[BiturboS4] More about turbos

[Edwards, Dave]

Phil,
I didn't reply to your original post below as I didn't see the digest. I now
know why: My MS Outlook junk mail processing 'feature' auto-deleted it!
Great software this Microsoft stuff!

I think that it's a common misconception that the turbos only spin when
you're boosting. This is not helped by the common phrase "when the turbos
spool up" which implies that they're normally idle. I'm sure you can imagine
that if the turbos were normally idle, there would be one hell of a lag when
you request boost.

Remember that the turbos are bolted to the exhaust manifold so that the
exhaust gasses are ALWAYS flowing across the impeller (or should that be
expeller?) That means that whenever the engine is turning, no matter how low
the revs are, the turbos will spin.

Even when the wastegate opens (a feature which limits the max boost by
diverting the exhaust gasses past the turbo), the turbo still spins because
some gasses are still flowing through it as the wastegate does not shut it
off.

The speed at which the turbos spin is directly related to the rate of flow
of the exhaust gasses. So as the engine increases revs, so do the turbos,
right up to the red line.

However, as Derek pointed out, it is not as simple as that: When you are
applying boost, you are stuffing lots more air into the engine, in
compressed form. Thus when the exhaust gasses come out, there is also a lot
more. The gasses expand and so the airflow is even faster. This makes the
turbos spin even faster.

(Of course this makes them produce even more boost; this is the positive
feedback that makes turbo engines so much fun!)

The bottom line is that whilst the engine is cold, you need to keep the revs
of the turbos low. This means BOTH low engine revs and LOW boost. However I
have no magic formula for this. As I wrote "I try to keep the engine revs
below 2500". I don't always succeed - especially when a BMW330* pulls up
beside me at the lights ;-) All you can do is your best to treat the car
gently until warmed up. I do feel for you guys who live in areas which get
really cold: It must be a real PITA!

Daniel,
Idling the engine when it's very cold is good, but idling for long periods
is not. The engine is designed to spin at much higher revs. That's why like
most manufacturers, Audi recommend driving off straight away. Of course
Germany is like the UK - we don't get some of the extreme cold that you get
in parts of the US.

If its not especially cold then I'd recommend driving gently rather than
idling. Experiment yourself - I'm sure you'll find that the engine reaches
the 'threshold' temperature quicker with light driving than by leaving it to
idle.

Cheers,
Dave

*Talking of these cars, I'm getting sick of them. I still have not had a
chance to test my car against an M3. The few that I've come across have been
driven by old geezers who haven't got a clue how good their car is. But I
come across loads of 330s, and some that have M3 "look-alike" body-kits on.
The other day there was one of these behind me on a road where we were
queuing up to a 2-lane road. It looked so much like an M3 that I thought it
was. So once on the 2-lane road, I put my foot down and left the BMW behind.
I was disappointed as I assumed that he wasn't trying. I stopped
accelerating at 95mph (I never go 100 or above - that's instant loss of
license over here if you're caught). Eventually the BMW caught up and
overtook at about 110-120mph. That's when I realised it was a 330, and
seeing the look on the driver's face I realised that he HAD been trying!

--__--__--

Message: 8
Date: Thu, 10 Oct 2002 06:55:27 -0700 (PDT)
From: daniel chisholm <dbc112 at yahoo.com>
Subject: Re: [BiturboS4] Turbo failure modes
To: Philip Pace <pjpace at yahoo.com>, biturbos4 at audifans.com

--
[ Picked text/plain from multipart/alternative ]


Would it make sense to let the engine idle and heat up?  The turbo will not=
move and oil will circulate faster when you first drive..  Philip Pace
wrote:Awwwww man... per your last paragraph, I didn't know about them still=
spinning fast as the RPMs go north. My problem is that I try to build _no_
boost bef= ore I hit 175 (the second tick). I set my own max RPM at around
3500.

Your explanation is excellent, Dave. Thanks for the input and I'll throw th=
is at my girlfriend and see what she has to say about it NOW. philpace