So.. I broke a piston in one of my rear calipers.

Im sure I can get another one cheap.. but I also have 914 4 front calipers as well (also 320I calipers.. but thats another story)

What would it take to get the 914 4 front calipers on the back? will it bolt on? If I put the spacer in it that I currently have on the backs, will it work as well?

I have humungo rear tires, and I dont think that the stock caliper will do all to good of a job with it.. Even with my racing pads. (i've been locking up the fronts pretty darn good...)

I have A calipers up front. with racing/street pads.

Andrew

Brake caliper size is about weight transfer. It has little to do with tire size, especially in the rear. If you have Ms up front and Ms in the rear, its going to brake like crap unless you have a proportioning valve and you do some testing. It really is excessive to have bigger brakes in the rear then dial them down, but if its for the sake of budget, my guess is that it will work.

Perhaps boxster brakes up front with Ms in the rear? I dont know about bolt up fitament, but this may be a good opportunity to get dirty with cutting oil. Many community colleges have machine shops you can practice in.

Andrew,

I have M calipers on the front of my 4 cylinder 2.0L with billet 4 bolt hubs and vented SC rotors, running Porterfield R4se pads. On the rear I run 914-4 front calipers with stockish mintex or textar pads. I do not currently have them spaced so I am using a stock 914-4 rear rotors. I have removed my proportioning valve. Running a T fitting the the brake lines to the rears.

Lots of folks said it wouldn't work, but it does. I use my car for AX and TT/DE with absolutely no problems as long as you balance the brakes with the pads.
With your A calipers up front you may have even less problems with balancing the front rear bias.
Don't remember the piston sizes....do a search on the site under brake caliper piston sizes. I believe the A calipers may have a larger piston than the M calipers IIRC. Actually I believe the cars that came stock with A calipers (Carreras and SCs) had rear calipers with piston sizes close to if not equal to the 914-4 fronts.

Hope this helps.

They bolt right on...you will only need to re-bend your hard brake line to go in the back side of the caliper and it is a pretty tight fit....but it can be done if you take your time and go carefully.

Your caliper spacers should fit the fronts OK....check with Eric Shea on that one. the bolt spacing is 3 inches just like the original rears..
this is something I have been considering doing so I could run vented rear rotors...I would have to have a set of 914-6 rear hubs redrilled for 4 bolt...kind of the reverse of what lots of folks are doing.

Matt, I quite beg to differ here. Caliper sizes front to rear (i.e., brake balance) have a LOT to do with tire size, since it's ultimately traction that determines the limit of the brakes. More traction means you can use more braking force. Since Andyrew's car is also a V8, it also likely has more rearward weight bias than a /4, so it can also use more rear brake than a /4.

You got me. All the cars I've calculated braking forces for have had equal tire size.

Too big of brakes in the rear will cause the rears to lock up before the fronts (not good). So more meat in the rear will help prevent that, so you can increase your total braking force.

If you're interested, calculating brake forces can teach you a lot about how braking system performance works. You need to know your center of mass in the car, your desired weight transfer (based mostely on tire data or reasonable approximations), know a few basic dimensions of the car, the coef. of friction in the pads at your desired heat range, size of the pads and rotors, the line pressure based on the master cylinder size, piston size in the calipers (usually what you're solving for) and probably 2 sheets of 8-1/2x11" paper.

Thanks! Just what I wanted to hear.

A caliper pistons are bigger than the M calipers, cant remember by much... but good enough for me. I DO have plans on going with a 944 turbo caliper on the front, or maybe some nascar calipers... Really dont know the extended future..

I have poterfield pads as well.

255 40 17's front 285 35's on back. for the street. On the track, I think hooser 225's and 255's, on 16's. then we have some victoracers.. but I'd rather go with some real slicks...

Honestly I dont think im going to have much "clearance" problems with that much space between the wheel and caliper.. But thanks for the heads up!

Also I think I will put a proportioning valve on for the rear.. Im planning on ditching the stock pedal assembly as well.. lol

Anyways.. Thats all I needed to know. Thanks guys!

The spacers you have in the rear 914/4 calipers will not work, you need spacers from the vented 911 rear calipers I think (69 to about 77). Not sure about thickness the 10mm spacers from the 38mm piston rears and the 14mm spacers from the 42mm rears may be too thick.

You won't have an emergency brake either. Probably better to use the 911 rear calipers as is rather than trying to fiddle with the 914/4 fronts. And it also depends on which version of the 914/4 fronts you have, they are quite different.

