jansenm

Hi!
I'm want to upgrade my '74 G model to Boxster calipers. I have difficulties finding an adaptor with the short 3" spacing for the early 911. Anybody did the conversion? Any knowledge of who does the adaptor?

Bill Verburg

What do you hope to achieve here?

There is a BMW E28 caliper that can be bolted on but not sure why you would

jansenm

I do not have sufficient braking power when on track with the small brakes

Bill Verburg

well a caliper change is only a partial solution and then only if the the caliper piston areas are larger than the stock S/M/A 48mm ones

brake torque is a function of
piston area
line pressure
pad co-efficient
effective rotor dimeter

the 911s w/ 3" front brake mounts came w/ M calipers that have the same piston area as the 3.5" mount S and A calipers

they all had 3" mount M rears w/ 38mm pistons until '84 when the front went to the wider A w/ the same 48mm pistons and the rear went to wider M w/ 42mm pistons

S & A all have the same effective rotor dimeters@ 282x20(24 from '84) effective diameter 228mm, an M will have a slightly greater effective diameter on the same rotor so will provide a marginal boost to brake torque at that end
rears are 290x20(24from '84) effective dimeter 244mm

so w/ the same pads the front brake torque is 1391nm @ 70bar line pressure all have the same brake torque result and rear brake torque is 933nm(1183nm from '84)


f/r bias is 1.491 1.220 from '84(necessitating a 33bar p/v on the rear)

the bigger front A & S pads compared to a front M actually reduce brake torque by a small amount due to the larger effective diameter, they mostly increase brake pad life

the thicker rotors used from '84 only increase the thermal capacity of the system and have no effect on brake torque

The most usual complaint wrt 911 brakes on track is heat issues, the fronts in particular can get very hot, boil the brake fluid w/ resulting long/soft pedal, no fun!!
solutions
loose as much weight from the car as you can
learn not to use the brakes so much
use only fresh(the newer the better) highest quality brake fluid, Castrol SRF is the best that I know of though there are a few others that have higher dry boiling points, they all fall behind in time
add deflectors to the A arm to shoot extra air to the calipers and rotors, ducts are almost always way too small to do any good
use pads pads that have higher Mu at higher temps

wildcat077

Listen to Bill's advice ... he knows 911 brakes !

Your best bet is to use top quality brake fluid and get some sort of cooling ducts for the front brakes.
I DE'd my 89 911 in the black PCA run group with stock 911 brakes for a few years before i changed the brake system completely , but the stock brakes were just fine.

Cheers
Phil

Uwon

If you are bent on replacing the front calipers, I have a spare set of beautiful Brembos with 3” ears. Bolt on. Text me if interested.
cheers

jansenm

Very interesting feed back - thank you for that. I will reconsider my project and probably go with the listed solutions.