May 2002
911 Brake Cooling - The Early Years
By Steve Grosekemper
Last month, we discussed the need for additional brake cooling on 964 and 993 model Porsches. Since that article came out, several early 911 drivers have contacted me with similar brake cooling problems. There was a clear common denominator on the early 911's called into question; they all had either engine transplants or modifications resulting in large horsepower increases. As we all know, when horsepower goes up, not only do the straights get shorter, but the brakes seem to get smaller and smaller.
While the earlier ('65-'89) cars can be much lighter than the later models, they also have brake systems that are only half the size of that on a 993 (210 cm2 for a 911T vs. 422 cm2 for a 993). With these smaller brake systems, those driving high horsepower early cars must exert more pedal pressure for longer periods of time to achieve the desired braking performance.
All of the cars in question already had their rotors replaced with cryogenically treated gas slotted units and their brake pads were replaced with Pagid orange racing pads. This combination gives about the best stopping power from stock components. It also creates more heat in the braking system, so the stock backing plates had been removed to get more airflow around the rotors. On the hydraulic end, steel braided hoses had been installed with ATE blue racing fluid to keep the pedal steady under the most extreme conditions.
So how did the cars stop, you might ask? Great! All of the drivers thought their cars stopped 30-40% better than with the stock brake set-up. Sounds great so far…until the cars see a few high-speed, continuous lap, 25 minute run sessions. The repetitive use of maximum braking potential now generates a ton of heat. When you can't get rid of all the heat being generated, the brakes start to suffer. You will have problems like increased brake pad and rotor wear, as well as warped rotors, which can lead to severe pulsations in the steering wheel and brake pedal. Under severe conditions, the brake pads can even crystallize from the heat and come out looking like charcoal briquettes
The solution seems simple; more heat in the brakes requires more air to cool them. But the factory doesn't have any cool little ducts to scoop fresh air from the bottom of the car as it does for the later cars.
One search on your computer will lead you to the answer. AJ-USA
sells a very high quality brake ducting kit for the 65-89 911's, Part # AJR
600 911, for $319.95. The kit consists of 2 scoops zip tied to the bottom
of the front A-arms, 2 ducted backing plates that feed air to the center of
the vented rotor and 2 hoses to connect all the pieces together. While this
kit is of very high quality, there is one vital piece missing. When the kit
is installed, air is forced to the center of the brake hub with the intension
of sending it through the center of the vented rotor. The problem is that
this type of rotor is bolted to a hub with 5 huge openings in it, see figure
1.
Figure 1.
The pressurized air wants to take the path of least resistance, which is through the holes in the hub instead of through the center of the rotor. To correct this problem, we must block off the holes in the center of the hub. This will force the air to exit through the vented center of the rotor and greatly increase our brake cooling efficiency. There are 2 ways to accomplish this task. One is to buy some very cool rotor block off plates, part # 691 011, from Smart Racing Products ($35.00/set). They bolt to the back of the rotor, fitting like a puzzle piece, making sure the air goes through the center of the rotor and not out the center of the hub, see figure 2..
Figure 2.
Another way to accomplish this task is to cut some plates out of .060"
thick galvanized sheet metal. While this is less expensive, it does require
a compass, a 3 ¼ -inch hole saw, a hole punch, and a pair of expertly
controlled tin shears. Dimensions are 3.25 inch I.D. with 5.685 inch O.D.
Use the rotor as a template for the five 8mm bolt holes, see figure 3.
Figure 3.
The sheet metal version will conform to the shape of the hub without having
to have those cool cutouts that the Smart Racing unit has, see figure 4.
Figure 4.
If you do not fancy yourself a fabricator, give Smart Racing a call; it might well be worth the price to have the block off plates fit just right.
Now that the rotors are prepared and ready, we need to install the ducted backing plates. They consist of a backing plate center with a 2½-inch duct tube welded in just the right spot, see figure 5.
Figure 5.
The next step is to install the air scoops on the bottom of the suspension
A-arms. The kit supplies a set of tie wraps to install these scoops with,
but I have found that they are no match for an orange cone at high speed (not
that this has ever happened to me). I like to use 6mm allen bolts and thread
inserts in the bottom of the A-arm to attach the scoops , see figure 6.
Figure 6.
With the kit installed, your brakes will be ready for just about whatever torture you'll be able to dish out.
This kit is intended
for 65-89 911's with vented front rotors but the same principles apply to
early 944's, 914's and 356's when the 911 style vented rotor is used.
Good Luck!
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