Originally Posted by bushy
...was mainly to keep the oil supply up to the turbos until they slowed down...
Not a bad point. I do recall raising that with one of the engineers I discussed the run-down with. From memory his idea was once again, having throttles closed or at low-idle from somewhere over the threshold until reaching the parking/shutdown area was more than enough time in most cases for the turbos to be spun-down about as far as they would go on an operating engine. I recall reading something somewhere (wish I could find it now) about this; again from memory, the idea was in an aircraft with throttles closed to a MP
below ambient atmospheric pressure, the turbos were being driven
by the mass-flow through them, rather than
driving the mass-flow. Again the conclusion was of a rapid decay in turbo rpm at engine idle, with the consequent temp gain of an extended run-down negating any potential perceived benefits.
I'll try to find the document to post...
Originally Posted by bushy
...automotive people seem to think the same with turbo charged engines which idle for some time before shutting down...
I'm afraid this is one that I personally consider to be a furphy of the worst kind -in most cases.
I've owned a twin-turbocharged 3-litre V6 for a bit over 11 years now. She's my pride & joy. I've had my share of grief with her over the years, but
none of it has been engine/turbo-charger related. Predominantly it's been the transmission that's caused me grief. My practice in running the car is the same as most normally aspirated engine drivers -when I get there, I turn her off. Simple. The
only exception to that is, if I've been running her hard out on the open road and need to pull up for a slash, a bevvie or whatever, I will either leave her running for a short stop or run her down for a minute or 2, no more.
One major difference in the way I operate all my cars (turbo or not) is I run then
up! First start of the day, I'll start her then go back inside & make a cup of coffee. By the time that's done, she's generally warm enough that she's off the auto-choke fast-idle, and the oil pressure gauge shows it has come back down to what I know to be her 'normal' pressure. I don't drive her until then. Once she's warmed, it's generally 5+ minutes driving in town before she's in a place that she can be opened up -by which time the transmission is also warmed. Then she can go to work, without worry. There's no need to put the car on the turbo in town
at all; anything under 90kmh I'm in 3rd gear too, never above.
I am
strongly of the opinion it is
far more important that you look after and care for a
cold engine than a hot one. To date, I have not seen any evidence that my theory is in error -in cars or in aircraft.
To further illustrate: there was a young fella lived just down the hill from me that owned a Godzilla, with the ceramic turbo. First thing in the morning you'd hear him start, then before the rpm had even stabilised, he'd be off down the road with everything howling. Of a night he'd arrive home in a similar fashion then leave his car running on the front-lawn for a good 20-30 minutes -I **** you not- before he'd shut it down.
One day I was washing mine on the street and up comes himself. First question was "it's a turbo isn't it?" Yup says I. "How long have you owned it?" was next -about 5 years at that point. "How many turbo's have you blown???" came next. He looked a bit miffed when I told him none. Seemingly he'd gone through 3 in little over 6 months... I don't know much about Godzilla's (usually only see them in the rear mirror and then not for long
) but suggested to him that maybe his engine management was a bit arse-about-face. Don't know what the outcome of that conversation was... either he or the car moved on not long after.
One advantage my car has is a boost gauge. I can
see (and hear) when my turbos are working. If the needles at the bottom of the gauge for 2-3 minutes before I get there, just shut her off. Easiest engine-management of the lot.
My point of course is that you get
one chance to look after your engine of a day -and a lot of people seem to do it at the
wrong end of the day!!! Stacking heat back into your engine is counter-productive, whatever your justification.
I'd love to see some turbo-
RPM vs temp data for the turbo's -I reckon it'd tell a
very interesting tale from TOD to shut-down! Anyone know where I'd find some?