A big part of building any car for drag racing is fixing its shortcomings. In the case of the Fox-body Mustang, flex is a big issue (we addressed that last issue, “War Pony, Part 2,” Jan. 2014, pg. 18). The next big problem is focused under the dash, namely the brake pedal/master cylinder assembly and the clutch linkage arrangement.
Most late-model cars are power brake only, although 1979-85 Mustangs with 2.3L normally aspirated engines were available with manual brakes. Factor in a healthy camshaft and that can spell trouble. You either have to incorporate some sort of vacuum canister assembly, which adds complexity, or switch to a manual brake arrangement. And when you swap to a non-boosted master cylinder, you’ll need to consider the brake pedal ratio.
What’s the big deal with pedal ratio? It’s extremely important—critical, in fact. The brake pedal acts as a lever to increase the force applied (by you the driver) to the master cylinder. If you examine a brake pedal, you’ll see the pivot point (where the pedal swivels) and the mounting point for the master cylinder pushrod are quite often different. By varying the length of the pedal and/or the distance between the pushrod mount and the pivot, you can change how much force (from your leg) is required to energize the master cylinder. This is the “mechanical advantage” or pedal ratio (hydraulics are a separate issue).
Here’s a formula that will help you figure it out:
Input Force X Pedal Ratio
————————————- = PSI
Brake Piston Area
When this formula is interpreted, it’s the amount of force exerted by your leg times the pedal ratio divided by the area of the brake system pistons. The typical adult male can exert approximately 300 pounds of force in one leg (maximum, as in an “I’m absolutely terrified,” panic stop). If you’ve ever tried single leg presses at 300 pounds, you’ll know that’s a bunch. As a result, when you’re figuring out the right pedal ratio for your race car, shoot for a number in the order of a third or even half of that figure. Maximum Motorsports points out that the brake system of most cars is usually designed for a maximum hydraulic operating pressure of 600-1,000 psi, and that pressure isn’t required for the master cylinder to be effective. The pressure out of the master cylinder is rather irrelevant because other things in the system can be adjusted to get the desired brake torque for a given pedal force. The brake system designer’s goal is to design the system to provide the brake torque required to stop the car without excessive pedal effort. The maximum pedal force designed for is typically no more than 150 pounds.
What really matters is the area of the caliper pistons on one side of the caliper multiplied by the pressure from the master cylinder. That product, in conjunction with the brake pad cf and rotor diameter, is what determines the brake torque.
For example, a brake system could be designed for the pressure out of the master cylinder to be 1 psi or designed for 10,000 psi. If the brake calipers had the correct piston area, for each of the two pressures, the resulting brake torque could be correct in both cases. The master cylinder pressure doesn’t matter, since other things in the system can be adjusted to get the correct brake torque for a given pedal force.
Now you have to translate 100-150 pounds of leg force into 600-1,000 psi. While changing the overall length of the pedal is possible, it’s usually easier (and far more practical) to shorten the distance between the pivot point and the master cylinder pushrod mount location. If you go overboard in the pedal ratio department, the brakes can become over-sensitive. The trick is to get the right pedal or lever ratio that is appropriate for the master cylinder you’ll be using.
Maximum Motorsports adds its insight: “Tuning the system by adjusting the brake pedal ratio must be done very carefully because the pushrod must be aligned properly with the master cylinder piston to avoid damaging the master cylinder piston seal. So, if the pushrod pin is moved on the brake pedal, the pivot point on the pedal box must also be moved for the pushrod to retain the correct alignment with the master cylinder piston. On Fox Mustangs, Ford put two different holes in the pedal box to accommodate the two different pin heights of the pedal arm: one for the power-assisted pedal ratio and one for the manual pedal ratio. Our pedal arm was designed to maintain the correct pushrod alignment to the master cylinder by properly locating the pushrod pin on the pedal arm relative to using the correct manual brake pivot hole in the Fox pedal box. Other than the unique case of the MM pedal arm with its adjustable pedal pad, it is almost always easier to change the master cylinder diameter and/or select the correct caliper piston diameters to alter the pedal effort.”
The next issue is the clutch linkage, or more correctly, the lack of it. Fox Mustangs have a big Achilles heel, and that’s the cable clutch linkage arrangement. It’s not so much the cable, but the way it’s fed out of the cockpit. They use a flimsy quadrant mounted to the pedal box to feed the clutch cable. The factory quadrant is equipped with an equally flimsy pawl that is engineered to self-adjust the clutch over the life of the car. Both the quadrant and the pawl are manufactured from plastic. Factor in age, heat and a heavy clutch and that spells trouble.
Ask anyone who works on these cars and you’ll find that working under the dash is a nightmare. There are a couple of ways to access the under-dash area: one is to remove the driver’s seat and lay on your back to work upside down; the other is to remove the entire dash and steering column. We chose the latter and it was easier.
What wasn’t difficult was picking the replacement parts, thanks to Maximum Motorsports. They’re pretty much the Mustang performance headquarters offering both a comprehensive manual brake conversion kit and a well-engineered cable clutch setup.
We started with the brake package. Essentially, the Maximum Motorsports pieces include a billet aluminum adapter plate for the firewall, a correctly sized plastic reservoir, aluminum body master cylinder, a new pedal with the correct geometry (and it even has an adjustable pedal pad, more later), a new brake light switch, all connectors and all hardware necessary for assembly. They even included a custom-bent set of brake lines to hook up to the factory brake distribution bracket.
As far as clutch setup, the Maximum Motorsports pieces we selected include a new billet aluminum quadrant, a new cable and billet clutch adjuster. Instead of using the pawl to automatically adjust the clutch, the Maximum Motorsports pieces provide adjustment right at the engine side of the firewall. The clutch cable provided with this package is an OEM style, meaning it has OEM life expectancy.
So how do the pieces fit and how tough are they to install? Remember, we took out the dash and that helped the process immensely. Check out the accompanying photos for an overview of how it went together.
Fox Mustangs use this device to operate the clutch cable. The bigger plastic piece is the pawl and the smaller plastic part to the right of it is the clutch adjuster. This isn’t the most effective setup when adding clutch pressure.
Instead of adjusting the clutch under the dash (with the plastic auto adjuster), the Maximum Motorsports setup adjusts the clutch manually at the firewall. The rubber clutch cable grommet at the firewall (shown here) is removed first. It’s replaced with these knurled adjusters (on the right).
Here’s a photo of the adjuster installed. You must drill an extra hole in the firewall for the adjuster because it uses two, while the stock rubber grommet has one. Once this is done, hook the Maximum Motorsports cable to the OEM clutch fork. If you refer back to the photos of the new clutch quadrant, you’ll see an aluminum spacer. That’s a spacer for the cable at the quadrant. It may or may not be needed, depending on your clutch setup.
Check out this comparison between the OEM brake pedal and the Maximum Motorsports piece (below). Not only is the pedal ratio correct for a manual master cylinder, it’s a beefier piece. The pedal is adjustable up, down, in or out. The adjustment makes it easier to drive the car. If you have an automatic and are foot braking it, you’ll really appreciate the adjustment capability.
The new pedal assembly makes use of a rod end on the pushrod side of the pedal (as shown here). In the second photo, we dummied up the pedal assembly along with Maximum Motorsports master cylinder to show how it all goes together.
Text and Photos by Wayne Scraba