Front brake only upgrade
 

I plan to upgrade the brakes on my 912 -- Boxster calipers, new rotors and larger master cylinder. Will I cause myself problems if I upgrade the front only? What do I need to consider? Thanks..

There may be bias issues. Big brakes up front and iddy bitties in the rear. Are you adding an adjustable bias device? Going with a bigger master cylinder?

Thanks for your reply. I planned on using a larger master cylinder. I could use a biasing device if necessary. Does the master cylinder make the bias issue worse?

If you leave the rear brakes standard and fit a larger master cylinder you will move the 'bias' even more to the front of the car than just changing the calipers.

The larger cylinder will increase the pedal effort but will reduce the pedal travel.

You may need the larger cylinder to gain sufficient fluid volume for the larger caliper but it will have a negative impact on rear braking.

If by a 'bias' device you mean an adjustable hydraulic pressure relief valve in the rear brake line you will only make the situation worse.

These valves normally have a fixed 'bias' ratio depending on the device you buy. The adjustment doesn't change the 'bias' it only changes the pressure at which the valve starts to operate.

If you wind it all the way in you will reduce rear braking effort at a very low pressure and its impact will further reduce rear braking. If you wind it out it will not operate until higher pressure and you will have the same brake bias as that determined by the master cylinder and caliper sizes.

These devices cannot increase the braking effort.

If you fit such a device into the front brake circuit it really defeats the object and is not something I would recommend.

If you can already lock up the front brakes then fitting larger front calipers may well increase your overall stopping distances due to a reduction in rear braking.

Not to overlook the obvious, but why upgrade to Boxter fronts? They're waaaaay overkill on a 912. What's your application, competition?

Right now you have front brake bias. That's the way Porsche designed the car. If you change out the calipers you're going to be adding even more front brake.

Is there a reason you want to increase the front braking power?

If you really want to play around with bias you can fit different pads on the front and the rear. In that case you need to talk to the brake pad company and explain what you're trying to do. Follow their recommendations.

A good way to determine your current bias is to do about a half dozen hard stops in a large open parking lot and then compare front and rear brake temperatures.

Richard Newton
How Hot Are Your Brakes?

This is very informative and I appreciate the comments.

I want to upgrade my brakes simply because I want to get modern performance and the shortest stopping distance my tires will allow. I plan on Boxster brakes simply because I found them cheap and there are almost off the shelf solutions to make them fit. I planned on front brakes only because I reasoned that's where most of the braking force comes from. My reasoning could very well be wrong. It has been before ��

What is my bigger mistake: Boxster brakes or front brakes only? What would you do if you were me? Thanks again!

I just sold a set of Boxster front calipers and cooling backing plates I had used on my 85 911 for DE.

comments above are relevant.

I changed the MC for a turbo due to the pedal feeling too soft and travel too much. boxster 4 piston calipers take a little more fluid that the stock.

in my case the kit worked with my stock carerra rotors and came with adapters to mount. but be cautious about such a kit fitting a 912.

I replace the stock rear bias valve with an adjustable.

the limitation of this is it does not do much for heat disapation. but with bigger R compound tires on the track the stock brakes struggled. upgrading the fronts helped a lot, along with fully opening the rear bias valve.

I do understand you interest in such an upgrade. a local friend doing a 912 based outlaw and installing 996 caliper front and rear. :eek:

good luck on whatever you do.

Can your current brake setup lock up your wheels when you slam down on the pedal? If it can, I'm unclear on how switching out calipers/etc. would improve/shorten your stopping distance. Generally, folks look to upgrade brakes to reduce fade when tracking their cars. Fade's caused by the (relatively) limited thermal capacity of stock calipers/rotors/etc. As discussed above, increasing front bias through a front-only caliper swap could well have the unintended effect of INCREASING stoping distance. Seems it's best to consider brakes as a system (Master Cylinder, front and rear calipers/rotors/pads) -- one interesting option to consider would be upgrading your entire braking system to that used by a 911sc or carrera -- That way you'd be taking advantage of the incremental advances Porsche made over the intervening decade while still retaining an integrated system.

Where's Bill Verburg when you need him? :D

So if I upgrade to something less potent than Boxster brakes, what parts do I need to make the SC or Carrera calipers fit my 1969 struts and arms? Stomski makes an adapter to bolt up Boxster calipers and use stock hubs, which is pretty beautiful.

If your 912 brakes are in good shape, then your stopping power and distances will be limited by your tires. The stock brakes on that car are very good.

If they feel weak/spongy, you might find that replacing the lines and bleeding the system does the trick. If you want better bite, pads'll probably do it. I would probably rebuild every caliper before messing with brakes from another car.

Are you sure that the adapters will work on your struts?

When I looked into doing a Boxster brake upgrade I was told that the only adapters were for the 3 1/2" caliper mounts.

I ended up changing out my front struts to newer ('74) Konis and installed Aluminum S calipers. I am very happy with the result.

May I ask why you feel the need to upgrade your brakes? I would get the stock system 100% (there is a guy here - Eric at www.pmbperformance.com - that rebuilds the calipers to like new if you don't want to), a good high temp fluid flush, and pads known for good bite when cold and be done. I see this brake upgrade stuff a lot, and a Porsche stock brake system is more than adequate for 95% of all applications as designed for the car they came in. I ran a full race SC with stock calipers/brake system with better fluid and pads, no added cooling, and never had an issue during a race or track day event.. and even with no fancy flexible stainless lines too!

early brembo calipers
 

With your early struts you have 3.0 bolt spacing and the only caliper that I'm aware of is the early brembo caliper. They do use larger pads which could give you more stopping power. There are a few posts about them on the 912 forums

BMW 320i calipers is a popular 914 upgrade and they also have 3 inch spacing. You may need to do some machine work on the bolt locations and chamfer the pads if I recall.

Swapping out the 912 front end with an SC set up struts is an idea....gets you the 3.5 space, vented rotors and more options. Boxster calipers will require adapters in any case AND Carrera rotors.

Contact Eric at WWW.pmbperformance.com he can discuss options, he is a Pelican vendor/poster. Bill V is also very knowledgeable. Give either one a PM or call Eric's 800#.

What tyres are you using? and the real question is how easily does it lock up.

If it runs 6" wheels then even an early M Type front caliper should be capable of locking up the front.

What pads are you using as this will also have an influence.

If you can't lock up the brakes then they aren't working very well or the pads are poor.

If you experience fade then upgrading to vented rotors will help and converting the existing calipers with spacer kits is quite easy.

It must be worth considering a change of fluid, fitting some good flexible hoses to help with pedal and and some decent pads such as Porterfield.

The later 3.5" calipers with a larger pad area will help with fade and pad life but won't have a massive impact on stopping power for a given piston size.

All standard 911 Two Pot calipers have a 48mm diameter piston and generate a similar clamping force and pad area has little influence on braking power.

For a given clamping force a larger pad dissipates this force over a greater area so the pressure of the pad against the rotor reduces and so does the friction.

If you do change the struts for later parts then be sure retain the early style camber plates fitted to your existing struts as changing to the later design will limit the amount of caster you can set and this will impact negatively on the steering.

If you consider all the variables then tyres with more grip could be the best place to start.