hello,

I am currently upgrading my 914-6 to vented rotors. My hope was to take a Front "M" caliper and mount it on the rear using the 911 (69-83 Rear) vented rotors. I mock fitted the caliper and rotor and it looks like I need to place a small spacer between the caliper and the mount on the rear trailing arm (not a problem) The issue is the 911 rotors get larger as you get closer to the brake surface and rub on the "M" Calipers. Has anyone else had this issue? I realize they are larger for the parking brake, is there a parking brake delete option? 914-6GT vented rotors, suppliers?

Thanks for any help, back to the drawing board!

Steve

I think it is easier to put the spacers between the two halfs of the original 914-6 caliper and retain the e-brakes! Someone told me you can mill the rotor diamiter????

Steve,
That is the set up I am running on my car. I bought the rear rotors from Steve at Air Cooled Racing in PA.. I used harden washers for the spacers and longer bolts from NAPA. Works great for me.
Van914

48mm pistons on the rear?

OK... I give up.

Hey, send me your 914-6 calipers and I'll give you a freshly rebuilt pair of M-Calipers for the backs.

Thanks for the reply Jeff. I would think shrinking this raised area would weaken the integrity of the rotor (see attached picture)? My car is mostly a track car with a cage, so e-brake is not to important. Also I want to use the M Calipers because I can than match the racing brake pad compound of the front calipers wide "A". I am using Hawk Blues, and currently Hawk does not make a pad for the rear 914-4 calipers that I know of.

Hi Van,

where id you get the vented rotors for your car?

Thanks
Steve

Hi Eric,

I left you a confusing message to your business number. I just rebuilt the M calipers and thought you might have a rotor that would work.

steve

Use "Rear" M-Calipers with the #31 pad (Hawk makes those for the front and rear of a 71 911 as an example). You will have a bolt on solution that will not "totally" ruin your brake bias (it will be the same bias the car you got the front calipers off had). You'll need a 5mm spacer for under the ears.

This is what ever 914 racing in the HSR 2.0 liter challenge uses. Frank Beck goes faster than anyone I know in a 914. It works well and prevents them from visiting the wall backward.

Eric,

I have a J-west brake bias valve, so I will not be using the stock proportioning valve. will a 5 mm spacer help my caliper from hitting the raised part of the rotor made for the parking brake?

What M calipers are you wanting to use on the rear? 48mm 911 front, 42mm 914/4 front, or 42mm 911SC rear, or 38mm 911 rear?
All take the M pads, but there are big differences in the size of the caliper.
I would not use the 48mm fronts, they are just too large. I don't know of any 911 or 944 variant that uses such a large piston area on the rear of the car; it just unbalances the braking so much.
The 914/6GT rear rotor is a perfect fit for the rear when a 20mm thick rotor is called for.

Dave,

The calipers are in my dads garage but they were fronts of a early 911, I am thinking they are a 48mm but I could be wrong. Is the 48 MM to big if I am using Wide "A" calipers up front with a proportiong valve to adjust? I think I might have jumped in the deep end without doing enough research, I will measure the calipers tomorrow and see.

Thanks
Steve

No. It's not made for that application.

As Dave says..., all of those calipers take the same pad. The early 914 front is the same casting as the 911 rear.

So... the caliper you want to use that won't fit, has the same pad size as the one you should use and will fit. The one you should use has a proper 38mm piston and will get you closer to perfect brake bias.

You can still use the fancy biasing valve if you'd like. You'll just have the proper caliper for the job. All good, no downside.

I've led the horse to the water.

Eric,

Sorry I read your reply fast and did not see that you called for rear "m" calipers. ahhh... what to do??

Eric do you want to trade a set of rear M calipers for my rebuilt front M calipers

Thanks for all the replys

Install rear M-Calipers and fret no more, especially if you want not for a parking brake. Best possible caliper for the job, it fits, pads are plentiful, they're a dime a dozen and you can now rest peacefully.

Like "other" 911 items, we use them as donors to supply 914-6 calipers with GT spacers. None in stock.

