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Front Brake Questions

Getting further along on my slow restoration of the 75 1.8... slowly because it drains the bank slower and also because it is just too fun to drive the machine to have it off the road too long... In really need to buy a second 914... one to drive and one to work on... but that aside... to my question

The front discs are under spec so they are going to be replaced. What are the opinions out there on splurging for the cross-drilled rotors as opposed to the less expensive solid.

I figure I will also rebuild the calipers as long as I am digging around in there... the price of the rebuild kit from Pelican is pretty cheap.

Are there differences in the choice of pads if you go with the drilled rotors? This machine is a daily driver and also used for tooling around in the San Gabriel Mountians here in So. Cal where I live. So there can be some resonably serious and extended periods of braking at times.

I will probably stick with the current master cylinder at this stage and keep the current prop valve unless convinced otherwise by some pearls of wisdom or experience.

Is there some way to check the proper functioning of the existing prop valve?

Comments anyone.

Old 09-22-1999, 12:12 PM
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Since $$$ is a concern, I would stay with solid rotors as the drilling will not do much good since they are not vented too and just drive up the $$$$. The prop valve is a debated issue, check other posts here for info. It works okay if the pedal feels firm after you hit the brakes and the car stops okay. They are hard to bleed though and that was one of the reasons I got rid of mine (both cars). As for pads PPs sells several good ones. I use Metal Masters on the dual purpose car as they wear well, stop well and are easy on the rotors and carbon on the race car as I don't care about the squeeling. Good luck.
Old 09-22-1999, 02:17 PM
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There is another issue with cross-drilled rotors: CRACKS. They *will* crack, it's just a question of when. If you don't catch it early, the whole rotor can fail when you need it most, and then you'll be seriously unhappy!

With that risk, and little or no benefit, it isn't worth it.

If you feel the need to throw money at your brake rotors, you could have them cryogenically treated. According to the sales lit, it results in better stopping and better durability. I have no idea myself, but if you want to try it please report back to us with the results!

--DD
Old 09-22-1999, 05:28 PM
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From what I've gathered reading other posts and newsgroups regarding the 914's braking system. The way to go is to replace the master cylinder with a 911 unit. The piston and cylinder in the unit are larger than the stock 914.

I'm not sure what gas slotting is. There was some advertisement in Grassroots Motorsports from a company that cross drills 914 rotors. What makes them special is that they claim that they cured the cracking that occurs in the drilling area. Motorcycles not withstanding, I dont think solid disks have any business being cross drilled. There is too much going on, they dont have any thing that will help them dissipate heat. Heat leads to brake fade because the fluid boils that much faster. Cross drilling is meant more for making the disks lighter to reduce reciprocating mass than to dissipate heat. On race cars the rotors arent drilled to dissipate heat, they are cross drilled so the rotors dont weight so much and that makes it easier to rotate the whell faster.

I've always wondered if that floor pan area where the pedal assembly is bolted to was a little bit more stiffer if that would improve the braking performance of the 914. Once I had the car up on my grandfathers lift and had him press down on the pedal. Like in an emergency situation. There was, what seemed to me, significant movement of the master cylinder.

I'm crazy enough to consider ultra-freezing the rotors, the claim that it moves the molecules closer together and creates a more uniform surface appeals to me. But that requires dollar$. In my current setup I have steel braided hoses and an original type cylinder that I bought when I was 'restoring' the 914 and I didnt know any better.
I hear that the braided hoses are a no-no for street application and can lead to instantaneous brake failure. I wouldnt recommend them because everyone else says so. But I havent had trouble with them yet....I hope that yet lasts until I rebuild my calipers.

Old 09-22-1999, 05:50 PM
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The cold treatment really works as witnessed by most weekend racers are using now. These are the ones that can't afford to replace the rotors every race, etc. The gas slotting is just that, slots ground at an angle across the rotor about 10 degrees from radial. Not worth the $$$ on a 914 either. I wouldn't worry about the sheet metal in the pedal assy area moving too much unless it is weakened by rust.
Old 09-22-1999, 06:30 PM
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The 19mm master cylinder, contrary to popular myth, will not improve the braking system's performance. All it does is change the effective "gearing" between your foot and the brake pads by about 25%. This means you have to push 25% harder for the same clamping force. This is percieved as a "harder" pedal because you have to push the pedal harder for the same effect.

The only real excuse is if you want to change the "feel" of the brake pedal, or if you add components to the braking system that require you to move more fluid. (E.g., calipers with bigger pistons.)

--DD
Old 09-22-1999, 09:35 PM
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Very interesting responses...

Brian you have a good point, it seems to me also that holes in the rotor may do little to add to the cooling properties without the venting to provide an immediate place for air to escape? Still, Pelican sells them and says,"These rotors are cross-drilled for improved brake cooling and better brake performance." I will probably chat with Tom or Wayne on that one.

