Turbo monitoring

[quk@sievers.com (Phil Payne)]

I've seen a few incidents over the last year or so that have given me cause to 
think.

The original ur-quattro (WR) engine's boost level was regulated by exhaust 
pressure vs the wastegate spring.  Boost level measurements gave a fair idea of 
turbo condition.

Leaving aside for a moment some of the performance kits available for the WR 
engine (e.g., the Dialynx manifold) a major change was made for the MB engine 
in the 1988 ur-quattro; the wastegate is controlled by turbo output vs 
wastegate spring.

(I simplified - I know about the boost regulator valve.)

>From the MB engine onwards, this is a servo system.  If, e.g., a small hole 
appears in the turbo -> intercooler hose, the system compensates by spinning 
the turbo faster.  Boost measurements made off the inlet manifold will not 
indicate a problem, because the wastegate is controlled by the very pressure 
that's being measured.  This can mask the development of certain problems - if 
such a leak as described above develops and continues to grow, the turbo will 
be spun ever faster to compensate, significantly increasing turbo wear and the 
risk of turbo failure.

(The car's driving characteristics will also change.  Turbo lag will increase, 
etc.)

The driver's manual, even for the MB engine with its water-cooled turbo, states 
that the engine should be idled for 40 seconds before being switched off "if 
the vehicle has been driven energetically".  The original reason for this note, 
for the WR (oil-cooled turbo) engine, was to prevent the buildup of oil 
products in the turbo's oilway that occured if the engine was switched off 
with the turbo still so hot that it cooked the oil left in it.  However, with 
the MB and subsequent engines, the turbo might have been working a damn sight 
harder to maintain boost than the driver realises - the note might be 
applicable even if the car has not been driven "energetically".

I've seen lots of cars with boost gauges - has anyone ever given any thought 
to an instrument that would show just how hard the turbo is working to achieve 
this?  Wastegate position for nominal boost?  Turbo RPM?    

--
 Phil Payne
 phil@sievers.com
 Committee Member, UK Audi [ur-]quattro Owners Club