By purchasing 320i calipers from an auto wrecking yard the cost of this upgrade should not exceed $200 plus an hour's worth of time in a machine shop. Caliper rebuild kits, (new) pads, new brakes lines and brake fluid are all included in this estimate. Even with rebuilt calipers from an auto supply company, the cost should not exceed $250.
It is assumed that the reader is competent to perform the operations described below in a safe manner. Also, the details of many operations (e.g. brake bleeding, brake line bending, proper tightening of fasteners) are not included -- rather, it is assumed that the reader is already aware of the proper way to execute these operations.
This upgrade will fit 1973 and later 914-4's. The earlier cars have slightly different front brakes, rotors and struts. This upgrade should fit those cars, too, but it has not been verified.
Please read the SAFETY WARNING below and the disclaimer located at the end of this TechTip before proceeding. Also, please read the entire TechTip at least once before doing the conversion.
Death is final. Don't neglect your brakes, don't abuse them, and make sure you understand them fully if you plan on changing anything. If you can't follow these simple directions, don't do this brake caliper conversion.
There are many simple bolt-on upgrades that can be applied to the 914-4 -- a bigger master cylinder, upgraded pedal bushings, improved flexible lines, slotted and/or drilled rotors, high-performance brake pads, etc. Some are applicable to both street and race cars, others are recommended only for racing vehicles.
I used my 1974 914-4 2.0 as a testbed for this brake system upgrade. Prior to conversion to the 320i front calipers it was equipped with all the brake upgrades mentioned above, plus it had new rear calipers and rebuilt front calipers. Apart from these brake system upgrades, the car's only major deviation from stock are the 195/50VR15 tires.
In my opinion, falling short of upgrading the front calipers does not provide adequate braking performance for street use. Simple physics dictate that the rear calipers don't contribute much (a lot less than half, to be sure) to vehicle braking anyway. Hence the search for better front calipers. In order to keep costs down, I did not consider switching to 911 front struts, rotors and brakes (and wheels) a viable option.
For all-out, no-holds-barred and cost-no-object 914-4 performance, there's admittedly little point in holding onto any of the stock components. Raiding the Porsche parts bin, combined with some diligent parts shopping and talented machining, can provide near bolt-on solutions that hugely increase the brake size and stopping power of the 914-4. This is especially critical when fitting "monster" motors in the 914. However, the expense of the required 911-style struts and trailing arms, brake calipers, vented rotors, pads, wheels, tires and other miscellaneous items can quickly exceed the value of the car itself. Such upgrades are not covered here. The reader is directed to consult Porsche-oriented magazines and brake part suppliers for more information on such upgrades. Also see FOOD FOR THOUGHT FOR PURISTS -- RAIDING THE PARTS BIN below.
Also, verify that the car is properly aligned, preferably by a computer-based system (e.g. Hunter) that can provide you with a printout. A misaligned front end can reduce the brakes' ability to bring the car to a quick and safe stop.
If everything's working correctly and the car's braking performance is still disappointing, then there are some simple and relatively inexpensive upgrades you can try before committing to the 320i front brake caliper upgrade. They are presented in no particular order.
The 19mm master cylinder upgrade should be done without question. This unit was stock on the 914-6. The upgrade may be a little messy, since the space the master cylinder lives in is a bit cramped and brake fluid is likely to leak all over while installing it, but on the whole it's pretty straightforward. Bench bleed the unit before installing it. Don't forget to reset the indicator on the safety sender when done. 911 master cylinders start at $100.
When upgrading the master cylinder, consider rebuilding the pedal cluster with improved (non-plastic) bushings at the same time. The plastic bushings swell with age and often cause the pedals to bind, which may confuse the driver into thinking there are problems elsewhere in the brake system. Bushing kits start at $15.
Flexible rubber brake lines often deteriorate with age, becoming plugged and impairing brake performance. This can lead to unpredictable problems, among them brake binding and difficulties in bleeding the system. The only solution is to replace them in a timely manner. They should be replaced with new factory lines. Teflon-lined, stainless-steel braided brake lines also exist. These "stainless" lines resist expanding and flexing, and many feel they improve the brake pedal feel. WARNING: Stainless lines are for "off-road use only" (i.e. for racing), and there have been instances the stainless steel braid can wear through the Teflon lining, some with catastrophic results (i.e. deaths). The only way to avoid this is through frequent replacement, as there's really no way to inspect these lines. In fact, the specter of sudden, catastrophic failure with no prior warning has led a number of serious racers to abandon them altogether and return to using (factory) rubber lines. Use stainless lines only on a race car (which receives frequent attention & parts replacement), and on street cars only if they are truly D.O.T. certified -- good luck finding any that are. Rubber and stainless lines start at around $80 for a set of four, and should always be replaced as a set.
