RCinXS

Previously, I upgraded my suspension and Brakes to 1991 S4.

I did not put in the ABS system as I hate the ABS on my other car.

I'm thinking about changing my Master Cylinder and Booster to the later model as well, while I have my Cam Towers off. Plenty of room at that point.

I'm told the Booster is the same and the Master will bolt in with no problem but has a slightly shorter stroke.

What I am wondering is if I should change the Brake Bias Regulator to the later model.

My 84 has a pressure switchover of 33 Bar. The later one has a swichover pressure of 18 Bar.

Any thoughts on the effects of going to the lower swichover pressure are appreciated.

RCinXS

Bump

the flyin' scotsman

With a stock S4 brake system I went with PBR ceramic pads, cast in hole rotors and changed the bias regulator to 33 bar.............like hitting a brick wall with little nose dive.

The Bilstein/Eibach suspension and Pilot Sport tyres all add to the experience

RCinXS

So, just keep the the 33 Bar I already have.

I would be disapointed in the 18 Bar from the S4?

Scotsman, When you changed did the rears on your car lock up sooner or later than with the stock 18 Bar?

the flyin' scotsman

The rears never lock with either bias regulator although the rear rotors do get warmer with the 33. I checked temps with a IR gun when first installed to confirm the seat of the pants feeling of squat vs nose dive.

There is a 54 bar regulator used on the older 928s but I read that rear lock up is highly likely.

Jim M.

Porsche went to the 18 bar valve for the USA cars only, IIRC the ROW S4's had the 33 bar valve. It was driven by the fact US drivers want to get a lawyer involved when their driving skill were not up to the cars capability. Most high performance drivers here have switched to a higher bar valve in our 87-95 928's.

The valve restricts the pressure to a max of 18 bar (261 lbs/sq.") to the rear brakes. This makes the car understeer rather than oversteer in high braking force events.

With both my 928's I experienced ABS intervention on the front wheels with heavy braking using the 18 bar valves. In both cases I switched to the 55 bar valve. The ABS almost never engages, nose dive is greatly reduced and braking is more controlled. The rear brakes are now adding more to the overall braking effort. When I added the 993TT rear brakes I found it moved the bias even more to the rear (rears could lock up first) and I went down to the 45 bar valve. Now, my front brake pads only wear out twice as fast as the rears, instead of 4 or 5 times faster. Again because the rears are doing more work than with the 18 bar valve.

RCinXS

You guys are great!

Basically, I should just leave my 33 Bar installed.

Now I wonder if I should really change out the Master Cylinder.

When I got the 91 suspension I also received Master and Booster, etc.

The wrecked 91 only had 53 K on it, so I figure the MC and Booster are in much better shape than my 84. I've never replaced them. I've had the car since 1985.

Bore and stroke of the 91 parts are only slightly different.

84 S:
Bore Size: 23/19mm
RR Port: 10x1 BBL
Frt Up Port: 6x1 BBL
Push Rod Depth: 1.83 inches

91 GT:
Bore Size: 23/20mm
RR Port: 10x1 BBL
Frt Up Port: 6x1 BBL
Push Rod Depth: 1.84 inches

PET shows that in early 32 V cars that ABS was only an option. MC from 1985 - 1995 was the same. Brake lines for 85 without ABS are identical to 84. Booster is identical to 84.

The 91 MC should bolt right in.

Biggest reason for consideration of the swap is my winter front and top reseal / WP/ TB, etc.

Lots of room to do the MC and Booster at this point.

Anybody done this sort of swap before??

Thanks again for all the input!

the flyin' scotsman

CDN cars got the 18 bar valve also probably because they were included in the North American cars not for the number of lawyers.

Amazing what the car companies did to sell more cars rather than improve the drivers made the cars more idiot proof

RCinXS

That's why I didn't put the in the ABS. Takes the fun out of it!

Car makers realized this in more modern cars, where you can at least disable the traction control.

tveltman

I'll chime in here too. I did the swap to the 33 from the 18 in my '88. Way less nose dive, it's nice. Definitely keep the 33

mark kibort

ive raced with both. on and off. there is no difference , unless you dont catch the lockup, then abs can help. its like a safety net. it only works when you didnt.
however, its not useful for stopping unless you are on ice or snow, its very slow compared to real racing abs

mk

123quattro

ABS is very nice. It's just that the ABS on 928s is 25 year old technology. The new stuff is SO much better.

the flyin' scotsman

Very little diff between the two.

I haven't done the swap but I'd be thinking if the 84 MC is working well with regular fluid flushes leave well alone.

The 91 system is an unknown even with the low miles its been sitting for years which is worse for any car system. If this fails and you've everything back together what a **** off that'd be.

Jim M.

The 91 system is an unknown even with the low miles its been sitting for years which is worse for any car system. If this fails and you've everything back together what a **** off that'd be.
__________________
Malcolm


+928

RCinXS

All good points.

Maybe I should rebuid it and put it on the shelf for future use.

Are there rebuild kits available for these Master Cylinders?