Notes on brake "upgrades", What's worthwhile, what isn't. Options

[lapuwali]

A bit bored this AM, so I'm rambling a little...

The 19mm MC used with otherwise stock 914/4 brakes: this results in a reduction in braking force for a given pedal force (forcing you to push harder to stop), and the actual amount is 16%. Some people like the "firm" feeling this provides, but it doesn't mean "better braking".

The 19mm MC used with BMW 320i calipers OR Porsche M calipers (same size). This results in a 12% decrease in pedal effort for a given braking force. So, you will feel an improvement, but it's not a very big one. The downsides to this swap is the bigger calipers are heavier (unsprung mass, too), and of course you're doing a lot of work for a pretty small gain. You're also doing nothing to help brake fade. You're likely making it more likely, since you can now load a bit more heat into the brakes more easily. Happily, this also won't have a huge effect on front/rear brake bias, however, since the overall effect is so small.

A 17mm MC used with M calipers would produce quite a big change in pedal force v. braking force: 33%. However, this MAY cause a problem in that the 17mm MC is small enough that you couldn't move enough fluid with it before you run out of pedal travel to fully press the pads against the rotors. You'd certainly have to push it a long way to get the brakes to come on at all, and the brakes would likely feel touchy and mushy, but powerful. Rather like overboosted power brakes...

btw, "17mm" and "19mm" (and "42mm" or "48mm" for caliper piston diameters) aren't strictly correct. Even in metric countries, brakes are actually specified in 1/16ths of an inch, so 17mm is really 11/16, or 17.45mm, and 19mm is really 3/4 (12/16) 19.05mm. 42mm is really 26/16", or 41.275mm, and 48mm is really 47.625mm. Germans just list the sizes in the nearest metric size, whereas the Japanese perversely use both systems: 10mm for some parts, and 0.50" for others, for example.

[lapuwali]

More rambling:

When "improving" brakes, you can really only do threethings: reduce the chance of fade, reduce pedal effort, change brake bias front/rear.

Reducing pedal effort can be done two ways: change the hydraulic ratio of master cylinder to calipers, or add a power servo. Adding a servo to a 914 is possible, but pretty difficult. Altering the ratios is certainly doable, and can make a difference. There's a sweet spot on ratios, which ranges from about 18:1 at the low end and 24:1 at the high end. Lower than 18:1 (master cylinder too big), and the pedal feels "wooden", there's very little pedal stroke, and you have to push hard enough that you end up losing the ability to modulate the pressure. Higher than 24:1 (master cylinder too small), and the brakes feel mushy and touchy. Pedal stroke is quite long. You also run the risk of running out of pedal stroke before the calipers are fully pressurised, since the small MC can't push as much fluid. These numbers are strictly empirical, and it really is a "feel" issue before it starts to become a safety issue. On a 914/4 with stock calipers, the front brakes are 22.3:1 with a 17mm MC, and 18.8:1 with a 19mm MC, so they're both in the sweet spot, but at opposite ends of it. 20:1 would likely be the most pleasing to most people, but there's no off the shelf combination of Porsche parts that will get you there. And yes, the front and rear brakes are considered separate systems, since the 914 uses a dual MC. The combination of the rear brakes will also affect feel. The ratio at the rear should be less than the ratio at the front, or you need an adjustable proportioning valve to reduce the pressure at the rear calipers, so they don't lock first.

Reducing fade is usually the most worthwhile goal in improving brakes, and certainly what you need to do if you're going to track the car. This cannot be accomplished by just swapping calipers. You must either change the pads (the material CAN make a difference), or enlarge the rotors, either with larger diameter rotors, or vented rotors (or both). The downside of rotor swaps is that they add unsprung mass AND rotational mass, the former making the suspension work harder, the latter making the brakes work harder. If you're a zillionare, you could go with ceramic or carbon composite rotors, of course. Using lighter aftermarket calipers can help offset the additional mass of the bigger rotors, though that won't help the rotational problem (which I need to characterize).

Improving brake bias is, I think, the overlooked upgrade for the 914. I wish I had more data, but I get the feeling that the 914/4, as stock, is actually underbraked at the back. This is a "safe" engineering decision, since preventing premature rear lockup is important in a street car to be driven by non-experts. However, I get the feeling some useful braking is left on the table by an overly aggressive front/rear proportioning. With the usual adjustable proportioning valves on the market, you can only get a useful change in brake bias by also fitting much larger rear calipers, since all these valves can do is reduce the braking to the rear. If you don't have enough braking at the back to begin with, you end up with a valve that's merely open (i.e., a Tee).