Onto brake pads. Ate, Jurid and Textar are typical factory-supplied pads. Ferodo, Repco/Metal Master and Cool Carbon are all aftermarket performance brake pads that are likely to improve the car's braking performance over stock. Some of the more exotic pads provide impressive deceleration, but require a proper warm-up to be effective. These pads are for racing use only, and must not be used on the street. Ask around and choose those pads that other owners recommend. Slotted pads provide a means for gas to escape from between the pads and the rotors -- important for heavy use. Pads start at $30 per caliper pair (front or rear) and climb into the $150 range and beyond. NOTE: experienced racing drivers are known to vary front and rear brake pad types as a means of controlling braking performance. This TechTip does not address this issue. Use the same pad type (same manufacturer and pad compound) front and rear.
Worn rotors should be replaced if they are warped, badly scored or under the minimum specified thickness. There are no commonly available vented rotors for the 914-4, nor will there be, as the calipers cannot accommodate the wider vented rotors. Choose rotors are from a reputable manufacturer (e.g. Ate or Zimmerman). Gas-slotted and cross-drilled rotors are a step up performance wise, but cross-drilled rotors have their own problems, including less mass to dissipate heat, and a tendency to crack and ultimately fail. For most drivers, stock rotors with gas-slotted pads are ideal. Rotors are fairly expensive, starting at around $45 each. Rotors should be replaced in pairs.
Lastly, there's the dreaded 914-4 proportioning valve. This valve controls the brake fluid pressure applied to the rear brakes, preventing them from locking up before the fronts. The valve is located just behind the driver's seat, in the engine compartment, below the engine sheet metal. The proportioning valve is adjustable, and is set and locked at the factory. This adjustment is critical to safety, as a car that locks its rear brakes first is VERY DANGEROUS and can quickly become uncontrollable. For a stock 914-4 brake setup, there isn't much point in adjusting the valve -- just bleed it carefully (see above). Adjustment and replacement of the valve are covered below in THE PROPORTIONING VALVE PART OF THE EQUATION.
To summarize:
The Ate BMW 320i caliper used in 1978-1983 is an excellent candidate for a 914-4 front brake upgrade. This caliper has the same 3" caliper-bolt spacing, uses the same size bolts, is also a fixed-caliper twin-piston design, has larger pistons and a has pad that's 40% larger than the stock 914-4's. It's also easily obtained at wrecking yards, as the 320i was a very big seller for BMW -- a common price is $50 for the pair (left and right). Some of these calipers have "BMW" cast on the outside. Obtain a complete (matched) pair, with pins, pad retaining clip and old pads on it. Also make sure you get the Ate units -- some cars had Girling calipers. Rebuilding the calipers is highly recommended and the kits aren't too expensive. Don't settle on a basket case pair of rusted-out calipers unless you're a brake caliper rebuilding masochist. Now is the time to disassemble them and verify that the pistons and bores are in sufficiently good condition to warrant a rebuild.
In addition to the calipers, obtain:
NOTE: The author has heard third-hand that the 320i caliper will fit 1972 and earlier 914-4's without machining. If you have a 1972 or earlier 914-4 and plan to do this conversion, verify the need for machining by removing the stock caliper and attempting to mount the 320i caliper. If it does not mount on the strut with the rotor perfectly centered in the caliper and the caliper flanges flush with their mounting bosses on the strut, then machining will be required. Also note that the dimensions below are verified only for the 1973 and later 914-4.
The 320i caliper does not have the correct flange-to-center dimension (too small) for the 914-4 struts, so you will have to machine the inside face of each mounting flange (not the face on the outside of the caliper, where the caliper bolt head is seated) to 0.587" +/-0.005" inches. The flange thickness "out of the box" should measure 0.712" +/- 0.002". This means that 0.125" of material must be removed from the flange. This will leave the caliper with a flange-to-center distance of 1.025" (this dimension is difficult to measure), matching that of the original 914-4 caliper. What you're effectively doing is moving the center of the caliper away from the strut mounting towards the outside of the car. Before you have the caliper machined, convince yourself that machining the outside flange faces would do nothing to re-locate the calipers properly about the rotor -- this way you won't be confused as to which flange faces (the inside ones) need to be machined.
The recommended machining setup is on a mill, with the caliper's outside mounting flanges resting on two (large) gauge blocks on the table, i.e. with the outside mounting flanges face down, and the inside ones facing up. Typically one clamp is pressing down on one inside flange, and the other flange is unobstructed and ready to be machined. Another, much taller clamp presses down on the caliper near the ends of the bolts that hold the caliper together on the same side as the flange to be machined. It is extremely important to ensure that the caliper is being held down "flat" against the mill table (via its outside flanges and the gauge blocks), otherwise the caliper will be skewed relative to the rotor when its mounted to the hub. Considerable clamping pressure is required to immobilize the caliper. Fill or cover the caliper orifices with something to prevent metal shavings from entering.
