Turbo cooling, previously Re: [Biturbos4] S4 Biturbo newbie

Charles Wurts cwurts12 at mindspring.com
Wed Dec 22 22:20:04 EST 2004

I am attaching an old post I made to the list a while back.  Sadly, for 
me, it did not get much attention or response.  Here it is again should 
anyone care to take a poke at it regarding cooling and after run pumps:

I find this to be an interesting subject as well. In my previous
experience with several 5ktqs and a UrS6, the after run pump and fan
would run after almost any normal driving other than winter. I always
found this to be very comforting. I tried very hard (with me ears) to
detect any evidence of after run activity, but never could, even on the
hottest of days. I dropped by AWE one day and asked Todd about this
question. I also asked if I could change a tstat to bring the after
run pump on at lower temps. His response was interesting, and I invite
him to chime in if he is here.

He said the issues surrounding the old days of after run pumps were
different 10 years ago. The  concern then was primarily coking, where
today he said the the issue surrounds metallurgy. If I understand this
correctly, the temps that coking occurs is much higher today (because
of chemistry and synthetics) and the after run tstat is set higher than
it used to be, and is higher than the normal operating temp of the
engine. So apparently coking isn't as big a concern. Now the concerns
are with regard to the metal shaft through the center of the turbo;
when very hot it can sag and cool out of round leading to failure. So
the best line of defense is to leave the engine idling long enough for
the spindle to cool.

I wonder if Audi has made the change to the after run tstat? This
would be valuable information for all of us.

01 S4 Avant
90 V8

On Dec 22, 2004, at 8:55 PM, Grant wrote:

> Just to be clear - while we all have favourite ways to achieve 
> cooling, there is no debate about the basic issue:  heat soak cooks 
> oil, and cooked oil shortens the life of turbos, in several ways.  So, 
> absent instrumentation to show us all exactly what is happening (like 
> the heat sensors someone cleverly suggested), we make our best guesses 
> about what is satisfactory, and trade-off convenience for perfect 
> cooling.
> As noted, I do exactly what you do. I just want anyone reading to 
> understand that heat is the enemy, and flowing oil is the main thing 
> controlling heat at the bearing surface.  Even all the water cooling 
> in the world just keeps the casing temperature down, so that it can be 
> a better heat sink.
> Grant
> On Dec 22, 2004, at 9:42 AM, northrup wrote:
>> there is quite a debate about turbo cooling... essentially what i do 
>> is to
>> stay off the boost as i near my destination, thus letting the oil and
>> coolant do their job of cooling down the engine and turbo parts.
>> Following this, and depending on the situation, I may idle for 30-120
>> seconds... or not.
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