Swapping proportioning valve for "T" fitting... Options

[Wanna9146]

I read on this forum recently about someone swapping out their proportioning valve for a "T" fitting.

Does anyone know the size/thread count of the correct "T" fitting? Any fabrication required?

[Eric_Shea]

Does anyone know a sucessful setup for up a system like mine (with much larger calipers in front) short of a front proportioning valve (I don't like that idea) or dual MC / balance bar setup?

You have what are called "Wide A-Calipers" up front. This is a standard A-Caliper (very similar to what Jon has as well) with a wider spacer to accommodate the 24mm wide rotor. The standard A-Caliper that Jon has rests on the same 20mm 911 rotor that the M and S-Calipers went on. I reference Jon because he was asking a similar question over the phone a few minutes ago. While I personally think the A-Calipers are heavy, I like the 20mm vented rotor for many reasons.

I'm not a fan of the Carrera system because I think it is too heavy for a 914 as there's generally too much rotating mass (24mm rotors) for such a light car BUT... that being said and the fact that you have the system and 914-6 rear calipers; let's review.

The "bias" is generally set from the factory with piston and pad sizes. This is something that you've fallen off a bit by installing the Carrera calipers up front. The Carrera was the first car to get a larger 42mm piston in the rear, yet it was also the first 911 with a p-valve. The p-valve in a 914 will simply act as a safety feature under extreme braking. By turning the screw counter-clockwise you will take some preload off the regulator spring. This means your rear brakes will be engaged longer in the braking cycle before being shut off (not engaged) -or- you'll have to push harder on the pedal to get it to shut down (stop from braking) the rears. Ideally, you'll want your rear calipers to come loose as soon as the fronts shut down or lock. You never want your rears to lock on a 914. They should ride that ragged edge for best braking.

Because A, S and M-Calipers with their 48mm pistons have been matched up with the rear M-Caliper with it's 38mm pistons on all 911's from 1969 through 1983 I still think you're OK with the brake bias that you have (your rear 914-6 calipers have the 38mm pistons). You just have huge front rotors. Depending on the master cylinder you have, you may want to back out the regulator spring adjuster however... the factory manual shows a lower changeover pressure of 525 psi for a 914-6 (vs. 685 psi for the 914-4). Again, they are comparing 19mm MC vs. 17mm MC. The 914-6 has 48mm pistons up front and 38mm pistons in the rear. The main difference with your set-up is the pad size on the (wide) A-Caliper; it is larger than the 914-6 front M-Caliper. So again... you may want to back the regulator spring adjuster out a bit to compensate.

This is tricky stuff and these are your brakes we're talking about, approach with knowledge and caution. I would have a friend with a video camera set up in an abandoned parking lot. Set up a panic stop staging area and let the video roll. Have a 5mm hex and a 13mm socket available to adjust the valve so the rears are never locked when the fronts are. You will have to break off the JB Weld type sealer the factory has around the adjustment rod to get everything to move (it is very brittle now and usually chips right off). I would back it out one turn at a time until the rears begin to lock. When they lock, screw it in 1/4 turn at a time until they no longer lock up. Once you have it adjusted, JB Weld it back in place and LEAVE IT ALONE.

E.

P.S. Fasteners and spacers from a 911 rear M-Caliper will make your 914-6 rear calipers 914-6/GT calipers. Vented rotors all around. Might be a nice upgrade. Remember... heavier rotating mass again (slight though).