A4 Brakes in Rain + Vacuum on a turbo engine

["P-O Selander (EUS)" <EUSPOS@am1.ericsson.se>]

Hairy green toads made Brett write:

>Umm, you can't have vacuum assist in a turbo car.  Think about that 
>one really hard.  Turbo charger cars spend a fair amount of time with 
>a manifold pressure that is NOT a vacuum.  You'd need some sort of 
>check valve system, and a very large reservoir, and if it runs out, 
>oh well, no brakes, eh?

While it is true that the manifold pressure many times is positive on a
turbo engine, that is AFTER the turbo charger. Prior to that you certainly
have vacuum, a good one (when the turbo charger tries to get more air in
through the air filter etc) and even better on WOT. Remember however that
the "vacuum" can not be greater than the atmospheric pressure since "vacuum"
is just the differential between atmospheric pressure and the lower pressure
inside the manifold (in a turbo car, prior to turbo charger). Hence, in a
perfect environment (as in outer space), the maximum vacuum would be
somewhere like 1000 milliBar.

In the brake booster, it is actually the atmospheric pressure that PRESSES
the diaphragm and provides boost assist. It does that since the harder you
press the brakes, the more air is being removed from the "secondary side" of
the diaphragm into the manifold. And since the diaphragm on most cars is
relatively large, you do not have to have much vacuum in order for the
brakes to have a decent assist (large area, a few pounds pressure per area =
large assist) . Do not compare this to trucks (big rigs) where the brakes
are actuated by high pressure air and therefore have a separate compressor
and storage tanks. On big diesels, this one is many times mechanically
driven. These setups also requires an "airdryer" so you don't get is ice
glogging up your brake lines.

Some what off topic, but I figure someone might have been interested.

P-O Selander, Bellevue, WA