svp928

There has been some discussion here recently regarding big brakes on 85-86 cars, and I've had a set waiting to go on my car for an eternity- So, I thought I'd share the experience with anyone who may be interested...

Front:
13" dia, 1.280" thick Coleman rotors on aluminum hats
Brembo calipers from an F-40 Ferrari
4130 steel adapter bracket for 85-86 spindles

Rear:
S-4 4- piston rear calipers
S-4 std. vented rotors

The front calipers are dimensionally similar to S-4 fronts, but they differ in having titanium pistons, internal fluid porting ( no crossover tube), and are cut for 13'' rotors. The rotors themselves are std Colemans with a 12 bolts on a 7" bolt circle. We machined the gas slots and opened up the inside diameter here. All the rotors were cryo-treated to improve wear characteristics. Bolts are from Coast Fab, ANS-6300 high- temperature with 9/16 grip length, using 6-point Jet-nuts. The hats are 2024-T651 aluminum, hard anodized. The brackets were originally intended to adapt S-4 calipers and rotors to an 85 spindle, and required little modification for the 13's- mostly just needed spacers under the calipers to move them out far enought to go over the rotor.

Calipers were stripped of paint for cooling purposes, so they are ugly. The grey stuff in the rotors is cold-zinc for rust proofing. Hats and brackets were made in my shop. This is obviously not a concours setup.

The hat-rotor-bracket combination clears the lower ball joint by at least .125" with the suspension compressed.

These (front) parts weigh 3 lbs less, per side, than the parts I took off- 26 lb vs 29, and about 2 lbs less for each rear. Total about 10 lbs less unsprung weight. This is good...ceramics would be better...too bad.

Installation was very simple- Unbolt the stock stuff, bolt on the new stuff. In front, the stock backing plate has to come off, so might as well repack the front wheel bearings. New stainless lines were used, made for an S-4. No other changes were needed. Same for the rears, the S-4 rotors are larger than 85, so the backing plate must come off, and my car needed a 2mm shim btween the caliper and rear upright to center the caliper over the rotor. Otherwise, straight across swap.

Had to make a dummy master cylinder cap with a hose barb in it to use a power bleeder, and then only pump it up to about 5 lbs, because I was worried that it would blow the hose to the clutch master off if higher pressure was used. I plan to put in a seperate reservoir for the clutch in the future.
Net result was a brake pedal that felt like it was just bolted solid to the firewall- this car has never had that hard of a pedal feel. Amazing.
After bedding in the new pads, the power and feel of these brakes is vastly superior to the stock setup. A much gentler touch on the pedal and lots of feel for modulation, where the old ones were a big stomp and hope for the best.. Modulation was how many pumps before the big stomp. If you stomp these, it locks up instantly, and you are rewarded with smoke and flat spotted tires. (Dammit!) So now, I need to use some of the "squeeze" technique taught in most racing schools. Right now, the fronts are overpowering the rears, so some adjustment to bias will be needed to make most use of both ends Can't wait to get on the track and see how they work on the clock- for now, I wil be happy with how good it feels drive a set of brakes that match the rest of the car's performance.

Sorry for the lousy pics- I had to make them small to go on here.

Steve

Jon B.

Very nice! I was hoping to do the same type of upgrade when I had my car. I just would have done without the slots. I'd love to see some full on side shots of the car if you had them. I could use some good looking older 928 pictures on my computer.

Jon

928SS

nice work! I was going to warn you about the ball joint clearance, but yours made it fine. did you mess w/the brake bias valve yet? iirc, you can put in a 55bar valve that really helps the rears get into the game and it really helps the fronts/gives a more balanced feel w/the bigger units up front...

svp928

I'll see if i can dig one up-- right now, I'm trying to figure out how to edit the post- the last photo is not the one I wanted. And it would be nice to put captions...

Jon B.

I thought that all the OB's had the 55bar valve on the car from the factory?

Jon

Peter F

Looks great, and probably feels great too

/Peter

svp928

928SS- If I read it right, I have 33bar valve now, and when talking with mark A., he suggested trying NO valve at all. I think these work like a pop-off valve, in that they don't allow pressure into the line till till it sees more than 33 bar, so, if that is the case, right now my fronts are seeing 33bar before the rears even come on. That's way too much front bias, so I think I'd want make that threshold lower, so a smaller number valve would be in order, hence trying it with no valve at all.
It's free, so I'll give that a go and see what it does for me...flat spots are already there, so what the heck! Maybe I can flat the rears so they match...

BC

Looks like the Big Blacks would also work in this situation - do you agree?

Very nice. Now we have to get the dimensions of that bracket.

svp928

I'd have to get my hands on a Big Black or Red one to see, but I think it would work...

