vacuum with blower

[jpearl at qsi.com]

>> someone please tell me how a turbo/super-charged system
>> gets vacuum for the vac-assisted brakes and other vac-
>> operated accessories ?  Is the vacuum that is produced
>> during idle and decel stored and used?  My knowledge
>> of SC and TC systems is limited.

>There is a check valve in the brake booster to prevent pressure from
entering the booster on many cars.  Under boost >you loose all effects of
the brake booster as there is no vacuum to draw from.  Unless you drive
with your foot on the >brake at full boost, you are not likely to ever
experience this.  As soon as you let off the gas pedal, you have >instant
vacuum to power your brake booster.

My $.02 - my car has NO such "checkvalve" and no special considerations
were made concerning boost and it's effect on the brakes. It's a Twin Turbo
5.0 (was actually) and I too wondered abut the effects I'd receive under
boost. Simply put - the pedal pushed BACK at me when under boost and
braking! Most of the boosted cars out there probably don't have this
problem (say a Vortech car) but with a turbo I could place my fot on the
brakes (hard) and floor it and get enough boost for the pedal to rise
despite my foot being jammed against it. Was rather disconcerting but since
I seldom did this (datalogging my DFI) it was never an issue. As others
noted, when you want brakes you don't often have your foot on the gas!
(grin) Was kind of interesting to get up to about 25-30 mph with my foot
hard on the brakes and have the tires begin to spin as I came up on boost!

jvp at qsi.com