Thinking Brake Upgrade

[Bill Verburg]

Another thought for Mike, several of the CVR guys run a 23.8mm m/c even w/ the stock brakes. The reason is that the pedal goes from 35.328 to 26.427, It makes it much harder to over use the brakes

[VFR750]

930 brakes = cubic dollars

Which I don’t have. 😉 but I agree would be very nice

Interesting thought on bigger m/c.

[911tracker85]

Not sure that I understand the point here
for an '85 w/ Boxster front and stock rear the bias is 1.554, right about where you want to be w/o a p/v, if the stock p/v is retained you are losing out on the rear under most braking conditions, so yes if you installed an adjustable one then you would want to run it wide open.

the point is it had the stock PV. I would have just removed the stock PV if I knew what the results would be. sold that car last year.

944 has very different brake needs from a 911

indeed.

930 is what Mike(VFR750) ought to be shooting for.

my project is a 78 SC/930 conversion. the PO only did a full metal widebody slantnose and stuff a built 3.3 turbo motor in. old 915 almost destroyed, and stock SC brakes and suspension.

for my project, it had to be 930 brakes, but not going floating front rotors.

If you haven't already you will want a 23.8mm m/c

after installing the boxster front calipers, I upgraded to a turbo MC. the extra fluid the boxster calipers needed made the brake pedal travel way longer than I was conformable with using the stock MC on the track. jump on the brakes and you were not sure it would not just go the the floor. putting on the turbo MC, short firm pedal, but a bit hard to modulate at threshold. retired it from track and reverted to stock calipers and MC before selling.

so now have a good turbo MC in the box and just finished cleaning the brake booster to repaint.

[LJ851]

An inline proportioning valve (like a wilwood) is just a delay valve, it doesn't permanently affect the bias.

Are the stock porsche units different in a way that can alter brake bias front to rear in steady state braking ?

[Bill Verburg]

Originally Posted by LJ851

An inline proportioning valve (like a wilwood) is just a delay valve, it doesn't permanently affect the bias.

Are the stock porsche units different in a way that can alter brake bias front to rear in steady state braking ?

While there is a delay, which is why we don't like them, that isn't how they work
Here is a p/v, the spring allows unrestricted pressure below a line pressure value called the knee, above the knee line pressure is reduced by a fixed %, resulting is a curve like this


This is what a 3.2 Carrera p/ does, it's spec is 33bar .46


A more general description of what is desirable




There is a lot going on in there

[VFR750]

Non trivial difference between stock 20mm SC rotor (20.35mm measured) and a 24mm Carrera rotor.


Full rings are slightly thicker and the cooling passage is ~2.5mm wider.

Both are Zimmerman coated.

[VFR750]

Bill,

if there is an “ideal” curve, that implies an increasing bias front to rear under hard braking.

For cars without a proportion valve why is it undesirable to have more that 1.5:1 bias?

Do you think the presence of ABS change the answer because the system will correct for excess bias?

Just thinking about the curves you posted

Also, it would make some sense that a hysteresis loop occurs due to compressibility of the brake fluid and the elastic expansion of the flexible brake lines.

[Bill Verburg]

Originally Posted by VFR750

Bill,

if there is an “ideal” curve, that implies an increasing bias front to rear under hard braking.

For cars without a proportion valve why is it undesirable to have more that 1.5:1 bias?

Do you think the presence of ABS change the answer because the system will correct for excess bias?

Just thinking about the curves you posted

Also, it would make some sense that a hysteresis loop occurs due to compressibility of the brake fluid and the elastic expansion of the flexible brake lines.

It depends, breaking from speed to zero is a different animal from braking from 130 to 75 for big bend

So sticking to track performance there is an initial big surge in normal force at the front, this is partly aero and partly weight transfer, as you get to the end of the heavy braking zone you are trail braking into the corner, weight goes back to the rear but also is transferred to the outside wheels, then you get back on the gas and the weight goes to the rear wheels.

During all this line pressure is varying based on driver input and feed back through the hydraulics and seat of the pants.

