Water Jacketed turbos

[QSHIPQ at aol.com]

> can't resist a smile at the inclusion of sae papers in this context, but i
> digress....

>>Some of them are interesting, like the graph of component temperatures
>>after engine shutdown for air-cooled and water-cooled turbos included as
>>Figure 9 in Audi's SAE 860103.

The more interesting part of that graph is the component temps *before* 
shutdown.   Makes you think twice about that quote for installing "afterrun". 


>P.S. Yes, air-cooled and not oil-cooled.  Oil plays no major part in
  >   heat removal - the key is airflow.  The way to cool a non-water
  >   cooled turbo is not to stop and idle the engine for 40 seconds,
   >  but to coast for the last 40 seconds of the trip and shut the
   >  engine off when you arrive.  With the car stationary, there's no
   >  airflow over the turbo and it cools mainly by radiation and
   >  convection, i.e., slowly.

Maybe Ned can help you here.  What you are doing by idling, is allowing the 
heat buildup in the centerbearing to be absorbed by the oil.  IF you think 
the above is true, you should try thermocoupling the hot side of the turbo, 
and try testing your theory.  A car at idle (not moving) will drop temps 
dramatically within the first minute, and within 3-5 have the lowest shutdown 
temp you will see.  NOT shutting the car off is the key, a glowing turbo with 
no fluid thru it (try this at night sometime) is a recipe for disaster, WJ or 
oil centerbearing.

Up for a whole bunch of nitpicking I suppose, but certainly this list could 
use some help in the what's and whens of heat and turbos.  Without question, 
idling for a minute or so after a hot run has great advantages, and is the 
most practical thing to do in terms of reality.  In a water jacketed turbo, 
the heat loss is quicker than in an oil only turbo.  IN terms of hot run and 
shutdown temps, the WJ turbo reduces these signficantly.  In terms of 
afterrun hardware, save your money.

Idle, coast, pop the hood and blow on it...  Bottom line:  a 1-2 minute turbo 
cooldown procedure on *any* turbo can extend the life of it.  A better mod 
than the afterrun hardware is a properly functioning bypass valve in terms of 
turbo life extension.

HTH

Scott Justusson