PCCB brakes conversion -> 911

[petevb]

I posted some info about this in my build thread some time ago, but I thought I'd go ahead and capture some detail in a separate thread for those that are interested. This captures what I learned putting PCCB brakes on a '69 912.

What: 350mm Gen 1 PCCB brakes front and rear onto a 1969 912.



Why: Fair question... Gen 1 PCCBs are known to have issues with track use. They are not much lighter than a 930 brake, they are enormously expensive and they force the use of a larger wheel. Thus the benefits, or at least cost/ benefit of this upgrade are debatable to some extent. Due to the track cracking scare, however, I was able to source a complete set of takeoff calipers, rotors and pads from a Ruf with less than 2k miles on them for less than 4.5k. Given that I believed my 997 cup motor would be too much for 930 brakes in track use, these did seem lighter and higher performance than the alternatives I was looking at, and the price I got the set for made it seem a practical enough proposition. (And of course they are cool).

Regarding the cracking, I was informed that the major source of the issue is either going off the track sideways into gravel or overheating the rotors. I felt comfortable that I can keep the rotor temperature significantly lower in my 2200 lb car than the 3300 lb GT2s and turbos the brakes were originally designed for. At least that's the theory.

How: I designed the the adapters and hats required under considerable guidance from Hayden at wevo, who did a good job making sure I didn't screw up too badly or do something stupid. I sourced the motorsports "green" pads recommended for track work, and we hooked them to a dual mastery cylinder setup with cockpit adjustable bias. Jeff Alton managed to supply 17" three piece wheels that will clear the calipers, limiting wheel weight from the upgrade.

Details:
As received: 6 piston front, 4 piston rear calipers, 350mm diameter disks.

Total front corner weight: 24.9 lbs
Front rotor: 12.2 lbs with hat
Front caliper w pads: 12.7 (w/o brake line)

Total rear corner weight: 20 lbs
Rear rotors: 12.2 lbs with hat
Rear caliper w/ pads 7.8 lbs (w/o brake line)
Rear Calipers w/o pad: 5.4 lbs
Rear pads: 2.4 lbs

930 brakes for comparison:
24.3 lbs front
27.6 lbs rear

Design for the adapters was fairly straightforward, with the exception that a greater factor of safety than I'd imagined is required. Good practice assumes 2Gs of stopping with 75% of the car's weight on the front (as might be encountered in a bump on a race course) and then a very healthy factor of safety over this. I used the exercise to learn my FEA program better:


A little cad and front and rear aluminum adapter were done.

The rear hat was even easier- turns out the mounting and contact areas are essentially the same size as an original 911 brake rotor. With the correct caliper adapter you can essentially bolt the rotor on and go. Even the original handbrake would work- gotta love Porsche for keeping parts interchangeable over decades.

For the front the brake design has changed over the years- old rotors sat behind the hub, the new one goes over it. I decided to keep the old mounting location, but in the process bit off making a new hat, the toughest part of the conversion by far.

The PCCB brakes run hotter than iron, so the thermal issues are of particular concern. Material selection is critical, considering conduction, thermal expansion and especially high temperature strength. I learned that machining the part from bar stock is important- the grain structure of the metal prevents it from turning into a pringle with thermal expansion, as plate stock want to do. (Who would have thought? Hayden, obviously)... Geometry is kinda important too.



Another hard won and as far as I can tell unknown little tidbit:
Porsche balances the ceramic brakes by installing lighter titanium bolts and nuts in the rotor where less weight is required. The assumption is they can't machine the rotor to balance the assembly, but by using the correct bolts in the correct places when attaching the hat to the rotor they get the assembly balanced. Lesson: keep track of which bolts and which nuts go where when you take they rotor apart. This is why the rotor is not available without the hat from Porsche.

One upside of making custom hats is that they are smaller and hence lighter- they saved about a pound per corner up front.

With everything assembled caliper clearance is tight under the 17" wheels:

We went with dual master cylinders, .625" and .7". The pedal box is custom, mechanical advantage is about 7:1, and of course no power assist. The pedal effort seems nice- not as stiff as some outright race cars, but still stiff enough with a nice short travel and pressure-plate feel.

The PCCBs with these pads do like a little temperature. They are not terrible from cold, but on the autocross if they start cold the second stop is significantly better than the first. They haven't been used on the track extensively yet, but I think it would take a jell of a lot to strain them. 350mm rotors front and rear on a 2200 lb car is a lot of brake, and the pads are absolutely ginormous. I don't see having to do tons of pad or fluid changes, which was part of the point.

Even under the 18/ 19" street tire wheels they look pretty big...

Overall I probably ended up a pound or two lighter than 930 brakes up front, more like 7-8 lbs saved in the rear. Once they are warm brake feel is better, pedal feel is pretty good.

I believe Steve at Instant G can sell a kit to get PCCBs on an early car and I've heard of a few others. Mine was a one-off. Learned a lot, pretty pleased with the results. Not sure I could recommend it for most people, but if you can find a deal on the calipers and rotors...

-Pete

[a914622]

Very good post. thanks for the inspiration.