Use a large end mill (e.g. 1 5/16") at very low speed to remove the proper amount of material from the face of the inside flange. Be sure your cutter is big enough to maintain the large surface area of the inside face of the mounting flange. Do your work slowly and check it as you go. Try for +/- .005" tolerance, but +/- .002" would be better. Unfortunately each caliper has to be setup twice (once for each flange) due to the fact that the only place to clamp it to the table is the very surface you're trying to machine. The whole job can be done in under an hour.
Left untreated, the newly-machined flanges will quickly rust. You may want to spray them with WD-40 or smear them with anti-seize before mounting them. Now would also be a good time to rebuild the calipers. Absolute cleanliness is a must when working on a brake system.
Mounting the calipers on the car is a cinch, although you may need to dog-ear and bend the splash shield (top and bottom) a little bit to clear the calipers. Once each caliper is torqued to the strut, visually check to ensure that it's not skewed relative to the rotor. The bleed screws should be at the top of each caliper. The 914-4 brake lines will not fit, and you'd need to bend them so much to fit that they might fatigue -- very dangerous. Instead, fit two new rigid lines with a minimum of turns between the strut and the caliper. Do not attempt to make large bends (over 15 degrees) by hand -- use a proper tubing bender instead. Install the pads, fit the pins and spreader spring, bleed the system and you're ready to bed in the pads properly -- follow the manufacturer's instructions.
With factory Fuchs 5.5"x15" alloys, the 320i calipers clear the wheels with roughly the same clearance as the factory calipers.
After the conversion, increased brake pedal travel is one of the first things you'll notice. This is because of the 320i caliper's larger piston diameter. Do not let this alarm you. The "feel" of the pedal should be relatively unchanged. Brake pedal travel will shorten somewhat as the pads are bedded in. Be sure to follow the pad manufacturer's recommendations for bedding them in -- usually this amounts to 10 or so smooth but firm stops from 30 or so mph, with time inbetween to let the pads cool off.
After 300-500 miles of "easy driving" inspect the pads and rotors to ensure that the pads have bedded in properly. If they haven't, there is probably something wrong with either your strut, your rotors or the machining job on your calipers, and this needs to be addressed before proceeding. If the pads have bedded nicely and the wear surface on the rotor is nice and even, then you can begin to try out your new brakes
I've has already expressed my displeasure with the stock 914-4 front calipers, and only on rare occasions have I been able to lock the front brakes. With the 320i calipers, it's a whole new ballgame. Not only do the brakes slow the car down much better, but the front wheels can be locked at will at low speeds. With these brakes and a firm foot on the brake pedal you will begin to appreciate why Porsches are renown for their brakes, even if this is a Porsche/BMW hybrid system. On my car, the inertial reels on the safety belts will lock with firm brake pedal pressure at nearly any speed. The switch to 320i calipers was the single biggest improvement to my 914-4's braking performance, and by a wide margin.
There is one downside to this conversion -- the material on the pads is a wee bit too large (near the pins), and will wear "around" the outer edge of the rotor. If there's a pad that fits the caliper and rotor more exactly I'd like to hear about it.
BM, who followed this TechTip in its formative stages, remarked after doing the conversion "All in all, well worth the improved brakes." His parts and labor costs were under $170.
"The -4 type proportioning valve starts to limit at 48 KG/CM2 (683 PSI) and maintains a differential of 9.2 +/- 2 KG/CM2 (129 +/- 28 PSI) with 65 KG/CM2 (924 PSI) applied to front brakes. With 100 KG/CM2 (1420 PSI) applied the differential of the -4 increases to 23 +/-3 KG/CM2 (396 +/- 42 PSI)."
If the front brakes lock first, you just keep going in the same general direction (understeer). If the rears lock, you will spin (oversteer). The valve is located just behind the driver's seat, in the engine compartment, below the engine sheet metal. Turn the adjuster "out to increase rear bias." There is a jam nut to lock the adjuster in place -- original valves may have some locking compound still on the nut and/or adjusting screw.
Since the stock 914-4 proportioning valve is designed to work with the stock braking system, it's worth discussing its function in a 320i-caliper-equipped 914-4. Simply put, the presence of the larger front calipers means that adjusting this valve to increase the fluid pressure applied to the rear brakes will further improve the car's stopping performance. Be sure to check the system at low speed on an empty road before venturing out into traffic -- the front brakes should ALWAYS lock before the rears. Better adjustable proportioning valves ("racing brake balancer") are available and can be fitted to the 914-4, particularly the earlier "rear shifter" cars. Unfortunately none are a direct bolt-in installation, as they require adapters and/or modifications to the stock rigid brake lines.
