Updated February 6, 2000
Tech Article: 930 Brakes
Das Boost – The 1978-1989 911 Turbo
by Peter Church
The second generation (1978-1988) 911 Turbo’s differed in many ways from their non-turbocharged cousins. In addition to the obvious body differences, the 911 Turbo was fitted with larger brakes, different suspension pieces, and of course the 3.3-liter turbocharged engine. This article examines some of the maintenance considerations and interesting features of the brakes. It is not a complete “how-to” article. Porsches are carefully engineered machines. Some prior experience and the appropriate workshop manual are required to effect most repairs.
When the second generation Turbo’s were introduced in the 1978 model year they came equipped with brakes derived from those developed for the Le Mans dominating Porsche 917 race car. At Le Mans these brakes proved themselves by repeatedly hauling the mighty 917’s down from their 240+ mph top speed on the Mulsanne straight. Properly cared for, they should prove capable of helping keep you out of the weeds at Waterford!
The second generation Turbo’s were the first high volume production Porsche’s equipped with cross-drilled brake disks. On the 917 project Porsche discovered that drilling holes through the disk friction surface and into the hollow vented area lowered disk temperatures and significantly improved braking performance.
Photo 1 Photo 2
The right (passenger side) front brake is shown in the Photo’s 1 and 2. In Photo 1 the cross-drilled holes (and the oh so cool cast in PORSCHE lettering) can be seen. Proper maintenance requires monitoring the thickness of the disks. The thickness is determined by measuring several points on the friction surface. The front disks must be at least 30mm thick at all locations. The rear disks must be at least 26mm. The disks must also be examined for any grooves (anything that will catch your fingernail is unacceptable) or cracks (no cracks permitted). Front disks can be machined smooth if at least 30.6mm of thickness will be left after machining. For the rears the thickness must be at least 26.6mm after machining. Cracked disks must be immediately replaced in all cases. Machining of cross-drilled brakes should generally not be performed at the local auto parts store. If you look closely at the cross-drilled holes you will see a chamfer machined into the top part of the hole. The chamfer lowers the stress concentration at the edge of the hole. This drastically reduces disk cracking. When the friction surface of the disk is machined much of the chamfer can be machined away. Porsche dealers are equipped to re-machine the full chamfer back in after the friction surface is machined. Most local auto parts stores aren’t.
Photo 2 is a view of the front facing part of the right front caliper. In Photo 2 a cross-shaped wire piece can be seen. It holds the brake pads in the caliper and is specially designed to provide quick access to them. This Teutonic brake paper clip is another feature of the Turbo that was carried over from the 917 program. In the old days the 917 had more power than brakes, so it was necessary to change the brake pads several times during the race. The 911 Turbo style pad retainer is now used on many other Porsche models.
In Photo 2 an electrical cable can be seen passing through the right “arm’ of the cross-shaped wire pad retainer. This is a brake pad wear sensor. There are two wires in the cable. When the pads wear they become thinner. This allows the connection between the wires to be ground away by the brake disk. When the connection is broken the control voltage from a switching transistor is removed, causing the brake light to illuminate. The wear sensor will have to be replaced if the light has come on. A replacement sensor costs about $7.00.
Photo 3 Photo 4
Photo’s 3 and 4 show the basic procedure for gaining access to the pads. In Photo 3 the brake pad wear sensor cable connector is separated. There is also a plastic clip not shown in Photo 3 that holds the cable to the brake line. This must also be disconnected. In Photo 4 the center part of the cross-shaped wire pad retainer is squeezed with a pair of pliers. This causes the retainer to release from the caliper. The retainer is hinged on the top for the front brakes and on the bottom for the rear brakes, so it doesn’t flip out when it is released.
Photo 4 Photo 5
Once the retainer is released, the pads can be removed. I’m using the second most popular pit tool for this purpose, vise grips. There is a very fancy pad removal tool shown in the workshop manual, made by a German company called Hazet. Hazet tools make (expensive) American Snap-On brand tools look like a bargain.
Since the new pads are thicker than the worn out ones, the pistons that push the pads onto the disk will be sticking out of the caliper too far to allow the new pads to be inserted. They have to be pushed back in. This is a somewhat delicate operation. You don’t want to scratch or damage the aluminum caliper or the steel pistons while you are pushing them back in. The workshop manual shows another beautiful tool for this purpose. I don’t know who makes it.
Photo 6 Photo 7
If you are very careful you can improvise using the number one most popular pit tool, duct tape. If only I could have worked a large rubber mallet into this procedure. I would have achieved the coveted pit tool trifecta (duct tape, vise grips, large rubber mallet).
Photo’s 6 and 7 show how a large flat bladed screwdriver with two pieces of thin hardwood duct taped to it can be used to push the pistons back into the caliper without scratching anything. Turbo brakes use four pistons in each caliper (two on each brake pad). Two small pistons distribute the clamping force onto the pads better than one large piston. They also allow larger pads to be used.
Another detail of engineering on the Turbo brakes is a step cut into the piston (Sketch 8). The orientation of the step must be checked before new pads are installed. The workshop manual shows the proper orientation. When the brakes are applied the pad becomes very hot. This heat can be conducted into the piston, heating the brake fluid. When the brake fluid boils it makes the brake pedal feel “soft” or “spongy”. If a driver is surprised by this effect, they may wind up in the weeds. The piston step reduces the amount of area on the brake pad in contact with the pistons and thus helps prevent heat from getting into the brake fluid
Sketch 8 Photo 9
The white circle in Photo 9 shows what the workshop manual calls the spring plates. There are two plates, one above the pads and one below them. The edges of the new brake pads rub on these plates as they move towards and away from the brake disk. This surface should be cleaned with an old toothbrush and coated with a thin layer of high temperature anti-seize compound. Pads that can’t move freely make that annoying squealing sound.
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