Revving an engine before shutdown?

[RoninLB]

Originally Posted by 911pcars

Reving the engine with a hot turbo at shutdown creates this sequence of events:


4. Turbo continues to spin w/o oil pressure

i didn't mean for my reply to state it should be shut down at higher than idle.

[AngM018]

Originally Posted by DARISC

I thought you're supposed to let the engine idle for a period before shutting down a turbo motor to let the turbo cool down a bit or the oil in it will coke up in the hot, stationary turbo and lessen its lifespan - no?

This is correct. I always let the engine idle before I shut it down. usually I use this time to check the oil level

[AngM018]

Originally Posted by 911pcars

Reving the engine with a hot turbo at shutdown creates this sequence of events:

1. Rev engine
2. Turbo spins to XX,000 rpm
3. Ign. OFF
4. Turbo continues to spin w/o oil pressure
5. Turbo shaft spins directly on bearing surface w/o pressurized oil film
6. Remaining oil inside the turbo seal increases temperature while turbo spins down and creates carbon residue.
7. Turbo shaft and bearing wear follow as repeated

For some, comfort, expediency, nastalgia and tradition are reference points for vehicle operation.

Sherwood

I would think that the quick rev would increas oil pressure, and with the way the turbo oil line is situated, oil would still be gravity fed down into the turbo while it stops spinning.

[barney911rs]

On my carburated 2.7 race car, I let the engine idle for awhile after coming off the track to let the temps settle down Before I shut it off, I run the RPM up a bit to clear the plugs. I don't think it's necessary in a fuel injected car.

My $.02 on the subject.

[MFAFF]

Interesting stuff guys..

I guess I'm quite lucky in that my 'normal' 911 use allows about 5-10 minutes at low speed/ low throttle openings at either end of the trip to get engine temps up or stabilised before putting the car away...

I don't feel its necessary to blip the throttle before shutting it off...never have done so with fuel injected cars....and the carb ones I have driven were rentals .

Shutting down the turbo car (Audi S2) is a bit more painful.. it needs a couple of minutes at idle to bring the turbo to idle speed...and cool down a bit. So by the time I have stopped, radio off, unbuckled, gathered my stuff together that time has elapsed and so it can be shut off.. There is a specific cooling circuit on the car to cool the turbo down further, separate from the main cooling system and you can hear it kick in if you shut off too soon..sadly there is not a similar oil system. Cooking the turbo bearings is easily done and a full refurb is expensive and but at 125K miles not unexpected..

Similarly 'calm' use of the throttle until there is some heat in the system is adviseable...usually guided by the water temp guage and oil pressure guage.. the pressure drops as the engine heats up...