What is benefit of larger calipers/pads?

logician · 05-07-2005, 08:57 PM

I'm studying brake upgrades for my 911 GTS race car, and have a question. An affordable upgrade is to use 944 Turbo front calipers (mounting modified) with Carrera rotors. The question I have is: does this provide better braking than just using the Carrera calipers on the Carrera rotors? Or is the benefit only that the pads last longer because they are larger. Since both setups use the same rotor, both would have the same heat dissipation ability. Thanks in advance for any help.

-Juan

Eagledriver · 05-07-2005, 09:26 PM

Juan,

I think you are correct in your thinking. The problem people have had is the Carrera calipers failing from the heat. I haven't seen this on my car yet but I know that Rich used to rebuild his several times per season and the Stuart Ward was going through the same thing (stuart has SC brakes though).

Some people think that the bias on stock brakes is too much front brake. Some of the brake upgrades help this. Our rules used to only allow stock brakes but several original cars in the GTS class had big brakes so we legalized them. I didn't fight it that hard because I don't think it makes that big of a difference. I have Carrera brakes front and rear. This gives me a little bit too much rear bias but it's pretty good.

-Andy

logician · 05-08-2005, 07:01 PM

Andy,

Thanks for the reply. My SC brakes seem marginal, judging by the amount of fluid overflow that I'm getting, presumably from boiling.

Question: what do you have for brake cooling? Do you have the under the A-arm scoops, or do you have ducting to the front of the car, like to the horn grills?

Regarding bias with your Carrera rears, how bad is it? Does the rear end try to come around? Can you apply enough brake to lock the fronts, or does the rear limit your braking?

-Juan

Eagledriver · 05-09-2005, 05:19 PM

Juan,

I have ducts under the A-arms that I fabricate myself from 3 inch (I think it's 3, I never measured it) aluminum dryer duct. It's the accordioned stuff you can get at home depot. I re-enforce it with duct tape and hold it on with fat tie wraps. It lasts for about half a season if I don't go off road and then I put on new ones. I'll show them too you at the next race. I think they scoop more air than the commercial ones but they just blast the air at the center without any type of seal so maybe the commercial ones are better. I crush the leading edge into an oval so they don't hang down too low.

I think the best solution would be to duct the air from the grills. Another solution is the 993 air deflectors that some people have used.

I'm happy with my brake bias. It doesn't get squirrly under braking. I haven't locked a brake for a long time but I would guess that the rears would go first under perfect conditions (dry high grip pavement on level ground with no turning). Last year I tried using Hawk blacks on the rears because I thought they would have a lower coefficient of friction but they couldn't take the heat and were quickly destroyed. They made the rear bias worse due to the deposit of brake material onto the hot rotors as they were disintegrating.

I also have a theory that my brake set-up has caused my front brake calipers to last this long because a higher portion of braking is done in the rear than other set-ups such as yours.

I'm still looking for options on rear pads that will slightly lower my rear bias but still hold up.

-Andy

logician · 05-09-2005, 05:48 PM

Hi Andy,

Thanks for the reply. I like the idea of your drier duct scoops. I have the commercial ones that go under the A arms, and they don't look like they would scoop much air. where they are located.

I'm curious about the 993 deflectors. What are those? The only link I found through Google was talking about using them on an NSX, but it didn't show a picture.

For brake bias, why not just use a Tilton type bias valve? Brad Maker used one in his Li prepared Carrera, and was adamant that it was a great upgrade. You can dial in just the right amount of bias for your weight distribution and conditions, Probably you want less rear bias under wet conditions.

I agree with you that fully using the rear brakes is important. I'm leaning toward your Carrera setup, since I can see that it works by your results, and because I think it will achive a reasonable bias.

-Juan

Eagledriver · 05-10-2005, 06:28 AM

Juan,

Jack Olsen has the 993 brake deflectors and a search on the tech forum should show a thread on them. I don't have time to look right now. I don't do the bias valve because I'm too lazy to install one and because I'm still under the illusion that I'm in a stock class in PCA.

I think you need more rear bias in the wet because the weight doesn't transfer to the front as much under braking.

-Andy

logician · 05-10-2005, 08:57 AM

Andy, Thanks. I searched PP and found some pictures.

-Juan

KTL · 05-10-2005, 01:33 PM

I put the 993 deflectors on my '87 like this

Dunno how much they helped because i've only tracked my car once (last week) and never ran any laps without them. I'm sure my braking could use some improvement too.

Didn't fully boil the fluid (Valvoline SynPower) though. Pedal got soft, but didn't go totally away. I'll switch to ATE next time out and see what happens.

Dave at Pelican Parts · Dave at Pelican Parts's Garage

Do the 951 calipers have the same size pistons as the Carrera ones? If they have larger-diameter pistons, then the brake bias will be moved more torward the front with the new calipers.