No factory race car uses a p/v, this is something used as a crutch for inexperienced street drivers, the curve above is for that purpose.

The 1.5 bias # comes from decades of experience by 911 racers stock 911 torque bias though 1983 911SC was always 1.491, there are some guys that have gone as for as close to 1:1, I wouldn't recommend it, but it does get a lot more out of the rear as long as there isn't any lockup, I use 993 RS brakes on both my 911 and 993, brake torque bias is 1.426, these are the best stopping brakes I've ever used, heat is not an issue even in hour long runs at WGI.

The reason you want the rear to do more work is primarily heat, the more work the rear does the less heat has to be managed by the front, secondarily the big rear tires can contribute a lot more than is often asked of them in slowing the cars down. The utility of the rears is limited only by the normal forces seen at the back, better aero, less weight transfer from a stiffere lower suspension all help to keep weight on the back.

Street ABS on the street or track is sort of like a safety harness, you don't use it most of the time but it's nice to have when the driver errs or something unexpected pops up, it won't save you if a GT3 pops a coolant hose in front of you. I've driven a few Cup cars w/ a faster operating race ABS but never at 10/10, maybe it helps I really can't comment, for the rest of us amateurs ABS is nice but not necessary, a p/v adds hysteresis to the system and is generally undesirable.

bias >1.5 is safer but puts more thermal load on the front and less on the rear, bias < 1.5 is closer to the edge and gets the most out of both ends.

Porsche has increasingly gone to bias closer to 2 on the street car because of legal concerns, they lost a # of big lawsuits.

If you pay attention to their race cars a late RSR will have twin masters and adjustable bias in the range 1.2 to 1.8

[VFR750]

Originally Posted by Bill Verburg

Another thought for Mike, several of the CVR guys run a 23.8mm m/c even w/ the stock brakes. The reason is that the pedal goes from 35.328 to 26.427, It makes it much harder to over use the brakes

I would think it also gives less pedal travel, which in an SC is pretty long. A little more consistent pedal location when heal and toeing.

Perhaps a safety net too, with more potential volume of you get a leak.

I had to add a Leeds “Bandit” electric vacuum pump because I only have 9” vacuum at the manifold due to the cam overlap.

Behaves nice on the street and track so I have pretty light pedal pressure and consistent 20-22” vacuum at the booster.

Going to a 930 m/c might not be that bad with the strong booster

Things to consider.

[juanbenae]

in addition to the rotor block off and ducting I did the black and decker grinder side plate brake cooling set up.. post #157

the plunge!

[Jens L]

When I got my latest car (3,2 1984 with a 3,6) the C12 kit with the bigger discs came with the car.
I have used them for a year now, mostly on the Nurburgring.

Compared with stock on the same pads(yellow paid) and same tires( trope R) i feel two things. Better heat resistance, better feel in the early phase of braking.

Is it as good as my previous cars( 964 RS/996 GT3). No.
Is it better than stock, yes but not night and day.
Is it worth the cost. Hum, I think so, I feel its more consistent lap after lap. But that´s very individual. I have friends that drives way faster than me on stock brakes.

[Bill Verburg]

Originally Posted by VFR750

I would think it also gives less pedal travel, which in an SC is pretty long. A little more consistent pedal location when heal and toeing.

Perhaps a safety net too, with more potential volume of you get a leak.

I had to add a Leeds “Bandit” electric vacuum pump because I only have 9” vacuum at the manifold due to the cam overlap.

Behaves nice on the street and track so I have pretty light pedal pressure and consistent 20-22” vacuum at the booster.

Going to a 930 m/c might not be that bad with the strong booster

Things to consider.

a larger m/s does give a higher harder pedal and yes, a vac booster certainly reduces the required effort.. Most of the guys w/ stock brakes that I was referring to are the '72-73RS guys w/o boost, It works for them, but most would find the effort excessive.

If you get a leak best to not use the car til it's fixed, I had a m/c failure in a Mustang once, not at all fun.

