I'm planning on using 911 SC calipers on the front and rear of my 914 DE car. Is anyone out there using SC calipers on the rears and if so, what rotors are you using. No problem with the fronts, I'll be using the 911 setup. Also, I have been told that I should not need a proportioning valve since the calipers are balanced since it's a stock factory setup. I plan on installing the valve anyway though.

I am currently working on this problem and still have to figure out what is going on with the brakes. I am considering using either 19mm or 23mm master cylinder with the 87 Carrera braking system all the way around. There is a lot I do not know at this point, but some of the basics that come out appear to be the following:
The 914 original braking system is setup for mid-engine; whereas the 911 concept
is for a tail dragger (weight behind the rear axles.

The 914 proportioning valve is set less than 50-50 to control the front/rear braking
biasing to provent the rears from locking before the fronts (I believe understearing
and spinning if the rears lock first)

In computing the weight transferring and changes taking place (i.e. center of gravity)
the formulas i use set the braking to the front first and then compute the amount
needed in setting the rear bias.

In looking at the different brake rotors (ie calipers, too), it appears the rotors on
several mid engine cars are of equal size in front as compared to the back if
not the back being slightly larger (Need to verify this point)

It appears the best master cylinder available (non-assisted) is the 19mm, but with
the right calipers (i.e. the caliper cylinder bore) the 23mm (non-assisted) will match
the 19mm one. (Need to verify this point)

The smaller the master cylinder bore (19mm vs 23mm) the more pressure to the
caliper cylinder bores for a given configured system. What is meant by this state-
is it takes less leg effort over a sustained period to make the brakes work better
for the driver. An example: Normal street driving leg effort is somewhere around
20 to 40 lbs effort input on the brake pedal and gives braking fluid pressure
around 700 to 800 plus PSI; racers are applying initial brake input leg pressure
off somewhere around 80 to 100 PSI (hard to sustain over a period of time) which
is giving pressures at the caliper in excess of 1500 PSI. This basic reasoning
is why the non-assisted 19mm master cylinder is used for known Porsche brake
systems.

Always understand what the braking system does and how it works since your
life and other lives depend on it working properly

On anyone of these points other people will have differing opinions and knowledge
so find out for yourself; remember: You are the Driver!

The 23mm master cylinder I am considering is being configured in conjunction with
Carrera or larger calipers: AND FOR STREET USE WITH ONLY OCCASIONAL
RACING. I am planning on using the front calipers on the rear in conjunction with
911 e-brake and half-shaft setups. Yes, I am including the 914 brake proportioning
vale which can be adjusted for th front/rear brake bias.

Anything larger than 23mm (and probably the 23mm non-assisted) should be assisted
to prevent having to use a lot of leg effort/pressure on the brake pedal. In these
master cylinder ranges the brake pedal pressure requires alot of driver effort and
extremely hard/heavy pedal effort.

I need to find out if the front caliper bores different fromt the rear caliper bores on a
normally setup midengine car, Carrera setup, and front engine car braking systems

To speifically answer your question, I have not as yet done the actual test driving for the above configurations mentioned. I am still working on getting the car completed. Down to the wiring and connedting the engine.

My car is a 1974 914 converted to 1987 Carrera engine with reinforced longs, etc. Brakes are very important to me since the basic configuration of the car has changed.

I did one of the first 914/V-8 engine changes using a 19mm master cylinder, rear 914 brakes including the proportion valve, 914 front brakes.. They were adequate but the 19mm give a hard pedal and more leg effort than the 17mm. I changed out the front suspension inlcuding brakes to 911 SC which gave me more front end braking power but still needed to adjust the bias in the rear (i.e. proportion valve)

Under the present configuration I am considering I will be increasing the bore on the calipers. With this change I feel I have to look at the 19mm master cylinder and the 23mm master cylinder to match leg effort with braking and driving capablilities.

I do get "long winded" at times but I hope you can use the foregoing information including some of the background logic. I may be looking at things all wrong so take everything with "a grain of salt".

Steve Hurt

1970-1983 911 rears with either the rotor spaced out 5mm or the calipers spaced back 5mm with washers.

914's do not have proportioning valves.

"914's do not have proportioning valves."

Oh?

The Cap'n

Maybe it was aftermarket, but my '73 2.0 had an adjustable proportioning valve, bought it that way. Not really sure what you're saying Eric, and I know you know more than I do, but I've seen many 914s with the valves, maybe all aftermarket, but nonetheless, 914s with valves.

LOL, well mine doesn't...cuz I took it out

Anyway, I am using 911SC "A" calipers and rotors up front, and front 914 calipers in back. (No e-brake but it is not really a street car anymore.) I put a "tee" in place of the proportioning valve, but I have an adjustable Wilwood sitting here if I ever decide I need the bias adjustability. 19mm master cylinder, Pagid pads. I have never felt short of brakes thus far although many of our local tracks are not extremely tough on brakes relatively speaking.

It doesn't "proportion" anything. It's a saftey device that shuts the rear brakes off in a panic stop (535 ft. lbs. of pressure if I recall). Otherwise, it acts like a "T" passing fluid back and forth without a care in the world.

At least that is what I am led to understand after reading the page in the factory manual describing the unit and it's operation. I may not have read it properly...

It was one of the first things to go back into my car.

Click to view attachment

One went back in my car today... Thanks E!

Eric,

Yeah, I strongly suspect you are right about the function of this puppy. The main reasons I removed mine were (a) I had already seriously modified the stock braking system, (b) it was heavy and © I have no reason to limit or cut-off rear hydraulic pressure in a track car application, so far (might change though).

Snicker.......