Larger calipers should also serve as larger heat sinks and larger radiators for the heat than smaller ones do. Aluminum calipers should also be slightly more effective radiators of heat than steel calipers. No idea if there's enough difference to matter, though. But in theory....

I like those 993 deflectors! I'll have to check into them for the GF's 911SC!

--DD

TimT · 05-10-2005, 02:33 PM

Use 944T rotors with 951 front calipers. Its a bigger heat sink, you have to space out the whole rotor though, so that the rotor clears the control arm.

KTL · 05-10-2005, 03:02 PM

Typical 951, 964 calipers are 40/36. Using rear Carrera calipers maintains decent bias. Without a proportioning valve- if you have a Carrera.

Like Tim said, you need a spacer to use the '86 944T rotors (along with the machining of the caliper and caliper adapters). The spacer goes behind the collar on the spindle.

Par911 · 05-11-2005, 11:34 AM

Quote:

Originally posted by KTL
I put the 993 deflectors on my '87 like this

Dunno how much they helped because i've only tracked my car once (last week) and never ran any laps without them. I'm sure my braking could use some improvement too.

Didn't fully boil the fluid (Valvoline SynPower) though. Pedal got soft, but didn't go totally away. I'll switch to ATE next time out and see what happens.

Looking good Kevin!

I like the 993 deflectors, I may have to look into them. Did you order them from a dealer, or was it a used parts sale?
You will notice a difference by switching to the ATE, I know I did when I started using ATE blue. Now, I only use this brake fluid, and maybe some Performance Friction blue fluid. They tend to have a higher boiling temp. They are great for track driving IMO.

KTL · 05-11-2005, 12:21 PM

Yeah. I was lazy and should have flushed the Valvoline. Stupid on my part. Especially since I have two cans of ATE Typ 200 Gold in the garage!

It was actually kind of a test. I drove my car pretty hard, and while the Valvoline didn't pass with flying colors? It did not boil to the point that the pedal went completely away. Just got a little soft.

The 993 deflectors are a Porsche part of course. I just took the easy route and threw some $ Pelican's way for the new deflectors.

Left scoop is 993.341.083.00
Right scoop is 993.341.084.00

They probably helped cool the brakes a little? Certainly didn't do anything negative. AND! They survived my uphill spin-out without being torn off.

They do hang down a little bit, so i'm surprised they didn't get ripped off while mowing the lawn.

Bill Verburg · Bill Verburg's Garage

Quote:

An affordable upgrade is to use 944 Turbo front calipers (mounting modified) with Carrera rotors.

Thats not really an upgrade. You do get more clamping force on the rotors but that is not the usual issue w/ 911 brakes.

THE 911 brake issue is HEAT and its management.

cooling helps, correct pads help reduced weigh helps but bigger calipers in and of them selves do not help.

Neither does cheap belong in this sort of discussion.

Since the biggest issue is at the front just changing to Carrera calipers and rotors at the front is a nice little improvement

More

logician · 05-11-2005, 07:35 PM

Hi Bill,

Thanks for you reply and your great information page. I'm thinking that I will probably be OK with just the Carrera brakes front and back. I like the idea of being able to dial in rear bias.

Question about the thermal properties of the thicker 24mm Carrera rotors. I was thinking that the biggest benefit with those rotors might be the larger vents, which would allow them to pump more air, thereby cool off faster. Is that the case, or is the more important point the greater thermal mass?

The reason I ask is because I'm curious about what the best rotor to use with Carrera calipers (front and back). Should I be going with stock rotors that would have a lot of mass? Or should I look for drilled or light rotors of some kind? One racer I know told me he uses Wilwood rotors because they are lighter. I've heard that drilled rotors don't help much. Do you have a recommendation for what rotors to use?

-Juan

Bill Verburg · Bill Verburg's Garage

They do flow more air through them because of the larger air gap. But the chief benefit of the Carrera rotor over the SC is in the combination of increased mass which can absorb more calories w/ less temperature rise and better though still not optimal, air flow to dissipate it after the fact.

The rotors are a sort of thermodynamic shock absorber, they absorb the energy in the relatively short braking time, then dissipate it via radiation and convection in the longer time between braking events. You need a balance, not too much of one or the other, between the 2 or temps-will exceed desirable operational limits.

2 much mass means the rotors are too heavy and the pads never get to their design temps, too much cooling can mechanically stress the rotor causing cracking and pad issues.

Rally jon posted this in another thread showing the difference between a Carrera and an SC rotor(bottom 2)

Eagledriver · 05-12-2005, 05:20 PM

Juan,

I've heard of problems with cracking using drilled rotors. Maybe ones with cast in holes are ok but I don't think they're any better for effectiveness than stock. I use stock zimmermans from any cheap supplier. They last about 2 seasons. Do look at them closely when you get them as I had one that was machined off center and crooked. The vents were not in the center of the disc and the vents "wobbled" from side to side around the disc. I sent it back as I believe the varying thickness would case warping.

-Andy