[Healeys] Girling brake boost ratio

Bob Spidell bspidell at comcast.net
Thu Aug 20 10:29:04 MDT 2009


OK, now I got a question: is the 'boost ratio' a measure of the amplification--i.e. 3.25 applies 3.25 times the force applied at the pedal? 

Don't see how that could be stated so unequivocally, since the boost is also a function of the vacuum available at the time of application (no vacuum, no boost). 


bs 



----- Original Message ----- 
From: "Earl Kagna" <kags at shaw.ca> 
To: drmasucci at comcast.net, "Healey List" <healeys at autox.team.net> 
Sent: Thursday, August 20, 2009 9:14:47 AM GMT -08:00 US/Canada Pacific 
Subject: Re: [Healeys] Girling brake boost ratio 

David: 

Just went through all of this when re-boostering several local BJ8's. We 
decided on PBR (Australian) boosters. They are (or were) made in a couple 
of vacuum chamber sizes, and 3 or 4 body bore sizes to match them to 
whatever braking system the car has. 

For instance, on Healeys there is a different recommended booster for the 
BJ7 (and BN7 / BT7) and the BJ8, just as the original Girlings were 
different - boosters are sized according to calliper (wheel cyl.) size, 
master cyl. bore size, etc. 

I was advised by the supplier that the original BJ8 Girling boost ratio was 
3:81. We chose a PBR VH40 booster that has an appropriately sized vacuum 
canister and has a boost ratio of 3:25. The cars already switched over feel 
very close to the original boost - excellent braking feel. The use of DOT 5 
silicone brake fluid, which everyone around here seems to use, will slightly 
affect the braking feel on any booster. We are very happy with these 
boosters - Kilmartin in Australia even supplied mounting brackets for them 
so that they bolted right up to the cars, (with 2 new brake pipes being bent 
to connect to the hydraulic system). The booster even has a bleed screw on 
it's body, which is really nice for bleeding. 

Your boost ratio of 3:00 shouldn't feel much different, and you probably 
saved a lot of $$$. 

Earl Kagna 
Victoria, B.C. 
BT7 tri-carb 
BJ8 


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