[Healeys] Girling brake boost ratio

[Bob Spidell]

Vacuum is applied to both sides of the vacuum piston when brakes aren't applied; 
when the brakes are applied atmospheric air is applied to the 'top' of the 
piston.  The difference between the vacuum and atmospheric pressures, and the 
large size of the vacuum piston compared to the smaller size of the servo 
hydraulic piston is what supplies the boost.

So, the difference between the vacuum pressure (something less than atmospheric) 
and atmospheric pressure affect the boost (compared to the vacuum, the 'top' of 
the vacuum piston is effectively pressurized when the brakes are applied).

FWIW, 'standard' atmospheric pressure is 29.92 inches of mercury.  A 'vacuum' of 
20 inches means there's 9.92 inches of pressure (it's only a 'vacuum' in 
relative terms).  Less vacuum means atmospheric pressure is less effective in 
pushing the piston.  Servos don't work as well with really 'hot' cams (lots of 
valve overlap--less vacuum).

bs



Warthodson at aol.com wrote:
> As I understand the servo, vacuum pressure is applied to both sides of  the 
> piston. It is atmospheric pressure  that creates  the "boost". Does the 
> boost really vary with the amount of vacuum or  is it a function of atmospheric 
> pressure?
> Gary Hodson  

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Bob Spidell           San Jose, CA            bspidell at comcast.net

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