My 914/6GT is struggling a bit with brakes having uprated to a 2.8 twin plug with about 280bhp.
It currently has S calipers up front and some 914/6GT rears with Porterfield R4 pads.
As well as heat the sticky track tyre set up could cope with more brake torque as well. (215/50r16 front and 245/45r16 rear). It is not a single use car - It is used on road as well as competitive rally and some trackdays.

There are plenty of 911 uprate options for the front, but without changing to a 911 emergency brake on the rear, then I haven't been able to find something to balance things at the rear. The current set up is properly balanced and there is no point in uprating the front unless I can uprate the rear. I really don't want to mess about with the rear arms by fitting the 911 e-brake system unless I really have to.
Is there any alternative rear caliper out there with a cable operated handbrake mechanism, with a >38mm piston like the original 914/6 calipers ? Anything from an Audi or VW for example that anyone knows will work in this application ?
Something with a 42 mm piston (like the Carrera rears) or even slightly larger would be ideal. Or has anyone fitted larger discs to achieve a similar upgrade ? If I could uprate the rears, then I could fit some Boxster front calipers on 280mm Carrera discs or a Boxster caliper kit with larger 300mm discs which will still fit under my 16" wheels which I definitely want to keep.

Am I wasting my time looking for an uprate option for the rears which still uses the 914/6 style cable operated handbrake in the calipers.

The crucial things is not having to change everything over to a 911 style e-brake set up. Any advice/options very welcome.
Regards
Andrew

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I am looking into the same thing right now, having already got a custom 911 Carrera rear (and front) setup but no e-brake. My car has been 95% track-dedicated (basically a full racecar with a license plate) in recent years, but now I'm returning it to more a streetable configuration/usage.

Particularly as I live in the hills, not having a handbrake makes me extremely nervous. I keep multiple wheel chocks around at all times.

I know there are hydraulic (or maybe electric) 'spot calipers' as well as 'line locks' that avoid the 911 type of setup.

A key difference is I'm on 15" wheels.

I am hoping there is some rear caliper off some other car (Audi/VW/?) with a larger rear piston and lever handbrake mechanism that could be adapted with some spacer to fit the 914 rear arm and GT disc.
I'm sure we are not the first people to ask this question but a few hours of searching on line and I didn't find anything about this option.
Fingers crossed

"The crucial things is not having to change everything over to a 911 style e-brake set up. Any advice/options very welcome."

Why is this crucial?

"Am I wasting my time looking for an uprate option for the rears which still uses the 914/6 style cable operated handbrake in the caliper"

Respectfully, yes. If you do a search on this site, you will se the question has been asked many times, over many years and it comes back to the same conclusion. Porsche used an independent drum style e-brake so they could have the freedom to use the appropriate caliper for the car.

The easiest way to do it is to add a 911 e-brake and use Tangerine racing's e-brake cable adaptors. The 911 e-brake is a proven system with spare part availability and fits any size wheel. Tangerine's adaptor cables let you use stock 914 cables. A couple small tack welds to hold everything in the right place.

--- Or you can try to design your own system with an auxiliary caliper that very few people get to ever work.

PS - Many people say that line locks are not safe to use as an e-brake. Especially on a hill.

I’ve been kicking around ideas of milling out a stock 914 rear caliper for a larger piston. I would think this may be a fairly economical thing to do. The only wildcard in something larger than a 914-6 GT piston is what needs to be done to create/modify a piston for the e-brake mechanism and if there is enough meat to go larger.

I have 930 Turbo calipers front/rear. Plenty of stopping power. I picked up 911 backing plates/shoes/bits to use the 911 parking brake. The 914 trailing arm requires some grinding for clearance of the backing plate but it's easy as pie. I'm using 911 cables for my setup but there are options as mentioned.

I can attest to this. It's the thermal expansion of the fluid. As it cools, it will loose pressure and slip. You'll find your car at the bottom of the hill...

wilwood
http://flyinmiata.fruitionqa.com/

IMHO the best setup for your car would be:
Boxster calipers on all 4 corners, adjustable brake bias to get more rear braking, 911 style handbrake.

http://www.914world.com/bbs2/index.php?showtopic=170890

I'm partial to the Boxster calipers, they work very well for me and i'm pushing a bit more power than you ...

Before you spend money on calipers, try a more aggressive pad compound.