Not just "more meat", but more rear mass, too. Traction is a function of vertical force on the tire, as well as tire construction, compound, and size. If a car has, say, a 65% rear weight bias static, and the same CG height, then less total weight will be transferred off the rears than they would be if the car started with a 50% weight bias. Thus, the rears will be able to contribute more to overall braking before lockup with the 65% static bias than they would with the 50% static bias. This is true even with equal-sized tires front and rear.

To size the split between front and rear calipers, you need to calculate the dynamic weight bias (bias under maximum braking). You can approximate the max braking G as 1.0 to start with (most decent tires will allow this). You'll have to know the CG at your car's ride height, and you'll need to know the static weight bias. The weight percentages you end up with front/rear will tell you (roughly) what brake bias split you want to start with.

An adjustable proportioning valve would be very important to such experimentation (and easier than doing the calculations, too). If you fit rear calipers that are too big, you just dial them down with the prop valve until you get the fronts locking a bit earlier than the rears.

You'd also want to test braking into turns, as this will also change the weight bias numbers. Much of the weight will move to the outside front tire, with the outside rear being the second highest loaded. You can basically ignore the inside tires for this test, as locking either of them won't (generally) cause any huge dynamic changes.

No they're not. They're all the same size, it's the pad size that's different.

Also having a 43mm piston in the rear and a 42mm up front may work but I would take my brakes a hell of a lot more seriously if I were you. In Dan's thread I stumbled on something I think would work for a lot of teener applications that do not have or care to have a handbrake (judging from some of your posts I doubt that matters to you).

Right now Kelly (echocanyons) has a set of rear m-calipers for sale in the classifieds at a good price. If a handbrake is no concern then:

a ) Bolt them on with a 5mm ear spacer and run a vented 911 rotor.
b ) Take the spacers out of the middle and run them with a 914 or 914-6 rotor.

You will get a 38mm piston that was used in the 914-6, 911, first years 930 and Carreras up to 1984. That means that M-Calipers up front with a P-Valve would be perfect. A or S-Calipers up front with a T should give you excellent results.

Dan proved the application can work and he's a serious driver and knows what he's talking about with regard to bias. For me however, I would bias the system with the calipers and not the pads. Either way is acceptable but when I buy some Porterfield R4S's I kinda want them front and rear.

Also, slapping a 944 or NASCAR fronts on there without taking a system-wide approach to this can net you car with less than desirable results. This is literally your life you're playing with here... get some tire size, weight and hp info and check in with some of the minds here (like James etc.). You'll end up saving money in the long run and you'll have a better balanced and safer car.

eric....

if boxster monoblocks were used up front....

what would you use in rear?

boxster rears? M calipers?

Does this mean more momentum the wheel picks up under braking?



The cars I calculated brakes for were trying to shed pounds under strict class rules. Its probably not a big deal on a V8 car that doesnt fit in a class, so yeah... buy whatever is cheap, make it work, and get a prop valve and do some hard braking with warm tires.

Not, I'm not talking rotational inertia. I'm talking vertical load on the tire.

If I start out with a weight bias of 45/65 (front/rear), and under heavy braking, I get a dynamic weight bias of 50/50 (due to weight transfer), then ideally, I'd want rear calipers that provided the same braking force as the fronts. If the dynamic weight bias is 60/40, I'd want rear calipers that provided 67% of the force of the front calipers.

Fitting larger rear wheels and tires will change this, requiring more rear braking force to get ideal braking.

Calculating all of this is doable to get you into the ballpark, but experimentation is called for to get everything exactly right. If you want to make use of an adjustable prop valve, you actually need rear calipers that are too big, since all the prop valve does is close off the rear brake pressure. If they're too small, the prop valve is useless.

That said, I'd agree with Eric that fitting larger rear calipers than fronts is not called for, even with a 914-V8. Maybe a 911-V8, but even then, only just. At most, equal front/rear calipers with an adjustable prop valve are the better idea.

Given that equation, I'd use Boxster rears with a handbrake solution.

However, again, with the average wide-body 914 it's been proven by experienced track professionals that the A or S-Caliper coupled with Rear 914-6/GT or rear M-Caliper is all that would ever be needed. So... that's a lot of extra work and money for a nice system that will be basically eye candy.

not for me...ive seen people with boxsterds up front and like stock or 911m in rear...

45/65 front/rear will be very difficult to achieve . . .

hey andyrew,

Keep your E brake. It's a street car....

as a temporary replacement, you can have my rear stock calipers for $2.00... =-) they both worked fine at the time they were pullled.

Dr. I'll take you up on that.. How far are you from Mike? Im heading over there today (now).

Call my cell.
Thanks!

Andrew

A slip of the keyboard. 45/55.

I use Carerra calipers and vented rotors on the rear with my front Boxster monoblocks. No e-brake but it is a track only car. M/C is 23 MM.