Geoffrey

When I built my car I installed the hydraulic brake system from a 993 C4S with a GT2 EVO racing ABS control unit. This required me to rewire the system, but that is another story. While the brake system is good, it has never given me a consistent pedal feel and I have always felt it was overboosted. Here is some background

The 964 C4, C2 Turbo, 993 C4, C4S, Turbo all utilize the hydraulic brake booster with a power boost ratio of 4.8

The 964 RS, Cup, RS 3.8, RSR, 993 RS, RSR, GT2 all utilize the same hydraulic brake booster, but the power boost ratio is 3.6

For comparison, the vacuum assisted brake booster ratio is 3.0.

The 964 hydraulic master cylinders are 23mm and the 964RS, RS 3.8, RSR, and all 993 use a 25mm master cylinder.

I won't bore you with all of the different part numbers, and there are inconsistencies with some of the racing documentation.

After searching for nearly 3 years I found the proper hydraulic for the racing application with the 3.6 ratio. Changing it requires removal of the fuel cell and it is messy in that it will leak all of the fluid from the boost section of the reservior. I am hoping that this will provide for a better pedal feel and completes the brake conversion to all racing components.

Turbohead

Hi Geoffry
I have these parts if you need them and also the ABS brain
all from my 94 3.6T.

BTW I did put a 993 Master cyl in my track car which had the the Big Reds from the 3.6T as you suggested ... And you were right . What a difference in pedal feel ,it is rock solid.
Thanks
Elliot

Geoffrey

Elliot, I'm glad it worked out for you, hopefully we'll see you this year. The ABS valve assembly you have there is a 3 channel version which works well on the street, not so well on the track. The hydraulic pump will work in any of the cars and looks like it is in good shape, much better than mine.

Bill Verburg

vacuum boosted systems
all 964 n/b n/a use 20.6mm m/c
all 993 C2 & C2S use 23.8mm m/c

electro-hyd boosted systems
964C4, 964t & 964tl use 23.8mm m/c
964RS, 964 3.8RS, RSR, 993tt, 993RS, 993GT2, 993GT2evo use 25.4mm m/c

So what did you do, pick up a 993RS(993.355.904.80) or GT2evo(993.355.025.80) booster?

Geoffrey

The 993RS booster is listed as 964.355.904.80 and like I said, there are part number inconsistencies in the different service and parts manuals. The GT2 part number appears to be for the master cylinder, adapter, and booster combination. The specifications across the .80 boosters are consistent. I am using the 964 part number.

Bill, all of the 993 hydraulic systems use the 25mm master cylinder too.

Bill Verburg

Yep, I mistyped that
Yes, the only 993 I didn't list was the C4S which is the same as 993tt

Great find

Voith

A bit of thread archeology..

What car did this booster come from and which parts (pump) do I need to make this work?