Re: BTV Replacement that should work for everyone

Posted by HH56 On 2014/2/16 10:39:21

Biggest problem with any conversion is the pedal ratio. With the stock Packard location, the ratio is 1:1 because it is at the bottom of the pedal. Just about any modern system wants a minimum of 3:1 and even more in some cases. In an emergency situation with engine off and no power assist be it vacuum or hydraulic, any modern system with a low ratio would be unable to stop the car in any distance close to what would be acceptable.

The BTV gets around this low ratio by the unconventional construction of the master cylinder. It utilizes a rather small ram with a long travel squeezing or displacing the fluid rather than a modern cylinder using a large surface & short stroke piston to push the fluid out. Even with the differences, the BTV still makes for an interesting experience if the power assist portion quits -- but not as interesting an experience as that when the moving mass of rubber called a compensator valve fails to seal.

Craig's two replacement approaches have addressed the ratio problem by either modifying the pedal if a new master is in the original location or by moving the master up in this latest job to have the actuation point higher on the lever.

One thing I have found interesting is location. While some cars followed Packard in placing BTV's toward the bottom and had low ratios, other cars (Chevy, for one) placed it differently and had a high ratio to start with. Near as can be determined, other than possibly some later refinements in the pedal feedback and sensitivity, the basic units were of identical hydraulic construction and functioned equally well at either spot. Because of the placement choice, there is a direct modern bolt in BTV replacement for Chevy but it won't work on a Packard.

Without changing the ratio, I'd be concerned about any modern replacement including the Hydro-Boost or the Electro-boost being able to stop the car in an emergency. The Electro system would at least not be compromised if the engine alone quit but since people have been installing it in the original location, I would be just as concerned if a short should occur or something happened to the battery and the accumulator pressure reserve was all used up.