Jon Moeller

Jolly,
ABS does engage much more easily in the wet, due to the decrease in traction.

Braking systems are setup with bias to the front in almost all cars (dirt track cars run larger rear brakes for some reason), this bias is based on the knowledge that under braking, weight transfers to the front of the car, loading the front tires and providing them with a larger percentage of the grip. Variances in traction and the angle of the surface that the car is on (uphill, downhill) have a direct impact on how much weight transfer occurs. This is why I ended up with rear lockup going into turn 1 at WGI, but no lockup going into the toe (the first is downhill, the second is uphill).

You can think of braking in the wet as braking on an uphill, since all four tires have less purchase on the road, less total weight transfer occurs, thus a lower percentage of traction is gained by the front tires and lost by the rears when compared to braking in the dry.

I hope that makes sense. Honestly, I'm glad that I ran LRP in the wet and not in the dry with that braking setup, as the downhill would have been pretty exciting with all that weight shifted to the front wheels, and too much rear bias. (Yes, I brake for the downhill )

-Jon

Alpine951

Now you guys have me concerned that I may have made a bad decision in putting the 928 valve in my 86 951. I just put in 300# springs up front and 28mm tbars in the back and koni's all around with the stock rotors/calipers and Hawk blues for the track. I have not put in the racing pads yet but maybe I need to go to the seldom used on the weekend long straight in the industrial park that I use to bed the pads to check out the rear lockup.

Oddjob

Alpine,

I dont think it will be that bad of a decision. Look at it more like a test. Try it, you might like it, or you might not. I have a 33bar in my 87T and I can still lock up the front first. The car is a little more rear end happy when trail braking, but Im not sure that isnt due to the toe/alignment settings.

TheRealLefty

You're OK, man. These are not "totaly lose your cookies" kinda distinctions, IMO. I've heard/read in a couple places that the difference between the 18/5 and the 33/5 proportioning valves is something around 8% more grip to the rear during a braking manuever.

If you swap valves, there is a very short period of time between 18 bar and 33 bar in braking pressure in which the rears are now going to get 100% of the braking pressure, where they would previously been receiving 50%. Below 18 and above 33, your front/rear balance will be the same with either valve as I understand it.

I really like the 33/5 valve because it seems to help fight dive. Compared to the OE 18/5, my 89 Turbo with the 33/5 really felt like the tail was squatting down during braking. We all know that defies physics, but it does accurately explain the difference in feel from the driver's seat.

Since the difference is in the middle of the braking process, there is not a huge risk of early or late rear breakaway since the bias is unchanged at the lowest and the highest brake pressures.

Renn 951

I don't believe that is correct. What would be the point of putting all the braking pressure on the rear between 18 and 33 bar?

My understanding is that with a 5/18 valve, the front and rear brakes are getting the same hydraulic pressure up to 18 bar. Above 18 bar the rear pressure increases at 50% of the rate of the front pressure. With the 5/33 valve, the front and rear hold the same pressure up to 33 bar, after which the rear increases at 50% the rate of the front. The two-stage valve is compensating for the weight shift to the front under heavier braking forces.

My car has the the 5/33 valve, and bigger Turbo S (or S4) brakes up front and SS lines front and back and doesn't have ABS. I also run 255/40/17 tires front and back on the track, and I have 500# springs in the front. I don't have any problems with the rears locking up before the fronts as long as the front pads have been bedded in. I use PFC 97 pads all around, and with a new set up front but a bedded set of track pads in the back I have to be careful in the first run session of a DE or the rears will start to lock up, but once the fronts are bedded in the braking is just fine.

ninefiveone

You guys are saying the same thing. Just worded differently

TheRealLefty

Yeppers, sorry for my poor syntax, Renn. You would think that English was not my native language What I MEANT to say was the same thing as you have written more clearly. The 5/18 clips the pressure to the rear brakes by 50% at 18 bar. An owner who switches to a 5/33 is simply allowing 100% of the braking pressure through to the rears for a short period of time between 18 bar and 33 bar in brake pressure...after which the pressure is clipped to 50% going to the rears just like the 5/18. Right?

