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I'm sure the booster ratios you speak of are in the booster part of the master cylinder, not the pump itself.
Darren
has anyone confirmed were in this unit is that ratio diffrence? has anyone take it a part totally for compare and how does it really work technical?
what ratio has the Jaguar, Rangerover, Ferrari unit?
Boost ratio is set by the master cylinder. Hydraulic system pressure is set by the pressure sender. The sender switches the pump on/off to maintain 160-180bar running pressure, and triggers a warning light if the system pressure goes below 102bar. The accumulator holds up to 50-80bar. If I remember, the PDAS system won't engage the AWD locks if the system pressure drops below 102bar in order to reserve all the pressure for brake boost in a system pump failure.
This pressurized brake boost system was used in lots of late-80s/early-90s makes/models. I think it was originally a WABCO (Westinghouse) design, then it was picked up by ATE/Teves. Porsche, Ferrari, Jag, Mercury, Lincoln, Saab, Audi, Cadillac, Rover to name a few. The pump bodies have inlet/outlet variations, but I imagine the pumps are generally the same as the system pressures should be similar, though the Jag pump is reported to have reversed polarity. I think the pumps all pull a max of 14 amps. Here are a few cross references:
So the text here about the two different ratios is wrong? it is only master cylinder size differens?
what is there differens on the part numbers then?
i have read about that special RS/RSR/ GT2 brakebooster but cant figure out what differens it has
i have been in contact with ATE and the say the same as you, the booster cant make any ratio difference its only master cylinder.
Originally Posted by 964George
I've read elsewhere on Rennlist that there is a difference in the hydraulic assistance boost ratio.
964: C4/Turbo 993: C4/C4S/Turbo all utilize the hydraulic brake booster with a power boost ratio of 4.8 964: RS/Cup/RS3.8/RSR 993: RS/RSR/GT2 all utilize the hydraulic brake booster with a power boost ratio of 3.6 964 C2 as a comparison the vacuum assisted brake booster ratio is 3.0
Apparently the 3.6 ratio is the sweet spot, but the 4.8 lists both the 964 C4 and the 993 Turbo, so this can't be the only difference or someone has the details wrong
Originally Posted by -nick
Boost ratio is set by the master cylinder. Hydraulic system pressure is set by the pressure sender. The sender switches the pump on/off to maintain 160-180bar running pressure, and triggers a warning light if the system pressure goes below 102bar. The accumulator holds up to 50-80bar. If I remember, the PDAS system won't engage the AWD locks if the system pressure drops below 102bar in order to reserve all the pressure for brake boost in a system pump failure.
This pressurized brake boost system was used in lots of late-80s/early-90s makes/models. I think it was originally a WABCO (Westinghouse) design, then it was picked up by ATE/Teves. Porsche, Ferrari, Jag, Mercury, Lincoln, Saab, Audi, Cadillac, Rover to name a few. The pump bodies have inlet/outlet variations, but I imagine the pumps are generally the same as the system pressures should be similar, though the Jag pump is reported to have reversed polarity. I think the pumps all pull a max of 14 amps. Here are a few cross references:
So the text here about the two different ratios is wrong? it is only master cylinder size differens?
what is there differens on the part numbers then?
i have read about that special RS/RSR/ GT2 brakebooster but cant figure out what differens it has
i have been in contact with ATE and the say the same as you, the booster cant make any ratio difference its only master cylinder.
AFAIK There were only 2 different m/c used on these cars
23.8 maybe used in C4 and definitely turbo or 25.4 used in RS and RSR There are references to a 20.6 used in the C4 but not in the parts books so I am unsure of that, It does makes sense from an Engineering point of view for C4 to have 20.6 or 23.8 as they had the same calipers as a C2
using the 25.4 w/ the same calipers only increases the amount of leg needed for a given line pressure compared to a smaller m/c
for a 964 C4 the boost ratio is 3.0 w/ a 20.6mm m/c according to the spec books, This makes sense as the C2 w/ the same brakes(though different boost method) also used a 3.0 boost ratio
a 964RS boost ratio was 3.6 w/ a 25.4mm m/c and larger caliper pistons
It makes sense to raise the boost ratio w/ a bigger m/c to keep leg effort more or less the same, though the bigger caliper pistons in the RS counter act that effect.
I have no info about the boost ratios in the 964RSR and though I have part #s for the 993 Cup/RSR parts but I have yet to find specs, these did use different parts from the regular or RS versions
AFAIK There were only 2 different m/c used on these cars
23.8 maybe used in C4 and definitely turbo or 25.4 used in RS and RSR There are references to a 20.6 used in the C4 but not in the parts books so I am unsure of that, It does makes sense from an Engineering point of view for C4 to have 20.6 or 23.8 as they had the same calipers as a C2
using the 25.4 w/ the same calipers only increases the amount of leg needed for a given line pressure compared to a smaller m/c
for a 964 C4 the boost ratio is 3.0 w/ a 20.6mm m/c according to the spec books, This makes sense as the C2 w/ the same brakes(though different boost method) also used a 3.0 boost ratio
a 964RS boost ratio was 3.6 w/ a 25.4mm m/c and larger caliper pistons
It makes sense to raise the boost ratio w/ a bigger m/c to keep leg effort more or less the same, though the bigger caliper pistons in the RS counter act that effect.
I have no info about the boost ratios in the 964RSR and though I have part #s for the 993 Cup/RSR parts but I have yet to find specs, these did use different parts from the regular or RS versions
Any one taken apart a brake booster and compared Turbo vs RS?
To see what is it what’s doing the ratio difference
pedal feel mostly comes from the slave/master ratio
993 is 37.342
993RS is 33.654
993tt is 33.896
993GT2 evo '97 and RSR is 27.826
964 4/2 is 48.525
964 4/4 is 46.159
964RS is 32.786
the lower the ratio the higher and harder w/ better modulation and the more leg required(all else being equal -that's where the boost ratio comes in)
I was also always under the impression that 23.8 or 25.4 were the only hyd boost m/c, but as Geoffrey says there are discrepancies in the factory documentation that claim a 20.6 for the 964 C4
the vac. boost systems are 20.6 , or 23.8 ,both Porsche factory units and I've recently found a 25.4 that was used by Alfa
I have to find the C4 unit I pulled from my car to replaced with the 25.4 I installed. IIRC it was marked 23.8 in a cast circle. The 964 C4/turbo/RS MC are very different design from the C2 RSA vacuum units so not interchangeable. What I found shocking is current pricing. These were $500 not long ago now they appear to be over $1k new. When I purchased mine from Porsche it came in a TRW box marked with a Porsche sticker. No signs of OE plates or markings on the unit itself. I would guess that it is safe to say the TRW for less will do the same.
BTW this is a 928 GTS pump. It is located behind the left rear bumper cover so if you get crunched there it will be costly. it only controls the PSD like the unit in the C4 and 959. I looked it over and can't find anything to say it is different other than the way it is fed and the reservoir. The pressure switch appears to be the same as well.
is there any were to purchase the 25mm mastercylinder for a better price than the $1000
is there aftermarket parts out there?
is there restauration kit if buy one used?
12-06-2023, 04:04 AM