From Nascar.com article http://www.nascar.com/news/1999news/March/15/00985346.html
"At one time, racers cross-drilled rotors to both increase cooling and cut down on the weight of the rotors. However, because cracks can form around drill holes at high temperatures, the practice was discontinued. Now, surface grooves are often machined into the rotors. The grooves serve two purposes. One, they reduce weight; and two, they allow the hot gasses that can build up under the pads to be released. Such "modifications" are all but unheard of on a passenger car rotor. Some NASCAR teams are now using cryogenic stress relief on their rotors. In other words, the rotors are frozen to -300 F and then brought back to room temperature. What this does is permanently change the structure of the metal."

The last note seems to confirm Brian's note on slotting being the latest thing and Dave's note on the problem's of cracking and potential of the cryo treatment.

With some further digging I found: http://www.germanautoparts.com/rotors.html
Which shows pictures of slotted rotors for VW's and a note to call about Porsche ?? I may check with them.

Other places mentioned that cross drilling may only help cool at very high temps and at low temp, the missing metal will actually decrease performance!

Seems like the drilled rotors may offer little help and others have reported the dreaded cracks that Dave mentioned.... and may only be good for lowering weight and "looking cool"

The story with slotted rotors is that they allow some more air flow, push out water, push out gases from the brake material, clean glaze off the pads and don't decrease structural integrity and promote cracks like drilling does...

That is what I have found so far guys thanks for setting me in the right directions. If I find out more about the slotted pads, especially for 914's... I will post again on this thread. Or if someone else does... please add your comments.

- Dave
Old 09-22-1999, 10:06 PM
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Check out a company called "GTS Rotor". They "dimple" rotors, much like slotting it dosen't go all the way through the rotor.

Cracking on solid rotors can be reduced by chamfering the holes. On vented rotors it doesn't help as much because you can't chamfer the inside hole.

When I talked with Bear brakes awhile back the rep also brought up the point that cross drilling hurts because it reduces the thermal mass of the rotor, that's why they use cast iron.

I agree get new pads, new lines, and maybe rebuild the calipers and MC. Cross drilling, slotting, dimpling won't help enough to warrent the cost.
Old 09-23-1999, 01:02 AM
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Found a website that does the crygenics procedure on rotors.


I posted another thread hoping to gather info on cryogenics for engine parts and trans parts also.
Old 09-23-1999, 06:22 AM
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Cross drilling your rotors is primarily to allow the gas that comes out of hot brake pads and the dust someplace to go. As the gas and dust build up, it reduces the coefficent of friction between the pad and the rotor. It works whether your rotors are vented are not. Yes it does reduce your unsprung weight and rotational inertia, but not enough that you would notice. Cooling is minimal (and not the purpose), especially compared to a vented rotor. The vents act as a centrifugal pump and pull air from near the hub and throw it towards the wheel. Cross drilling WILL result in cracking. Chamfering the holes will help, although you actually loose a little of their effect without the knife edge, but nothing will prevent them from cracking.

Slotting is a better choice. I have seen slots like John Rogers describes as well as arced slots and two parallel slots, one on each side of the top hat. If you do go with slotted, make sure they are cut with a ball end mill. This will insure a full radius at the bottom of the slot. If they use an end mill you get a sharp corner, and like cross drilling, they will crack. But again, its purpose is not heat removal.

Given that you won't be on the track much, I agree that new pads, lines, FLUID, stock disks, and maybe caliper rebuilds are the way to go.

Bobbitt
Old 09-23-1999, 07:04 AM
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Hey... here is a novel approach I found at http://driversedge.com/rotors1.htm

"For the real brake-addicted, add water cooling. I fog water into the back side of the rotors which have the aforementioned rotor hats with a separate windshield washer motor. This pump runs continously on the track. With these simple modifications you can vastly increase your stock brake's performance without spending huge money."
Old 09-23-1999, 07:20 PM
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For those that are interested. I contacted Pure Performance, http://www.pureperf.com/ about slotted and drilled rotors for the 914 and here is their reply.


Yes we do have rotors available for your 914, front crossdrilled or gas-slotted are $96.99ea, front KVR carbon fiber pads are $36.99set, rear rotors are $103.99ea and rear pads are $31.99set. Cadmium plating is available on rotors in black, silver and gold for an additional $15 per rotor.

Thanks to all who responded. I'm saving the cash and going with the standard rotors on this mostly daily driver. I think I would have to upgrade to a larger caliper and vented rotors to get significant improvements over the standard setup. I will also replace the brake lines, pads, wheel bearings, rebuild the front calipers and put in the turbo tie-rod kit at the same time.

- Dave
Old 09-27-1999, 02:14 PM
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Sounds like smart choices.
Old 09-27-1999, 09:53 PM
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