I've had vented rear rotors on a 914....
absolute waste of time
but the way to do it is to take a set of SC rotors, turn them down, and then vent the caliper...


when I built my 2nd 914 race car I left them off!
solid rear rotors are more than adequate!

have you ever overheated your rear brakes?
neither has anyone else...
absolutely unnecessary..

but hey if you want to waste 20lbs or rotational mass fine with me
makes it easier for me to pass you!

I'm at 1837lbs wet with an all metal -6 racer now...
not something you can achieve with vented rear rotors!
(I'll be taking another 20lbs off this winter, by changing out to a mag engine case)

b

I ran SC rears with S fronts. The stock 4 pooper rears were just along for the ride with the S fronts....with the adjustable porp valve wide open.....after 5=6 months of use, the machinihg marks were still on the the -6 rotors.
These actually do some work. Bour a 1/8" (washers) spacers between the caliper & trailing arm mounting area.

Out brake me if ya can in that heavyweight, Brant.

I think I'm a bit outgunned JP!

at one of the races this summer
they had an evening event...
after many beers.... they had all of the drivers stand on a scale and fill sand bags until each hit 250#

then 4 equal go karts were lined up on the 2.5ish mile track
each heat of equal weight'd drivers went out side by side
the winner of the heat progressed to the semi-finals and beyond...

I didn't win anything
but it sure was fun being dead equal to all of your competitors in weight and power...

I'm thinking that I'd need to shed more than a few pounds to run with JP!

hope your doing well
brant

Brant,

My car is pretty well gutted. The car probably weighs a little under 2000 lbs. I have a 2.7 six that has been heavily modified and at Watkins Glen on the high speed sections the car seems to not have enough brake to stop the car without braking earlier than some of the later cars without running into them. I have had somewhat of a mushy pedal, we have added cooling, bled the brakes a bunch, new master cylinder (19mm), changed the proportioning valve that we thought was the problem. We also have tried several different brake fluids (racing) and the problem seems to come back year after year. I pump the pedal on the straights to make sure something is there because in the past it wasn't. Also, i have put a temperature gauge on the front and rear rotors and the rears are much. much hotter than the front. I get that this could be because they were not vented like the front, but thought the upgrade to vented in the back and a bigger master cylinder, and rear piston might help the problem.

steve

Agree... that said, there's only about 4.2lbs. difference.

Vented = 13.6lbs each
Solid = 11.5lbs each

what pads?
you can buy pads to run in different temp ranges

when you bleed the fluid, is there obvious air?
try upgrading the temp range of your rear pads
go with cool carbon and select the next temp range up
cool carbons, now called KFP's work great on sub 2000lb vehicles.
Pagids are better on 2500lb+ cars, but not as great as the KFP's on the light weight vehicles.

I go through rear pads at a 2 to 1 ratio compaired to my fronts.

have you ever added cooling to your rears?
It would be easy to do as a temporary test

I finally modified my S calipers, by removing the knock-back pins to eliminate a little of the spongey feeling. are you running aluminum calipers up front or steel ones?

b

Not really obvious air. maybe a bubble or two when bleeding after a weekend back to back sessions with my dad sharing the car. I have been running KFP's on the back and like you said they last 2 to 3 times longer than the front while running Hawks on the front calipers. I have run KFP front and back with not much gain.

no.. my front KFP's last 2times longer than my rears
I can get a whole season out of a set of fronts... only 3-4 weekends from a set of rears. and our car is a double driver too.. with my dad.

what compounds (what color) are your front/rear pads
try KPF gold color on the back or go up one step from there if you need to

which calipers do you use front/rear?

really?
my memory was a lot more
of course I don't have them sitting with me currently as you no doubt do
still we weighed them at the time and it was more in my head...
b

I was running M caliper fronts and stock 914-4 calipers in back both with KFP gold compound and found the fronts still wear 2X as fast as the rear.

I than upgraded to Wide A calipers on the front and still had 914-4 stock on the rear. I decided to try the Hawk HP on the front because they are much cheaper and people at the track really liked them. The rears were still KFP golds, as Hawk did not make HP pads for the rear. I did notice better braking with the upgraded front calipers, but the wear remains the same with the fronts wearing 2X faster than the rears.

