engine/turbo theory question

[Avi Meron]

Mr. Todd,
The 1.8 Audi turbo has a "cold engine light" it lights up till the engine is
warmed up.  The service manual says specifically to drive easy till the
light goes off!
Take care,
Avi

-----Original Message-----
From: quattro-admin at audifans.com [mailto:quattro-admin at audifans.com]On
Behalf Of Todd Phenneger
Sent: Monday, October 16, 2000 11:47 AM
To: Ameer Antar; quattro at audifans.com
Subject: RE: engine/turbo theory question

Ameer,
  I Highly suggest you crawl under your car after a 80mph
freeway jaunt.  Esp after going up a Pass or something.  I will
bet money its Red Hot.  Every Audi Turbo I've looked at IS.
When your just driving around its HOT.   Get on it for even a
short spell and its red Hot.  Its normal.
  In fact,  the turo is designed with tolerances that arent
right untill its hot.  A cold turbo will spin a bearing in No
time if you get on it.  Thats why its SO important to let turbo
cars warm up before you get on the boost.  THe turbo must be
thouroughly warmed for all the parts to have the right
clearences and the bearing shaft to properly float itself on the
oil surface.
  But, TO hot and you can break down the oil.  Thats why we have
oil coolers and run Synthetic oil.  ANd why we let our engines
idle befoer shutdown so we dont coke the bearings.
  Anyhow, normal driving will make an Audi turbo glow Red Hot.
But its so buried in the car most people dont notice it, but
look around (it must be Dark outside) and you'll see a dull red
glow back there.   Or maybe a bright red glow if you have been
hard on it.
l8r
  TOdd

--- Ameer
 Antar <ameer at snet.net> wrote:
> i've tried to stay away from this topic, but I can't....I've
> talked to
> turbo guys and this what I've picked up...
>
> turbo is the best type of forced induction b/c it has the
> lowest power
> losses, but there is a loss. Nature and engines follow the
> laws of physics,
> and physics says you can't get something from nothing. Power
> is required to
> turn the turbo, but the difference is similar to having a
> stock exhaust vs.
> a free-flow one. You only lose a few hp, when you put on the
> restricted
> exhaust, just like putting on a turbo. But the gains are
> tremendous, b/c
> the compressor side has big blades and can push lot's of air
> down the
> throat of yer engine.
>
> turbo operating temp, is normal engine temp. no turbo manuf.
> will want you
> to make it glow red. Heat doesn't make a turbo work better, in
> fact heat is
> dangerous for turbos, as the oil [the only thing in contact w/
> the
> bearings] breakdown and harden, causing the turbo to seize.
> You can put a
> turbo in an oven at 1000F, but it's not gonna start spinning
> and it will
> perform worse than a turbo at normal temp. The reason why some
> people
> connect heat and power is that the hotter the exhaust, the
> more expansion
> of the gases is occurring. In fact it is expansion, not heat
> is what drives
> the turbo. But again, more heat=more expansion. On the cold
> side, the
> opposite is occurring, the air is not expanded, but
> compressed. There are
> some that will torture their engine till the turbo is red hot
> to get every
> last drop of hp out of it. This is only for people who race
> and repair
> their equipment on a regular basis. I have never seen my turbo
> glow red and
> plan to keep it that way. Your turbo is at operating temp, as
> long as the
> oil is warm and lubricating the bearings...good luck...other
> comments welcome.
>
> -ameer
>
> At 12:46 PM 10/12/00, you wrote:
> > >       i was discussing turbocharges with a friend - well,
> > >       explaning them, actually.  he raised a question that
> > >       i never thought of:
> > >
> > >       is there any overall power loss that comes from the
> > >       pressure (backpressure?) required to turn the turbo?
> >
> >
> >The increased pressure in the exhaust manifold over the
> pressure on a n/a
> >car cost some power, a fraction of the power needed to drive
> the most
> >efficient centrifugal superchargers.
> >
> >Super charger is driven by power that the engine has to
> _produce_ while the
> >turbo _prevents_ the engine from producing a few hp.
> >
> >A properly setup turbo also produces power from just above
> idle, that is not
> >the case with a centrifugal supercharger. The lysholm screw
> superchargers
> >(not roots type) can produce boost at idle to maximum rpm but
> they need more
> >power to operate.
> >
> >To get a turbo quick spooling the exhaust turbine has to be
> pretty small,
> >the exhaust runners also have to be small to keep maximum
> exhaust energy to
> >drive the turbo. The pulsing of the exhaust is important to
> keep, the
> >exhaust pulses should 'mix' as little as possible, this is
> also for
> >minimizing the spool up time. Remember that most turbos never
> reach
> >operating temperature in city traffic. A turbo charger is not
> at operating
> >temp until it glows bright red.
> >
> >All of these design rules adds to backpressure and prevents
> the engine from
> >producing maximum power.
> >
> >For racing maximum power is more important then drivability
> of idle, this
> >makes it possible to stretch the design rules a bit, the
> first measure
> >usually are to make the runners and the exhaust turbine
> larger. By
> >sacrifising the turbo spoolup when the car is not driven hard
> the exhaust
> >backpressure can be lowered a lot, the car is still
> responsive when driven
> >hard.
> >
> >
> >Jorgen Karlsson
> >Gothenburg, Sweden
>


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