Renn 951

I've got an 88 951 without ABS and just had the S4 "Big Black" front brakes installed (the same front brakes on the Turbo S as I understand.) I didn't go with the "Big Reds" because I wanted to still be able to use my 16" track wheels. I haven't changed the brake proportioning valve yet, and to avoid too much "trial and error" I was wondering if anyone has experience with this configuration and can recommend the right replacement valve. If the car had ABS I'm sure if I got generally close the ABS system would take care of the rest, but I don't have that advantage. Anyone know if the 5/33 valve is the right upgrade from the stock 5/18, or if I need to go higher? Or do I need to change at all? And for the track, will I need to run a less aggressive (or stock?) pad compound in the rear if I have track pads on the front?

I've searched the archives and found some info on "Big Red" upgrades but I haven't found anything yet on the "Big Black" S4 brake upgrade. I suppose I could just install an adjustable proportioning valve, but if anyone has already found the "right" combo with a fixed valve I'd appreciate any recommendations to keep the remaining mods as simple as possible. Thanks!

David Floyd

"Big Blacks" are from a 928 GTS and need 17" wheels

S4's are from 88 S, 89 951 and 928 S4

5/33 is an upgrade that a lot of guy's use.

Russ Murphy

Yup. Got the same setup on my car. Don't overdo it with an even more rear-biased prop. valve than the 5/33. It's a bad thing when the rear lock up well before the fronts. Spinny spin spin.

Renn 951

Thanks, Dave, what I have are the S4 brakes. I saw the 5/33 mentioned a lot in discussions on "Big Reds", but most people also seem to have ABS and I'm wondering how much difference it will make and if anyone has direct experience with my configuration.

NZ951

I have the valve on a non ABS model. Much better braking. I have not had the chance to lock them up yet, but you can always take your foot off the brake when they do!

Mike Buck

John,
Did you see my post over on PT? I did the same upgrade over the winter, to an identical car as yours. Tried running a race pad front and rear, wasn't happy. WAY TOO MUCH rear lockup. PF97 Front pad and stock rear pad is the combination that finally worked for me.

David Floyd

I think the S4's are a very good upgrade, I like the idea of still being able to use 16" wheels and braking power is beyond what most of us can use.

Planning my brake upgrade now and I am leaning toward the S4's also.

NZ951

I thought it requred toooooo much adaptation to fit the S4's?

Sam Lin

Less than the Big Reds.

Sam

Renn 951

Mike:

Yes, I saw your other discussion thread and also discussed with Chris. Did you change your proportioning valve at all? Chris said it could take some experimenting to find the best combo of valve, pads, etc., and I'm just trying to see if anyone has already figured out the solution. It would be good to find a setup that would allow track pads front AND back for the track, and hopefully still keep things mostly balanced for street driving with street pads and tires.

Maybe the only way to do that without ABS is an adjustable valve, but that adds more complexity and $$$ to install. If someone has a simpler solution figured out I'm willing to give it a try!

NZ951

Oh, even for non ABS models Sam?

Renn 951

I had to buy a $300 adapter kit, but it wasn't that big a deal. After all, these are the same brakes that go on the 951-S.

smokey

I have the S4's with the 33 proportioning valve. I had also tried the original '86 front brakes on the back with the 33, but got some rear end lock-up at slow speeds, so I went back to the original rears. Plus the pedal travel was longer due to the larger pistons on the fronts at the back. I switched to the later master cylinder, which has a larger piston for the back brakes, so I have a firmer pedal now anyway. I was getting a lot of fade at Watkins Glen with the original (S4, 33, original back brakes) set-up, so I went from Hawk SP+ to Hawk Blues. The previous year, I didn't get fade with the front brakes at the back, but I was slower, so who knows? I know I had more overall brake with the front brakes at the back, but on threshold braking, the "chirps" would be at the back.
At this point, I'm not sure that my rear brakes are doing enough work, so I'm thinking of trying out the next higher proportioning valve, which is used on 964's with the same caliper/rotor set up. If I do, I'll post the result. As you can see, I don't have a definitive answer. For my first track event this year, I'll leave it as it is, because the consensus seems to be that what I have is the right set-up. Spring rates probably make a difference as well.

jerome951

Hey JG,

When I switched to the 5/33 prop valve on my TurboS I did notice better braking but my fronts still chirp from ABS lock up before the rears. That's using PF97s all around and a bone stock suspension which takes quite a bit of weight off the rear under braking.

I'd say try the 5/33 and bring your old one with you to the track (or try threshhold braking on the street). They are easy enough to swap at the track if necessary.

Renn 951

Jerome:

Thanks, I guess there's so many variables here that I'll have to do some trial and error. Chris wanted to keep the car a while longer to try to sort out the balance, but for a number of reasons I ended up having to pick it up Tuesday night. In the short test drive he did he said the rears were locking and he thought I'd probably have to run with the stock pads in the back with PFC 97's on the front. The car still has Hawk Blues on the back right now since I didn't change them after the SP event. When I tested the brakes on the way home I was quite surprised to see sparks flying up in the air from the front wheels in the dark! I figured it was just the paint coming off the rotors with the new pads bedding in, but it looked pretty spectacular in the dark! When I really stomped on them I got a little wiggle and chirp from the rear.

I'll play around some more with it over the weekend both with the Hawk Blues and with street pads and see how it does. It just seems to me that if the rears are locking up early then the last thing I need is to go with the 5/33 valve, because that will just put more bite in the rears.