928SS

my bad - I guess I put in the 33 bar - had the 55 bar... friggin holiday hangover that'll last till 4th of july... haven't thought about removing it entirely though/would've been cheaper, ha!

svp928

928SS- ya had me goin there for while...LOL!

BC

The UP/DOWN spacing may change, but the bolt spacing is the same.

RDS928S

Steve,
nice job.
Can you give us more detail on the adapter bracket for the 85/86 spindle?.
I assume you kept your ABS function by keeping the original spindle?.

I would like to find an adapter bracket for the S4 caliper and 993 rotor to fit the 85/86 spindle. I lost my ABS function when switching to the 78-81 style spindle.

Thanks Rich

svp928

Yes, then it would probably work. As long as the caliper isn't big enough to hit the steering arm below it. IIRC, that caliper is not much longer, but taller radially, and takes a pad that is also radially larger? Should work..

Rick Carter

http://www.nichols.nu/tip481.htm

Brakes


Increasing Rear Bias

In this month's Pano there is an article by Kim John Crumb on brakes. Part of the article describes a modification in which the rear brake pressure regulator from late model Porsches can be replaced with ones from earlier years to boost rear brake pressure (ABS will continue to function). He has replaced the regulator in his GT (18 bar of pressure) with one allowing much higher pressure (55 bar), greatly increasing rear braking. I was considering doing this to my GTS, replacing the regulator (18 bar) with one from an early '86 (33 bar). The intent is to increase overall braking abliity, decrease wear and heat from the front brakes, and decrease front nosedive under heavy braking. Has anyone out there with a GTS tried this mod. at the track?

---------

I did the 55 bar modification two years ago. I liked it so much that it's now on the GT. Regarding the benefits real or imagined here is what I have experienced. Mind you, I am not a dedicated track junkie, so my experiences are limited to about a dozen or so track events and about 100 autocross events.

The increased rear bias regulator works better that the 18 bar regulator in that it allows the rear calipers to contribute more toward the overall braking effort. This has the effect of reducing the overall braking contribution from the front calipers. The car is still braking at maximum efficiency, but it's doing so with more poise and balance. I find that the brakes have less fade than before. Best of all, the front pads and rotors are wearing twice as fast as the rears, where before they were wearing three or four times faster. YMMV.

Caution, the increased bias works well with ABS 928s. I do not recommend that you try this in a non-ABS car. The last thing I or anyone else would want is for the rear wheels to lock-up first. Such an occurance could easily lead to an oversteering condition that can then result in loss of control. (Gosh, I hate when that happens.)

Good luck and merry motoring, Ed.


Why did Porsche change the valve from a 55bar to an 18bar in the first place? This doesn't seem in line with Porsche's continuous improvement approach. Their must of been a reason for the change.

John Veninger
1990 928GT


Actually, the 928 series started out with a 33 Bar regulator. (Those of you with very early models, please correct me as I'm not absolutely certain that thy even had rear brake regulators.) When the S4 brakes were added to the 86.5 MY, the regulator was changed to 18 bar. The 45 bar and 55 bar regulators are from the 911 & 930 series. (I'm not absolutely sure of this, so YMMV.)

Why Porsche reduced the rear brake bias probably has to do with safety and avoiding or reducing law suits. Having too much rear brake may cause them to lock-up first. This could easily lead to an oversteering condition.

It is much safer to drive a car with lots of understeer. However, it's also less fun. Correcting an understeering condition is very easy. To correct, whatever the driver is doing to cause the condition, he/she should do less of it. If the steering is turned too sharply, turn it less. If the brakes are applied too severely, ease off the brakes, etc.

Correcting an oversteering condition is far more complicated, and usually not very intuitive. For example, you may have heard that you should not brake in a curve, particularly if your car is an older 911. The main reason has to do with weight transfer. When braking the weight shifts forward and adds more load to the front tires and less load to the rear tires. If the car is also in a turn, it is possible (and in older 911s, very-likely) to have a very small rear tire contact patch. With the engine hanging beyond the rear axle, older 911s will behave just like a dart that is thrown with the feather end facing forward. As you can guess, the heavy end will want to go first. 928s are more evenly balanced than 911s, but they can be induced to oversteer, especially in a turn while braking or with a sudden lift of the throttle. To keep the back end from coming around it is important that the rear tires continue to maintain maximum contact patch. If the rear seems to be coming loose, the correction may be a combination of moves to keep it from coming around. Usually, some counter-steer and some light throttle application will keep the rear tires from losing contact and thus keep the backend from coming all the way around. (With major capitol letters; YMMV!) Moreover, quick reactions are paramount if a spin is to be avoided. That's where practice, and lots of it, helps make one better prepared for such a condition.

Merry motoring, Ed.