Hydraulic System Concerns
in 914-4 Front Brake Upgrades

One of the continuing lines of discussion in the various forums concerns increasing the braking capabilities of the 914-4.

Quickly looking at a brake system we see that the driver presses on the brake pedal that is connected to the piston in the master cylinder.  The piston moves, compressing the brake fluid and creating pressure in the hydraulic system.  The hydraulic pressure forces the brake caliper pistons against the brake pads and the pads against the surface of the brake rotors causing friction that stops the car.  This is a force multiplier system, using a small diameter master cylinder piston pushed over a long distance (an inch or so) to move the large diameter caliper pistons a short distance (a few thousandths of an inch).   Mechanical advantage is generated by the ratio between the areas used for force input (master cylinder) and force output (caliper pistons).  There is also mechanical advantage in the brake pedal to master cylinder linkage.

Master Cylinder and Pressure

The mechanical advantage of the pedal linkage is constant regardless of which master cylinder is used.  A quick measurement of the pedal gives a bit over 5:1 ratio of distances from the pedal pivot point to the pedal pad, and the pivot point to the master cylinder rod connecting point.  This gives a 5:1 force advantage, yielding 5 pounds of pressure on the master cylinder piston from only one pound of pressure on the pedal.  It also produces a 1:5 motion disadvantage, meaning the master cylinder piston moves only one inch while your foot moves five inches.

Just to keep the rest of the numbers easy, we'll have the driver press on the brake pedal with 20 pounds of force.  This multiplied by the mechanical advantage of the pedal linkage will apply 100 pounds of force to the master cylinder piston.

Let's begin with the hydraulic pressure generated in the master cylinder.  Pressure is defined as a force applied over (divided by) an area:

The portion of the master cylinder piston that pushes on the brake fluid is a circle, and the area of a circle is the square of the radius multiplied by pi. 

A recurring comment is that braking force can be increased solely by the use of a larger diameter master cylinder.  There are three master cylinders generally used on 914-4's, the stock unit having a 17 mm bore diameter, the 911/914-6 master cylinder with a bore of 19 mm, and a unit from Mercedes used by some of the monster brake aficionados that has a 23 mm bore.  By running the numbers to determine the area pressing against the fluid we get:

Returning to our pressure calculation, we must use these areas to divide the force our driver is applying to the master cylinder piston (100 pounds). 

From these calculations it can be seen that with a specific amount of pedal pressure, increasing the diameter of the master cylinder bore actually decreases the pressure in the brake system.  This means to get the required amount of braking force you will have to push harder on the brake pedal if you install a larger master cylinder. 

What does happen when you use a larger diameter cylinder? You can move a larger volume of fluid!  In order to move your brake pads against the wheel rotors, the caliper pistons must be moved by the brake fluid.  By increasing the master cylinder bore you increase the volume of fluid displaced with each unit of distance the brake pedal moves.  To move the required amount of fluid the pedal will need to be moved less, and you have a higher brake pedal.  Thanks to the rear brake proportioning valve, 914 brakes are notorious for feeling a bit mushy, and this decreased movement of the pedal is easy to perceive as increased braking ability. 

Caliper Pistons and Force

At the other end of the brake system we have the brake calipers. They convert the pressure in the brake system back to a force to press the brake pads against the rotors.  In doing calculations we will be looking at front brakes only, and performing calculations for a single piston.  Actually there are two pistons in each caliper on the front axle and the same on the back, unless you have installed REALLY big brakes with four piston calipers.  As part of the brake biasing, rear calipers usually have smaller pads and smaller diameter pistons than front calipers. 

Our original equation was:    Pressure = Force divided by Area

To calculate the force being applied to the caliper pistons we rearrange the equation to read:    Force = Pressure times Area

And in this case the area being used is the area of the caliper pistons.  As mentioned previously, the caliper piston area is many times that of the master cylinder piston, giving a significantly larger output force, with a much smaller degree of motion. 

The most common upgrades are the modified BMW 320i (1978-1983) calipers or the Porsche M calipers from early 911's.  Both of these units have larger pistons than the stock 914-4 brakes.  A given pressure in the hydraulic system applied to a larger piston will produce a greater force against the brake pads, but having larger pistons will also require more fluid displacement to move them. 

The increase of fluid displacement will require a longer stroke of the brake pedal, possibly exceeding the available stroke of the master cylinder. 

This is where the increased volume from a larger master cylinder really comes in.  If you do things correctly, the force from the larger caliper piston diameters will more than offset the decreased hydraulic pressure from the larger master cylinder.

The 611 lbs. of force provided by the 914-4 brakes with the stock 17 mm master cylinder is the baseline used for comparing other master cylinder/caliper combinations.  Any combination giving a higher force will provide greater braking.

Other Considerations

This analysis was done only to provide a look at hydraulic pressure and force issues in the brake system.  It was not intended to cover other factors involved including the effects of larger brake pads available with the caliper upgrades, pad composition, or increased cooling capacity from the larger calipers, though a few observations on these topics are included:

Larger brake pads will increase the area of the brake rotors that are gripped, increasing friction and the effectiveness of your brakes.  It is my understanding that the M caliper pads are basically the same size as the OE 914 pads, while the 320i calipers use much larger brake pads.

Brake pad compounds have undergone significant changes in recent years.  Asbestos has been removed from friction materials, and organic, metallic and semi-metallic compounds are now in use.  Race applications use carbon composite materials almost exclusively, and carbon street compounds are available.  The coefficient of friction of carbon composites is much higher than that of other pad materials, but they are expensive and tend to squeal and produce a lot of brake dust.

An additional note on race compound pads, they are designed to work at much higher temperatures than are normally encountered in street or any but the most aggressive autocross applications.  Under any conditions usually (or safely) encountered on public highways, they will not stop your car as well as street compound pads.  By the time they heat up enough to work efficiently, you may be dead.

The original equipment brake calipers for the 914 were taken from Volkswagen applications.  There are many reports that they will actually flex under severe (race) braking conditions.  The 320i and 911-M calipers are larger and much more rigid than the stock units.  The larger calipers are also supposed to dissipate heat better than the OE pieces, though this should not be a factor for most street applications. 

For very high performance applications, upgrading the front suspension to 911 components will allow the use of many other versions of 911 and aftermarket brakes.  These are significantly larger and more powerful than the units discussed in this article.  To maintain front to rear brake balance the rear brake mounting system will also have to be reconstructed to allow use of upgraded brakes.

Inspect the general condition of your brake system.  Worn, soft, and degraded hoses will reduce system effectiveness and are just waiting to fail, replace them.  Worn or warped rotors, worn pads, and worn or stiff seals in the master cylinder or calipers should be replaced.  Old contaminated brake fluid should be replaced, ensure the system is properly bled.  Make sure the rear brake venting clearances are set properly.

Conclusions

To obtain the greatest force between the brake pads and the rotors you will need the largest caliper pistons you can get coupled with the smallest diameter master cylinder bore that will move the required amount of brake fluid without excessive brake pedal travel. 

You should also consider the fact that when you significantly increase the mechanical advantage in your brake system you will be able to produce very large pressure changes with very little difference in pedal effort.  The difference between a properly modulated brake application and major flat spots on your tires will only be a couple pounds of pressure on the brake pedal. 

Brakes are a very important system on your car.

If they fail or if they don't perform reliably you could kill yourself or other people.

Know what you're doing

Use good quality parts and materials

Inspect your work, again and again

Immediately correct any problems

Inspect your brakes on a regular basis

The life you save could be mine