The short version.

I just want to check to be sure that the bolt spacing on 914 rear calipers is the same as early (we'll say '65-73 or so) 911.

Thanks for your help.

Kelly

The long version.

I have '69E front struts and brakes. They were very cheap and work great. I am currently using them with 914-6 rear rotors (yes the car is 5 bolt) and 914-4 rear calipers. I have an adjustable bias valve.
My problem is I used to have a nice bias adustment range with the stock 4 cylinder brakes. Now the bias, even with the valve closed or removed is OK for the street, but I like a little more rear bias for autocross. Yes I realize you can get yourself into a mess in a hurry this way. I would also like to go to wider tires on the rear than in the front and my experience is this will make my bias problems worse.
The solution I'm looking at is using '69 911T/912 rear calipers. These have the larger pad area to match my '69E fronts, but will allow the use of my solid 914-6 rear rotors. It also means that I could swap back to my 914-4 rear calipers to use my e-brake when driving on the street if necessary. This is important to me as I drive the car all summer, everyday and I may need an e-brake if inspection laws change. I would also avoid having to turn down '69 and up vented rear rotors to the 286mm diameter. I have read elsewhere on the forum vented rear disks aren't necessary with a small six like my 2.0E.

Who out there has done this conversion? Anything I am missing?

Thanks for your help.

Kelly

I've often wondered how necessary it is to have rear vented rotors for most applications. Since most published numbers indicated that the front brakes do 70% of the work, do we we really need the extra weight of vented rear rotors?

this is decent article to read:

Do you really need to bigger rear brakes?

Back to your question, the rear trailing arm have the same 3" spacing as the front suspension of the 914 (/4 or /6), in fact some people when they install a 911 front suspension, will install the stock front 914 calipers onto the rear. If you want to still have rear parking brakes, you can install 911 drum parking brakes (do a search here, Timo I believe and a few others have done it...excellent pictures and descriptions)

Mueller,

Thank you for not only answering my question, but reminding about the -4 front caliper swap. I read about that a long time ago and totally forgot about it. Since I happen to have a perfectly good set of -4 front calipers sitting on the shelf and the pad availability for them looks good I think that is what I'll do. You saved me a bunch of time and money looking for 911 units.

That stoptech article is a very good summary of what's going on. Of course these cars with a bit more weight on the back use the rear brakes a bit more. Porsche did use vented rears on competition 914-6's but I believe it's mainly because they had them from the 911, turning a few mm off the OD wasn't a big deal for them, and heat management for a 24hr endurance car is a lot different than a sprint race car. I'll also add that a friend of mine is running stoptech's on the front of a fairly heavily modified 996 turbo with stock rears and they work very well for him.

Thanks again,

Kelly

QUOTE

I'll also add that a friend of mine is running stoptech's on the front of a fairly heavily modified 996 turbo with stock rears and they work very well for him.

I've spoken to a few people that have the monster front brake upgrades on thier BMWs or Audis and once they installed a big brake rear kit about the only benifit was that thier wallets are now lighter

QUOTE (Mueller @ Jan 17 2005, 12:27 PM)

QUOTE I'll also add that a friend of mine is running stoptech's on the front of a fairly heavily modified 996 turbo with stock rears and they work very well for him. I've spoken to a few people that have the monster front brake upgrades on thier BMWs or Audis and once they installed a big brake rear kit about the only benifit was that thier wallets are now lighter

Yeah, but BMWs and esp. Audis are relatively nose heavy even statically, and often carry their weight higher than a 914. The 914 not only has a slight rear bias statically, but the CG is so low that there's a LOT less forward weight transfer under braking than there is with most cars.

Nonetheless, I'd still question vented rotors on the back of a 914. I'd question vented rotors on the FRONT except for a track car (and I don't mean AX, but a full course track car where you're going to be doing 100-40mph braking once a minute or more), or a car that's heavier than stock (V8).

I can't imagine why the factory got it so wrong. Almost every 911 got rear vented rotors.

The 914/6 had almost the same calipers on the rear as the 911 but didn't get the vented rotors. The GT rotors were similar to the 911 casting but unique to that car. The front was the same as the 911.

QUOTE

I can't imagine why the factory got it so wrong. Almost every 911 got rear vented rotors.

I'm guessing that when paying that kind of money for the car, it is (and was) expected to have rear vented rotors

without a doubt the vented rear rotors do have a working practical use, but could they not be overkill for the majority of applications??

just cause the factory did, does not make it absolute
(sorta the same twisted logic of people saying that NASCAR or F1 does or does not use or do something )

QUOTE (davep @ Jan 17 2005, 02:02 PM)

I can't imagine why the factory got it so wrong. Almost every 911 got rear vented rotors. The 914/6 had almost the same calipers on the rear as the 911 but didn't get the vented rotors. The GT rotors were similar to the 911 casting but unique to that car. The front was the same as the 911.

Um, lots of apples and oranges here.

We've been talking about 914s, 911s, BMWs, and Audis, and trying to compare braking systems between them. They're all very different cars with very different weights and very different weight distributions. The 914 was, by and large, also a lot less expensive than the other cars mentioned, so it's going to come with a more minimal braking system.

The earliest 911s didn't come with vented rotors at either end. The earliest cars also weighed a lot less than the later cars, so it's likely they didn't NEED the vented rotors. A typical 914 weighs about the same as a very early 911, so there's a reasonably fair comparison. Even the early 911 needed much more brake on the back than a 914 simply due to weight distribution diffferences.

Discussing the BMW and Audi are really bringing in a set of oranges to an apple discussion, as they weigh a LOT more than a 914, and again have very different weight distribution from the 914 or the 911. Trying to extrapolate what's going to work on a 914 by looking at what works on a recent BMW or a later 911, is not going to work.

914s are light, and when you throw more money at it to add very light 5-bolt Fuchs on it, you're throwing away much of that advantage by hanging heavy vented rotors on it. On a street or AX car, I'd strongly argue that they're not necessary. If you've never experienced actual brake fade on the AX course, then vented rotors aren't going to buy you anything, and will add weight in the worst possible place.

If you want better braking, work on bias and pads. If you have an adjustable prop valve on the car and you still have too little braking on the rear, then the calipers you have on the front are too big (pistons are too big), or the pistons on the back are too small. You can adjust the bias by simply putting calipers with bigger pistons on the back, or calipers with smaller pistons on the front. With the former, pedal effort will be lower, but pedal stroke will be longer. With the latter, shorter pedal stroke, but more effort. It mostly comes down to a personal preference on which one works better for you. On an AX course, you want pads that work well cold. If you have track pads on the car now, you may not be getting top braking on the AX course, as such pads often work better hot.