Take A Brake - Wilwood Brakes master Cylinder
Gerald Lum, Rolling Hills Estates, CA: Many of the cars in Car Craft and other mags show brake systems sans the vacuum booster. I think they do it for a cleaner, better-looking engine compartment. But how well does this stop the car?
Jeff Smith: Great question, Gerald. The common misconception is that disc brakes require a booster to make sufficient line pressure to actuate the brakes. While it is true that discs require more line pressure to squeeze the pads against the rotor compared with drum brakes, it is possible to generate sufficient line pressure with a nonassisted master cylinder. The concept is very simple. A smaller master cylinder bore diameter will generate more output pressure in pounds per square inch (psi) than a larger piston version given the same amount of pedal effort. As an example, let's exert a force of 50 pounds directly to a 1-inch-diameter piston master cylinder. By calculating the area of the piston (Pi x radius x radius), we come up with an area of 0.7854 square inch. Divide the force by the area (50/0.7854) and the result is 63.66 psi of hydraulic pressure. Now let's do the same math with a 7/8-inch (0.875-inch) master cylinder (Pi x 0.4375 x 0.4375 = 0.601 square inch of area). Divide the 50 pounds of force by 0.601 and we get 83.19 psi, or a whopping 30 percent increase in line pressure. For the record, there is also a significant ratio gain from the pedal of usually around 6:1, so 50 pounds of force on the pedal quickly becomes 300 pounds of force applied to the master cylinder. If we divide 300 pounds by that same 0.601-square-inch area, we get 499 psi of line pressure with a 7/8-inch piston.
Of course, there's no such thing as a free lunch, especially with hydraulics. The price we pay with the smaller master cylinder piston is moving a reduced volume of fluid for a given piston stroke. In other words, converting to a smaller master cylinder piston requires moving the piston a greater distance to displace the same volume of fluid. If the entire braking system is sized properly, this usually isn't a problem, but it is an important point that can't be overlooked. As an example, we recently replaced a 1-inch iron master cylinder on our Orange Peel Chevelle with a 7/8-inch-diameter piston Wilwood Brakes aluminum master to save some weight. Many years before, we had converted the Chevelle over to disc brakes and mistakenly routed the brake lines with the front master cylinder piston connected to the rear brakes and the rear piston routed to the front discs. When we bolted on the Wilwood Brakes master, the rear master cylinder piston displaced insufficient fluid volume to move the front brake caliper pistons and pads against the rotors. As a result, the brake pedal just fell to the floor as if there were massive air in the system-which there wasn't. In most GM master cylinders, the front (or forward) piston assembly is designed to move a greater fluid volume to accommodate the generally larger pistons used in most disc brake calipers. Because we had routed the lines backward, total pedal travel created insufficient volume to move the calipers' pistons, and as a result, we had no pressure, even though there was no air in the system. Once we rerouted the brake lines, we had a very firm pedal. The first testdrive also revealed far less pedal effort necessary to generate the same braking force.