[BiturboS4] Turbo failure modes

[Edwards, Dave]

Phil,

I'm no expert, but I do talk to experts whenever I get the opportunity. All
have told me so far that the warm-up is more important than the cool-down.

I think you're right that turbo failure is most likely to actually occur
when the turbo is hot, especially when it's spinning fast. But wear is most
likely to occur when its cold, because the oil is not lubricating properly.
As long as the turbine is not spinning much that's OK. However spinning the
turbine fast whilst the oil is not lubricating properly is bad news.

AFAIK there are the following main failure modes:
1) Turbine 'seizes' up. Mainly due to lack of oil, probably from coking.
Reason is use of semi-synth or mineral oil, and/or not cooling down
properly.
2) Turbine bearing wear. Mainly due to improper lubrication. Reason as
above, but also due to not warming up properly.
3) Turbine blade stretch. Due to overspin. Reason chipping to far too high
boost (for given air flow).

The main point is that turbos often fail for a combination of reasons, not
just one. If you want your turbos to live long then you need to address all
the causes.

Regarding the 2500, yes I did not make myself clear. What I meant was that I
keep the boost AND the engine revs low. Yes I do have a boost gauge, but I
don't need it during warm-up: As you know, trying to drive the car at for
example no more than 0.5 bar boost is not easy as once the turbos are
producing such a boost, it's too easy to go higher. No, much better to just
be gentle on the gas and you know that there's probably no boost at all
without even looking at the gauge.

However my main point about the 2500 rpm is that if you gently accelerate
the car but don't change gear, you can get the revs well above 2500 without
registering much if any boost. As a result you might think that you're being
easy on the turbos, but you're not: The rate at which the turbos spin is
related to engine revs. They are still forcing air into the intercoolers,
however most of it is being recirculated by the DVs. Yes they are under less
stress because of the recirculation (lower pressure differential), but
nevertheless spinning the turbines fast whilst the oil is cold is still not
advisable.

As I said, I'm no expert and am ready to be corrected if I'm wrong.

Cheers,
Dave

--__--__--

Message: 15
Date: Tue, 8 Oct 2002 15:44:00 -0700 (PDT)
From: Philip Pace <pjpace at yahoo.com>
Subject: Re: [BiturboS4] Turbo care
To: "'biturbos4 at audifans.com'" <biturbos4 at audifans.com>

I agree, but was told recently that the warm up doesn't matter that much. H=
ow many turbo failures happen during the winter months in cold areas?
Warming = up the oil takes forever under those circumstances. If there IS
stress on the turbos when they're cold, wouldn't you think that more would
fail when it's cold out? All I could say was that cold causes them to wear,
but heat ultimately destroys them. She didn't buy that. Can you address
this?

Also, you say that you keep the RPMs below 2500? That's when you're likely =
to get the most boost! Do you have a boost gage? I was surprised at where
and = how boost is on the car. Maybe you already know. I've been leaving it
in a lower gear longer and overall boost is a lot lower during warm up time.
My 2=A2
     -Phil