Re: A4 Brakes in Rain

[Aleksander Mierzwa <alex@matrix.pl>]

At 11:15 99-02-05 -0500, Brett Dikeman wrote:

>Umm, you can't have vacuum assist in a turbo car.  Think about that 
>one really hard.  Turbocharged 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?
>
>Maybe your 2.8 a4 does, but I'll bet that the 1.8t certainly doesn't.

There is a fair number of turbocharged cars with vacuum assist. Without
searching too far, Renault 21 turbo is a fine example (I've got the Haynes
manual for this car, so I'm pretty much sure about this). I believe the
first generation (pre-84) Audi 200/5000 had vacuum assist as well.

Take note that even in N/A car, you have almost no vacuum in the manifold
at full throttle. In normal driving, though, you almost never use left foot
braking. You close the throttle before applying brakes, hence generating
instant vacuum in the manifold. Some cars (like 4ks ) use additional vacuum
pump to ensure there's enough brake assist even in these rare situations.
My mother used to drive an old Nissan diesel. Diesel engine, having no
throttle, also generates almost no vacuum. It had the vacuum pump
integrated into the alternator!

Aleksander Mierzwa
Warsaw, Poland
mailto:alex@matrix.pl
87 Audi 5000CS turbo (mine)
88 Renault Medallion wagon (mom's)
91 mountain bike (just in case both cars broke at the same time :-)