with a high load at revs which are too low is a better definition. It comes about through the engine not being able to deal with WoT at low revs and not being able to produce the torque to accelerate the load. In terms of reasons for this, the possibilities include not being able to get the ignition timing right or the valve timing right. There must be more than that. Maybe even insufficient oil pressure at low revs.

So if a modern engine with different/better design and more ability to vary the ignition or even valve timing can do better and produce more torque lower down then it won't be labouring. You'd probably not worry as much if was a wapping great diesel which is just a different design of engine.

I suspect the great minds at Porsche know all this and a lot more and the real answers and have designed the behaviours of the ecu/engine/transmission accordingly.

And finally, surely with your slushbox ther is a great soggy torque converter in there mopping up the pain?

Bert

[supersport]

I am sure I remember Baz from Hartech saying that it was exactly this that was more likely to generate bore scoring, and as such the autos were more prone to it?

[Magic919]

It's not likely that Baz meant the Cayenne when he wrote it.

[ean21]

In addition to what's been said already, I think an auto-box will help the engine a little as there is no direct connection from the engine.

At very low rpm each bang just pulses into the torque converter which absorbs it to an extent, but on a manual box each bang gets stopped by a hard connection to the driven wheels, and you feel this as 'labouring'.

I used to be able to run at around 1500rpm, below that felt like labouring. Then I changed my DMF for a RS flywheel and noticed it even more and now I try and stay away from the throttle below 1700-1800 rpm because the single-mass flywheel offers an even more direct connection from the crankshaft to the wheels, so now there is only really the tyre to absorb each bang.

[hartech]

The basic theory is right - high torque at low revs loads the piston face more. However - the Cayenne is underpowered/cylinder compared to a 996 or 997 - so the actual load/cylinder face is less - the cooling is not as unbalanced (all cylinders equally cooled), and the number of firing strokes/crankshaft rev is more - so delivery is smoother.

Overall the Cayenne should suffer less and very few have been suffering from nore scoring.

Baz