[Chilling in VT]

Mike, Bill is giving all the right answers. When I was just starting to do de's, I had 3"M front calipers. At Bill's suggestion I upgraded to 3 1/2" front struts and Carrera front calipers, and some better pads. This gave a great boost to the brake performance. After awhile, as I was going faster, I started to get a very long pedal by the end of my sessions. Bill suggested better fluid and better cooling and it helped a bit. The master cylinder was the next thing I was up against. On my new car (still a 73), it had the larger 23mm master cylinder that he was talking about. Night and day difference. It is a fairly stout pedal, but after you get used to the amount of pressure you need to use, you will never look back. The pedal feels the same all day long. Experiment with some different pad compounds and figure out what works best for you. I found that Performance Friction 97's work great for me. I can get about 12 track days out of a set. They are long out of production, but if you keep your eye out, you can find NOS ones. I bought 6 sets a few years ago at Hershey and found another couple of sets here on Pelican. Again, experiment. What works for one persons style of braking, and car weight might not work for another. Another part of the equation is tires. The more grip you have, the more braking force you can use before lockup.
I think that with a light car, proper cooling, and better pads, you will find that you have more than enough braking. I think the 930 brake setup is still probably the best one out there, but I agree with you about the price. I'd rather use that money for more track days.

[VFR750]

Downside of buying used calipers is finding they are seriously rusted under the rubber boot. $200 for the calipers, $28 for rebuild kits. Not a bad deal overall.



I guess the rubber boot traps moisture. All 4 pistons were stuck and 100psi air would not move them. Heat didn't do it either. Apparently the exposed bore of the caliper rusts. Rust expands and creates a strong lip that prevents the piston from moving outward. Pushing the pistons all the way in and scraping the rust off is needed. Then 100 psi can push them out easily. I use pads and a block of wood to keep the piston from shooting out of the caliper.

Took two days, but i was not going to be defeated by rust. Tonight: Rebuilt Wide-As

[Bill Douglas]

I don't want to rain on you guys parade. Because I fully agree with upgrades and having more fun.

But if you ferk about with the brakes and have an accident on the street; the accident investigators will sue your as off.

[GaryR]

Originally Posted by Bill Verburg

While there is a delay, which is why we don't like them, that isn't how they work
Here is a p/v, the spring allows unrestricted pressure below a line pressure value called the knee, above the knee line pressure is reduced by a fixed %, resulting is a curve like this

I simply gutted mine into an empty tube, problem solved.

[VFR750]

The beauty of brakes, especially non-ABS equipped cars, are they are a very robust and simple system.

Leaks and hose rupture are the biggest risks. Maintenance and proper assembly, historically reduces the risk tremendously to the point where the probablibilty is amazingly low.

Caliper failure is is even less likely. Bolts falling off, shame on the mechanic.

Rotors exploding only happens on the race track and those silly you tube videos.

Obviously on the race track, where energy levels and heat generation is massive the risk of systems failure goes up. Those tend to be brake fade or pad wear out. Manangeable during track days.

As seen too many times this past year in professional racing, failure of the rotors from excess heat happens.

Hence, track days are the events where not upgrading at least the pads and fluid is more risky than doing nothing. And the next improvement are larger better rotors and calipers.

All in all, if you pay attention, the risk of a brake upgrade specifically causing an accident on the street is very, very low.

[VFR750]

Surprising difference in the inlet side of the Carrera Rotor compared to the SC Rotor. Both are Zimmerman rotors. Significantly less blockage. This should improve the ability to get air to enter the cooling passages.

[Bill Verburg]

Originally Posted by VFR750

Surprising difference in the inlet side of the Carrera Rotor compared to the SC Rotor. Both are Zimmerman rotors. Significantly less blockage. This should improve the ability to get air to enter the cooling passages.

Do me a favor and weigh both rotors

[VFR750]

Stock sc rotor with 3000 miles of wear 11.5 lbs

New Carrera rotor. 13lbs

So the added thickness increases the rotor weight by ~ 1.5 lbs