HODY905

Does anyone know if it is possible to install a hydraulic brake boost on a RS America similar to the one on a 964 C4/C2T/RS models?

Cupcar

It is possible, several brackets have to be welded inside the trunk to support the hardware and wiring harnesses have to be changed along with most brake lines and the brake fluid reservoir.

An ambitious, very expensive project for sure.

garrett376

That would be a neat project. Fortutantely the system is mechanically controlled, so there's no interface with the DME or other systems. You could get all the plumbing, accumulator, pump, MC and booster from a wrecked C4, plug off the pressure line from the accumulator to the solenoids (since you won't have those) and you'd be set!

Fun project indeed!

Rich W

Is there some particular advantage to this modification?

Cupcar

Main advantage is it allows the fitting of a correct 25.4 mm size master cylinder for RS/ Carrera Cup brakes. Also, since the brake boost is no longer dependent on engine vacuum, pedal feel is more consistent.

One has to keep in mind though that the C4/Turbo master cylinder one would find used at a salvage yard is not the same as the RS master cylinder and it is around $1500 to buy new. The C4 master cylinder is the same as a 993 at 23.81 mm, so an advantage since it also is fitted by Porsche to Turbo/RS size brakes. Normal vacuum boost C2 has a 20.64 mm master cylinder.

Rich W

Thanks for the clarification.

Bill Gregory

And we know there are several Rennlisters who either completed, or are embarking on, fitting the 993 MC into C2 models. We know that we need the MC, vacuum cannister, possibly MC bracket, and 2 line adaptors.

garrett376

Maybe the catalog is wrong, but performance products says they've got the C4 MC for $461!

Cupcar

That's the C4 standard M/C in 23.81 mm. As far as I know, Performance Products doesn't sell the Euro RS stuff, but it is available at a Porsche dealer since it came on the American Cup car.

The RS parts are a little more money you need both the 25.4 mm master cylinder and the RS brake servo parts which are different than C4. Servo is: 964.355.904.80 at $1767.21 and the master cylinder is: 964.355.903.80 at $1031.24= $2798.45, list price, up from $1500 last time I checked. Discounts probably available.

Bill Verburg

The 993 C4 and 993 C4S and 993tt all use the larger 25.4mm hyd boosted m/c. It's only advantage over the 23.8 is the ergonomics of less pedal travel, this makes the brakes far easier to modulate

Cupcar

Also the larger cylinder, since it has a lesser hydraulic ratio advantage requires greater pedal pressure for any given rate of deceleration. The driver has to push harder to to the same amount of work to stop the car. The amount of extra force would be the ratio of the areas or 1.139= 13.9% more force for a given deceleration with the larger cylinder.

The RS servo also has a lesser boost ratio at 3.6:1 instead of the C4's 4.8:1, a difference of 33%. RS/Cup brakes require substantially greater pedal force for any given deceleration.

Bill- Does the 993 C4 have the RS servo as well as the larger master cylinder?

Bill Verburg

Yes, it is not the same as the RS unit though. The difference being the boost coefficient. RS has a boost coefficient of 3.6, 993 C4 is 4.8.

The 99RS will need more leg for the same brake torque as the 993C4

the 993tt, and C4s do use the RS parameters