engine/turbo theory question

[Jorgen Karlsson]

Ameer Antar wrote:

> 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.

Of course you are right and the center section of the turbo should be kept
at a temperature where the oil can do its work. Letting the car cool down
before it turned off is extremely important when the car has been driven
hard.

> 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.

Right again, it is not really the turbo that has to be hot, the exhaust
manifold should be hot, to prevent the exhaust energy to be cooled of before
it reaches the turbine. Unfortunally the turbo reaches almost the same
temperature as the exhaust manifold and they are built to handle it. The
temperature is not transfered very well from the turbine housing to the
center section, the axle is also cooled to prevent the oil from being burnt.

The early audis are pretty much alone with their cold running turbos, I
think that this has to do with the early turbo cars tendencies to shoot the
turbo after a few years, this was before water cooling of turbos was
commonly used. Five cylinder engines also has the problem with the exhaust
manifolds that can't take the heat, they crack and pull studs while they
cool down.

Here we come to the exhaust manifold again, one made of thin material will
light up imediately when you hit the throttle, you will have to run out of
the car to have time to see it before it cools down. Of course the turbo
does not have the time to start glowing under these circumstances but that
is not needed either. This kind of manifold can allow for a responsive turbo
system without having the whole manifold glowing all the time.

And as Ameer says, I don't think that it is a good idea to let the turbo
glow on a audi. But I only say this because of the manifold, the turbo can
take a lot of beating.

One other thing is that when you don't have sufficient fuelling on a
modified car the exhaust temperature will increase to extreme levels. When I
first found that problem on my car the manifold, turbo and downpipe was
glowing in a whitish color, the turbo was of course the coolest of the
three. The turbo took the beating without any problems, the manifold is junk
now...

Jorgen