914-4 Brake upgrade

[davemartin]

I am currently upgrading the suspension and brakes on my 1973 914-4 2liter and I have a few questions. I know the upgrading to 911 struts with 3.5 inch bolt spacing opens up the door to use 911 or even Boxster calipers but what calipers could I use if I stayed with the stock suspension or used 911 struts with 3 inch bolt spacing? I would be happy with 944 turbo (951) or 928 S2 calipers but I don't know what they would fit? I have a set of 911 M front calipers, what front suspesion setup would they fit? Lastly, what calipers fit the rear suspension and would I need adapters to make things fit? Any help would be greatly apprieciated.

Thanks.
Dave

[Dave at Pelican Parts]

M calipers fit the 911 struts with the 3" bolt spacing.

There are probably adaptors to put 951 brakes on one type or another of 911 strut.

I would go with the M calipers, though. They're quite good, and were built for stopping cars that weighed more than ours and had the potential to need to bleed off more speed than ours. They're plenty fine for almost any use I can think of on a 914-4.

The rear brakes don't do that much work, so keeping the stock rears (with an aftermarket proportioning valve or possibly just a T) is an excellent option. If you must have 911 rear brakes, search for posts in this BBS by member "wilco"; he's done the swap.

Remember that brakes only stop the wheels. The tires stop the car. The best upgrade to shorten your stopping distance is good tires. (This assumes the brakes are in good condition, though.)

--DD

[davemartin]

Dave-

Thanks. I've got my eye on some 911 front struts but for now I'm going with the BMW 320 front caliper option.

I do have a question though. I have a 21mm master cylinder but from what I have read if I install it, my brake pedal travel will be increase and my likelyhood of locking up the brakes is increased as well. I was told I would need to change out the brake plunger to a larger one to take advantage of the larger master cylinder. So far I haven't been able to find one. any suggestions?

Thanks.
Dave M.

[rfuerst911sc]

I'm a DIYer so take this for what it's worth . I have a 1975 chassis that I am converting to a /6 useing a 3.0 out of a 911. I knew I was going to need more brakes then the stock brakes. I did quite a bit of internet research and decided to go with early 911 struts ( 1970-1973 ) with 3 inch caliper spacing. I then purchased the correct M calipers for both the front AND the rear. I have to do some custom work to use 911 rear e-brake but I decided that M calipers on all four corners with a 19mm master cylinder and a adjustable brake proportioning valve was the way to go. Many folks get caught up with " size matters " meaning bolt on the biggest calipers and rotors you can. I like to go " just big enough " to keep the weight down. I will end up with 20mm vented rotors front + rear with plenty of clamping force. With the larger MC and good performance pads ( Porterfield R4S ) I'm sure the limiting factor will be the tires. If I determine I'm cooking the brakes during DE's I'll add some brake cooling. The way I look at it is I will end up with the same rotors as my 83SC with calipers that have the same size pistons as my SC but smaller brake pads. My SC did a DE two years ago at Sebring in July heat with no brake fade, I'm guessing the 914 will do as good or better with aprox. 500-600 fewer lbs. to haul around Hope this helps

[Dave at Pelican Parts]

Originally Posted by davemartin

... I have a 21mm master cylinder but from what I have read if I install it, my brake pedal travel will be increase and my likelyhood of locking up the brakes is increased as well.....

Somebody has things backwards.

The pedal will travel less with the larger master cylinder. However, you'll have to exert more force on it to get the same "grabbing force" at the calipers than you would with a smaller master cylinder.

In fact, the difference is proportional to the squares of the diameters. So a 21mm MC will travel 17*17/(21*21) ~=66% as far to get the same clamping force as the stock 17mm one. However, you will need to push 21*21/(17*17) ~= 1.5 times as hard on the pedal to get that same clamping force!

Unless you're going with brakes that have very large pistons in the calipers, stick with the 19mm MC. If you have stock brakes, the 17mm is a fine choice, though some prefer the stiffer feel with the 19mm one.

--DD

[djpateman]

If you have a stock or near stock 2.0 engine you don't require anything more that stock brakes in good condition. You could do a minor upgrade on the front with non-vented 911T calipers with 48mm pistons compared to the 42 mm you have now. Better is to get the 911 front struts and go with vented rotors, but that is a big expense to convert to 5-bolt systems. There was a machined adapter to put vented rotors on the stock 4-bolt strut. Vented rotors are a big improvement in heat dissipation. For the rear there is not much you can do without either going to 5-bolt, or drilling 5-bolt rotors for 4-bolt fitments. To use vented rotors on the rear you would need to either get something like my spacer kit for the stock caliper, or get big-buck 914/6 calipers and space them, or adapt other calipers and add a drum type parking brake.

While vented rotors are great for heat sinking and dissipation they do add weight and inertia. This means the engine has to work harder to accelerate, and the brakes have to work harder to slow you down. If you don't have a significantly stronger engine then you may actually be slower.

Where did you find a 21mm master cylinder? That is one I have never heard of. The biggest the 911 used before the power brakes was a 20.5 on the 911S from about 1969. With a 23mm master you are going to really want power brakes or the thighs of a bicycle racer.