In some installations, a non-stock combination of front and rear calipers makes the proportioning valve unnecessary, and a Porsche "T" fitting can be substituted for the proportioning valve. This has been done on a 914 race car with exactly the configuration described in this TechTip, and resulted in an "improvement [of] another 20-35% over just adding the front calipers -- it means you can *REALLY* use the rears which is not something that is possible with the stock setup." Note that this modification was on a RACE CAR! Again, such a modification can have a DIRE EFFECT if it results in rear-wheel locking under heavy braking. "Full speed stops from a variety of speeds, using a chalk mark around the tire to verify lockup or no lockup" are among the things required to guarantee proper brake bias setup. Consult an expert before making this modification.
Since the 914-4 has solid front rotors, 911-style calipers for vented rotors are not suitable, though they may have enough clearance to bolt on without interference. The author has read that it's possible to use these calipers (for vented rotors) with thicker, post-'73 914 pads on the 914-4. CAUTION: using any calipers with rotors substantially thinner than those originally intended (as here, where the calipers were designed for vented rotors, but the 914-4's stock rotors are solid!) may result in the caliper piston exiting its bore, with attendant total brake failure. Beware!
Another alternative is to use 911 calipers for non-vented rotors. 1970 911T "M"-style calipers (for solid rotors) are similar to the BMW 320i units, and must also be machined -- removal of 0.103" material from the inner flanges is required. These calipers have the same brake line mounting as the stock 914-4 calipers, so the stock 914-4 lines can be reused. These calipers use the same size pad as the stock 914-4 calipers. The advantage to using them is the stiffer caliper and the larger caliper pistons.
A 914-4 outfitted with 911 calipers ought to perform similarly to the 914-6, but without the heat-dissipating advantage of vented rotors. The BMW 320i calipers have the added advantage of much larger pads, all other things being roughly equal.
914-4 Brake Master Cylinder 914.355.012.00
17mm
914-4 Proportioning Valve 914.355.065.00 (1.7)
914.355.065.01 (1.8 & 2.0)
914-4 Front Brake Rotors 411.407.075 (1970-72)
411.407.075B (1973-76)
914-4 Rear Brake Rotors 914.352.401.10
914-4 Front Brake Pads Ferodo FDB2
2.43"x2.22" ('70-'72 pads were 10mm thick,
'73 and later are 14mm thick)
914-4 Front Rotor Thickness 11mm new, 9.5mm minimum
914-4 Rear Rotor Thickness 9.5mm new, 8.5mm minimum
Original Mounting Flange Thickness 0.671" +/- 0.002"
Rotor-Center-to-Flange Distance 1.025" (approximate)
911/914-6 Brake Master Cylinder 911.355.012.02 19mm 914-6 Proportioning Valve 914.355.067.00 Brake Caliper Rebuilt Kit Ate 13.0441-4815.2 911 65-75, 912 65-69 "T" Fitting (see warning above) 914.355.667.00
320i Front Calipers 34 11 1 150 253 (left)
'78-'83, solid rotor 34 11 1 150 254 (right)
Rebuilt 320i Front Calipers, 342-6041 (left)
from NAPA 342-6040 (right)
320i Front Brake Pads Ferodo FDB116
3.01"x2.58" Ferodo FDB192
(has cutout for wear sensor, may be
more expensive than FDB116)
BMW 34 11 1 150 527
320i Front Caliper Rebuild Kit Ate 13.0441-4825.2
BMW 34 11 1 116 618
Ate Caliper O-rings BMW 34 11 2 660 177
Mercedes A 112 997 04 40
Original Mounting Flange Thickness 0.712" +/- 0.002"
Final Mounting Flange Thickness 0.587" +/- 0.005"
(see machining instructions above)
Wilwood Lever Style Wilwood 260-2400 Proportioning Valve (has 1/8-27NPT threads)
THIS IS SERIOUS. For your safety and the safety of others, your brakes must work properly and must work well, all the time. This TechTip involves a change to the stock 914-4 braking system. "You get this wrong, you die and probably kill others. Check and re-check everything, come back and check it all again after a day, a week, a month."
Death is final. Don't neglect your brakes, don't abuse them, and make sure you understand them fully if you plan on changing anything. If you can't follow these simple directions, don't do this brake caliper conversion.
The opinions expressed herein are those of the author and do not necessarily reflect those of Porschephiles or any of its members.
This TechTip was written by Andrew E. Kalman, Ph.D. The author welcomes comments, corrections and further information. Please direct them to aek@netcom.com.