[jac1976]

Well, if they don't work out it is probably due to poor planning. Maybe next time you'll employ more than CAD and FEA.

[lgb240]

I re-read your build thread just the other day ... very, very nice work, one of my favorite cars on this forum.

Nice write up here as well,

Hat's off to you!!

[Flieger]

Awesome. A+ for Engineering.

[David]

Very cool! I love to see engineering and design used for something more fun than work.

[LARRY SHARP]

Nice write up Pete. Now let's try it the dry right?

[Bill Verburg]

Can we get some pics of the rears?

[sc_rufctr]

You certainly don't muck around... Well done!

I've thought about this but the cost always puts me back in my place.

[petevb]

Thanks guys.

Yes Larry, dry would be good. You can stop dancing now, btw.

Some shots of the rears:




That rear hat is really heavy with the provision for the handbrake. It would probably be possible to save a couple pounds per corner taking some material out if you were willing to give the handbrake up and attack the rear hats, but that looks tougher than the front.

[mnmasotto]

Amazing! I would love to hear about your motor transplant. I love your car! Elephant Racing was working on a PCCB Ceramic Kit for the older 911's but don't think anything came of it. Would love to get a update from Chuck.

Mark

[Bill Verburg]

Thats' interesting, so the stock 996/997 rear rotors(w/ hats for the 2 piece versions) bolt on to a 911 rear hub and the p/b works? the p/b shoes aren't hanging out?

[petevb]

Originally Posted by Bill Verburg

Thats' interesting, so the stock 996/997 rear rotors(w/ hats for the 2 piece versions) bolt on to a 911 rear hub and the p/b works? the p/b shoes aren't hanging out?

I am not using the parking brake- we pulled it to save weight. When we did the early test-fits that's what we thought we found, however- the stock parking brake would engage the rear hat. I'll take more pictures to confirm, or my model shows 56.5 mm from the hub face to the inside edge of the rear hat. Perhaps you know if that's enough to fully cover the rear parking brake shoes.

[Jaskas]

Really nice design and work. Suits your car great!

[Bill Verburg]

Originally Posted by petevb

I am not using the parking brake- we pulled it to save weight. When we did the early test-fits that's what we thought we found, however- the stock parking brake would engage the rear hat. I'll take more pictures to confirm, or my model shows 56.5 mm from the hub face to the inside edge of the rear hat. Perhaps you know if that's enough to fully cover the rear parking brake shoes.

Ok, I just measured a 996, 993 rear and a 911 rear rotor, from the mounting face to the edge of the parking brake structure
911 69mm
996 62mm
993 is 59mm

[Bill Verburg]

Here is a pic
from left 996, 993, 911

[spuggy]

Originally Posted by Bill Verburg

Thats' interesting, so the stock 996/997 rear rotors(w/ hats for the 2 piece versions) bolt on to a 911 rear hub and the p/b works? the p/b shoes aren't hanging out?

The Instant-G 996 kit uses stock 996 rotors front and rear. When a 996 rear rotor is fitted to a 911, there's a small amount of p/b shoe hanging out, maybe 85% of the shoe is engaged. Apparently there is a little less engagement with a 993 rotor (which Instant-G use with 993 calipers), but it will still operate as a parking brake.

My wrenches had no concerns with the engagement or fitment of the 996, although they felt that drilling a hole to access the adjuster for the parking brake shoes was a good idea.

[petevb]

Originally Posted by Bill Verburg

Ok, I just measured a 996, 993 rear and a 911 rear rotor, from the mounting face to the edge of the parking brake structure
911 69mm
996 62mm
993 is 59mm

Sounds like these rotors would have the parking brake pad hanging off significantly without modification.

The rotor itself sits a fair way back- 71mm form the mount face to back face of the rotor. However drum for the parking brake doesn't extend that far.

[Steve W]

Originally Posted by petevb

I sourced the motorsports "green" pads recommended for track work, and we hooked them to a dual mastery cylinder setup with cockpit adjustable bias.
-Pete

I believe Porsche stopped recommending the P50 motorsport "green" pads for use with PCCBs due to accelerated wear of the ceramics due to abrasion. Andreas Preuninger is now recommending using only stock pads such as the P40:

PCCB & Cast iron Brakes / pads technical information - Page 3



.

[Bill Verburg]

Originally Posted by spuggy

The Instant-G 996 kit uses stock 996 rotors front and rear. When a 996 rear rotor is fitted to a 911, there's a small amount of p/b shoe hanging out, maybe 85% of the shoe is engaged. Apparently there is a little less engagement with a 993 rotor (which Instant-G use with 993 calipers), but it will still operate as a parking brake.

My wrenches had no concerns with the engagement or fitment of the 996, although they felt that drilling a hole to access the adjuster for the parking brake shoes was a good idea.

That makes sense, based on my measurements ~5mm of p/b shoe or a little less will overhang.

when I put 993 rotors on my 993 the p/b overhang was too much so 911 p/b center section is pinned into the 993 rotors



It's good to know that the 996/997 stuff bolts up.

same sort of thing applies to 993 rotors on 964, you can use them if you forgo the p/b