TheRealLefty

PS Everything I post about performance driving must be considered against my DE instructor's most recent report card He kindly gave me an "A" in Car Control and in Passing, but only issued a "C" for Threshold Braking and Following The Racing Line. All the rest were gentlemen's "B's".

We jokingly shared that if I was good at the first two, the second two didn't matter all that much...but truth be told there is a big difference between autocrossing and track driving that requires some different skills, most significant of which is the use of the brakes effectively and safely at much higher speeds. At first, I really enjoyed having more time between manveuvers during track events but as soon as I started trying to go a little faster, it became apparent that decelerating at track speeds is a true art form outside of the autoX envelope.

Renn 951

OK, Lefty, now I understand what you meant to say! We are on the same page on this after all!

RKD in OKC

I put the higher rear bias on my 88 Turbo S because my driving style was cooking the front brakes. After installing the 5/33 I was able to hold the brakes a little farther into the corner and ease off the brakes as I turned in. The rear would come loose and rotate the car as I turned in with the higher bias to immediately put the car in a 4 wheel drift on turn in. It took some practice to get the timing right, but it made for much faster corners once I got the hang of it. The effect was the same for both autocross and track, but the timing was a bit different.

Under high speed braking I did have to keep on my toes on initial brake applicaton because the rear was a little floaty, but they did not lock up.

This was with stock S brakes and the same brand/type of pads all the way round.

RKD in OKC

Oh yeah, I did the same thing on my 90 928 GT. It had the 5/18. I upgraded to the 5/33 and it wasn't enough so I upgraded again to a 5/55.

TheRealLefty

Final thought on a useful thread. Many of the posters here make the same point in many different ways about the "perfect setup" for a 951. There is no such thing. There is no single "best" front brake spec, or sway bar combo, or brake bias setting, or brake pad, or shock absorber or spring rate.

There are however, a rainbow of factory developed parts available NOS or salvage, and a robust aftermarket to pick through to get your 951 where you want it to be for street or competition performance. The trick is to evolve your own understanding of the art and science of performance driving and performance handling...they go together, ya know. The better driver you become, the better chassis tuner you become.

I've seen more than a couple of 951's that have a large investment in added power and suspension bits that don't really perform very well when pushed. But, rarely have I seen a 951 in the hands of great driver that didn't drive great.

Alpine951

Just want to respond after my first track event this year. I spent Monday and Tuesday at New Hampshire Int'l Speedway. NHIS is tough on brakes and tires. You are on the brakes a lot. I use Hawk Blues and for the first time I used the cheap Ford heavy duty truck hydraulic fluid. My calipers and rotors are stock 86's. I was concerned that my car was going to be very tail happy with the 928 valve based on the experiences expressed in this thread. I figured I would take it easy the first day just to get re-aclimated to the track since it was my first event this year. I am signed off to drive solo in Blue. I just put 300# springs up front with Konis and adjustable ride height kit, plus 28mm tbars in the back with Konis. Recent Toyo TS1 on the car for track only. I locked up the rears a few times during the first session of the day but was able to modulate the brake pedal so I was in control. By the second session the idea of taking it easy during the first day was thrown out the window! It was just too much fun. I was actually able to apply less brake initially and the car was more level, braked well, and was never out of control when braking. What a blast! I actually like it. It is much more balanced.

APKhaos

I'm firmly in the 'tuning required' school.

With the 5/33 in my stiffly sprung 89, and with Hawk Blue pads front and rear, the rear end was quite unstable under threshold braking. Going to slightly softer pads in the rear cured this problem. With this tweak, braking is stable under all track conditions.

As others have said, this is unlikely to add much value as a street mod. If you rears are locking under street conditions, look elsewhere for the problem before making this change. [IMHO, YMMV, etc.]

Oddjob

Ran my F Class 87T a couple weeks ago at a Club Race. Had some more toe-in put in the rear (0.15 deg per side) and the car was very stable under heavy braking w/ the 33 bar regulator. This was with Pagid Orange pads all around. It would still chirp the fronts before the rears, and fine under trail braking. Had no problem staying with E-class 964s in the brake zones and did not lose much ground if any to E-class Turbos, so I was braking very hard.

Im surprised, I would have thought your Turbo S (with the greater front brake bias due to the 928S4 calipers) should not have much of a balance problem at all running the 33 bar valve.