I am trying to calculate the mechanical disadvantage that results from the A body and B body Power Brake Booster Linkage.

I am not an engineer (but play one in my garage) but I did some research and found the calculations shown on the attached diagram.

It looks like the A/B body booster linkage results in a 44% reduction in pedal force.

My car was originally manual brakes and has an unusually high brake pedal ratio of 8:1 and it looks like by using the A/B Body Power Brake Booster Setup I will be able to get the pedal ratio down to a reasonable 4.456:1 to use for power brakes. This is right in the range of the 4:1 to 5:1 that chrysler used on its muscle cars. (from an earlier post in this subject).

Is there anyone here that can validate my calculations and conclusions?

Thanks,
Scott

You have to use the power brake linkage set up for use with power brake booster. Doesn't work with manuel brake push rod.

the pedal ratio won't change with the stock setup...as they both use the same hole in the pedal.

The stock booster does decrease the pedal travel vs a manual brake setup, which is why the pedal sits closer to the floor. If there was no vacuum assist, it would be MORE difficult to create the same amount of travel at the master cylinder with the power brake setup. The vacuum servo is an assist that makes it easier to push and also creates more master cylinder displacement from less pedal displacement when compared to a manual system. So its kind of like being on the wrong end of a lever system without the vacuum servo.

Thanks for the feedback.
My car is a 56 Dodge Custom Royal Lancer (Coronet). I am converting to a manual Tremec T56 Magnum 6 speed and 4 wheel power disc brakes. The manual brake pedal setup has an 8:1 pedal ratio. This would probably make a great manual brake car but I prefer to go 4 wheel power disc if I can work it out. Because of the design of the brake pedal I cant just re-drill the hole for the brake rod to alter the pedal ratio.

The A/B Body booster and linkage is a direct bolt on to the original brake pedal. If my calculations are correct the mechanical disadvantage created by A/B body booster linkage will have the effect of creating a pedal ration in the 4.5:1 range, well within the recommended range.

What I cant control (or figure out how to calculate) is the pedal travel, but once it is all together I can play with the MC piston diameter to alter the pedal travel if there are any issues.

Scott

well the a-body booster works with the manual brake pedal ratio for an a-body anyway. There is only one hole. It will probably work fine then, if it bolts up otherwise.

If you already have the booster, bolt it on. The floor position stays the same while the "rest" position should be lower to the floor.