[BiturboS4] More about turbos

[Philip Pace]

Excellent. I bought that Corky Bell book (Maximum Boost), so I should probably
find the time to crack it open. Thanks for the good and thorough response.
     -Phil

--- "Edwards, Dave" <Dave_Edwards at iOra.com> wrote:
> 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!

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