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.

[woobn8r]

I have just acquired some nice alloy 4 piston Boxter brakes and adaptors for the 914 rear trailing arm and 3.5" 911 front struts.

They mate up with vented (and drilled) 911 rotors. This is an excellent conversion as it is way more brake than my 3.2 914 will ever need...even on the track.

The larger pad area (with greater potential clamping force) spreads out the load on a larger rotor surface which dissapates heat more quickly...the alloy caliper saves some unsprung weight to offset the increase in rotor weight....end result should be a brake system that requires less pedal pressure for equal clamping force...it will also have a greater potential range (which increases feel for modulation) and will dissapate heat more quickly eliminating any need for ducts, hoses or fans...oh, and as this system works at the lower end of it's potential, it should last longer as well.

The trick now is to choose a good pad that works in the right heat ranges without chewing up the rotors....and then set the bias.

Car not ready for a while...but I'll report when she is.