Turbos And Letting Them Cool [Archive] - Bimmerfest - BMW Forums

Turbos And Letting Them Cool

02-19-2012, 03:44 AM
Are you still supposed to give your turbos ample cooling time before shutting down or is that an old school of thought? I had a turbo timer installed in my 300ZX many moons ago and I still sit in my 650 for a few minutes after driving to let them cool before shutting the engine down.

And why is there no turbo whine? I love that sound but i dont hear it on the 650 lol. Dont hate :)

02-19-2012, 07:39 AM
No, bro that was old school.
No cool down period required.

I miss the whistle too. Used to love it going up winding hills in N Cal.

My guess is that the twin scroll brings in air differently than the old turbos.

I'm guessing again that the answer lay in the same manner turbo lag is dealt w more efficiently. Someone smart will chime in to explain. :)

02-19-2012, 07:54 AM
This is new to me - guess I'm showing my age lol. The scientist in me really wants an answer. +1 on hopefully a smart person who has some idea about this - my physics education didn't extend into engine mechanics.

02-20-2012, 03:59 AM
This used to be the logic:

If an engine was revved highly, the turbos would be spinning very fast. If you switch the engine off quickly, the oil pressure drops, and the turbo bearings run without oil forced on the sufaces before the turbos finally come to a halt.

This logic still holds.

I think that the twin-scrolls are more adaptable, and probably slow down much faster.

On the other hand this sounds like a perfect excuse to listen to that great engine for another 30 seconds or so...:D


02-20-2012, 04:43 AM
And I remember seeing my turbos glow red from heat when not letting them spool down sufficiently... Not good! lol (in my Fairlady, not the 6er)

Funny thing is, when you don't want the lag, it is there, but when you need them to lag when shutting down, they don't. Well if the turbos are still spooled when stopped for, say, 10 seconds, why is there still lag? The exhaust should be able to keep the turbine spinning at a pretty high RPM as soon as you hit the gas. Just some food for thought lol.

Still wondering where the turbo whine/whistle went. I'm pretty sure that is the sound of the compressor, like a gas turbine engine in a jet. A turbo and a gas turbine engine are essentially the same basic design (with the exception of the burner cans). That whistle is not present in a turbo prop which is basically a jet engine with a propellor instead of a compressor.

What exactly does twin-scroll mean/do? I know all the components of a turbo and how they work but not sure what twin-scroll is. Maybe I can understand better once I know what twin-scroll is. I guess I should google lol.

Edit- just googled twin scroll. In short, it looks like a dual entry point for exhaust gas to the turbine wheel making it more efficient. I would still like to hear that compressor working lol.