Racing916,
I bought my vented rotors here: http://www.aircooledracing.com/
Van914

There is a correct rotor for the 914-6 racing application from the Competition Parts Book. Part number 901.352.041.12(rear vent rotor). Last time I checked there were 46 rotors left in Germany. Special order item about $150 EACH.

Brake Update Pictures of the front and rears, thanks for all your advice. Wide A calipers in the front, M calipers in the rear (38mm), with T installed and adjustable proportioning valve.

steve

front

rear

Thanks for all your input Eric!

I always thought that if you need to pump the brakes more than once the wheel bearings were too loose and the pistons were getting knocked back from disk wobble. Also I would think that the venting clearance would be a factor in the temps the rear disks and pads see on the 914. Don't race on a track so I probably shouldn't even be here. I have two sets of the 48mm brembos from the alfa and am going to put 38mm sleeves in the rears or whatever the smart guys think it needs after I get the front and rear weights stabilized with the suby motor and tranny and boxter wheels and tires. My fieros use solid rear disks and a caliper very much like the stock 914 and they are a heavier faster car than the stock 914. The 88gt I have has a 3.4 v6 with bigger ss valves and chevys hottest cam, the ecu has been reprogrammed. I probably has 180-190hp. The stock brakes with good pads works good on the street and doesn't fade with hard street use. I can mash the brakes hard from 120-130 and bring it to a stop with no problems. Of course thats nothing like the track stuff. I know the brakes smell hot but the fluid seems to hold up ok. I use the dot 5 and most people say it's junk but I don't have any water problems and most of my stuff sets a lot between uses. The dot 5 is a little spongier than 4.

So is it worth it, and is it possible to get the rear m calipers & remove the spacers and make them "non vented" and use 914/6 solid rotors? Minimizing weight and keeping correct bias with front A calipers.

Andy, in Steve's situation... maybe. In most, no.

I agree with your 914-6 rear rotor assesment. And yes, you can take the spacers out and have them fit on a 914. Very nice set-up for a track car that doesn't need a handbrake or, a street car with a different handbrake solution.

You get:

38mm pistons - Same as the 911, 911RS, 930 (early), 914-6 and 914-6/GT. These bias well with any 48mm front pistons (M-Calipers, A-Calipers, S-Calipers, Brembo 3")

Large #31 pad size - Same as the 911, 911RS, 930 (early), 914-6 and 914-6/GT. Again, these balance well with any of the following fronts - M-Calipers, A-Calipers, S-Calipers, Brembo 3"

Slightly less rotating mass with the solid rotor in the back (as you mentioned).

Biases well with the A-Calipers (as you mentioned).

I have a set of those exact calipers sitting on the bench right now.

Click to view attachment

So what "m" rears are they: 84-89? for my front A's. Yes mine will be a lightened track car. I may have a line on a set of used ones near me. Do you have rebuild kits for those? What about o-rings for splitting the rears? How much for your pretty rebuilt ones. What bias vavle do I use? Stock 914/4,none, aftermarket? Thanks!

1969 through 1983. The 84-89 have larger pistons and I would not recommend them.

Rebuild kits are in stock. I've got the caliper 1/2 o-rings as well.

Oooooooooooops, forgot: I can't stress this enough; The stock "biasing valve" is NOT. It is a saftey shut-off valve that comes into the system at 535ftlbs. In a panic stop this shuts the rears down and slowly brings them back into the system preventing them from locking and causing a catastrophic spin.

Until the point where the panic stop is noticed, this valve ACTS LIKE A "T". It does not bias anything. The pistons and pads in a 914 are properly biased from the factory. This is just one of the many reasons I recommend staying with factory biasing options like:

43/33 -or- 48/38

This is also why I never recommend removing this valve (anymore... there was a time when we were all spouting the virtues of a "T")

Rear 914-6 GT vented rotors(901.352.041.12)were still available from Porsche about a year ago. Around $150 each and have to come from Germany . Said they were about 70 in stock.