1. are you running front S alloy[i] calipers ?

those have their own issues worthy of many other threads you can google - they "flex"

- i gave up on mine 20 yrs ago & went to SC calipers

2. "? balanced "? - are you still running the oem 914 rear brake pressure compensator? - if so, throw it away & install a wilwood or similar other

we need to know what your exact setup is

3. road use pads vs race pads is another lengthy discussion - are you complaining about road braking with race pads, or race braking with road pads???????? - you can't have both at once -

For the OP, I would also vouch for fiddling with optimizing pads, fluid, and cooling before changing more major parts. I've used Pagid pads for years and also have a Smart Racing Products cooling-duct setup on the fronts, which has been essential for me at certain venues even with the larger brakes I have vs. yours and full race pads/fluid. The front spoiler/airdam used makes a tremendous diff on cooling. That said, I did end-up going to larger brakes ultimately with it sounds like comparable power/weight and tire to you.

I looked into the Boxster setup and had the adapter kit, but wanted to stay more period/vintage correct plus I'm running 15" wheels.

Re. the e-brake, I am definitely not opposed to using 911 bits and in fact, would like to stay factory-ish. I don't have the parts so figured I'd survey other options before embarking on this.

911 e-brakes are ~$200, Tangerine adaptors are $130, stock 914 e-cables & wheel bearings.

So the question remains at least for me is why couldn't you machine for a larger piston?

DP

When you say struggling, what is the issue. Do you run better brake fluid, track pads, cooling ducts to your front brakes? Are you using your brakes too much?
For most people, the next move would be to change the fronts to Carrera's (larger rotors) and tick with the GT's on the rear or go all in with turbo brakes front and back $$$.

I’d think Eric would probably know if it could be done, but with liability issues these days, may not be advisable on a street driven car.

For the 914-6 caliper, ATE used a larger casting to go to the larger piston size. So at least their engineers (probably) felt it necessary.
THe early turbos used a front S and a rear M on a 3000lb car and the M/S caliper combo was a staple on many early Porsche race cars. It's a pretty strong combo for our much lighter 914's. But like I said, the Carrera front is often used as an upgrade because so much weight shifts to the front on braking that bigger rear brakes have very little effect.

There are a lot of posts asking questions most of which I think I answered in my first post. But to add -
Porterfield Race pads and race fluid already in use. Car used extensively on road and track so a compromise is essential.
Brakes are overheating and fading at the front at full pace within a few laps. Would like a bit of extra break torque as well.
Extra power in the car simply means higher speeds and therefore more breaking.
I have discounted simply changing to Carrera front discs and it is not significant enough a step.
Thought is that Boxster calipers up front with larger pad area on Carrera discs will give extra brake torque and heat dissipation I need. But no point in doing that unless I can uprate the rears by a similar amount.
I would like a bolt on solution and would prefer not to have to change everything over to 911 e-brake which is a big job what with full strip, welding, refurb of old parts, painting, geometry, etc. I would wouldn't be doing the work myself and labour is expensive.
930 Turbo calipers and discs would be fine but too many £s

Hence my specific question about a possible rear caliper with 42/43/44mm posting and lever handbrake mechanism that could be adapted for use on the 914 trailing arms.

I see there are rear calipers around with this larger size pistons used by OEMs but question is has anyone made one work on the 914.

The idea is the larger piston in the 914/6gt caliper is another idea but is that possible (given all the e-brake adjuster complexity) and has it been proved by someone to be reliable. I'd only go down that route if there was a package of available parts that are proven to work (pistons, seals, adjustment compatibility). I've never seen this idea mentioned before so thinking the alternative rear caliper with larger piston is a more realistic option that someone might have tried ?

I don't believe Boxster brakes give any more brake torque or cooling than Carrera's. Larger pads don't supply more brake torque but they do last longer. There is no more heat dissipation with Boxster calipers because heat is carried out by the rotors and the cooling air directed on them and Boxster brakes use Carrera rotors.

But unfortunately. there is no bolt on solution for larger than 914-6GT rear brake calipers.

I'm led to believe that the larger pad area helps to dissipate a little more heat even with the same disc. Pad surface temperature is kept slightly lower which delays the onset of brake fade which is the crucial issue. I'm also led to believe the extra piston/pad area of the Boxster caliper gives some more breaking torque. This 2nd factor is a nice to have.
Another option with Boxster front calipers I'm led to believe is to uprate to bespoke 300mm discs which are offered on some kits ? I haven't investigated this larger disc option yet as was going to try the std Carrera disc first which is an uprate on my SC discs anyway.

My main question on this thread is in relation to the rear brakes. If those can be uprated by a bolt on option then there are plenty of bolt on options for the 911 3.5" front struts.

Previously did design and release engineering of brake systems for OEM automotive.

Machining of a brake piston bore isn't as simple as you might think.

Tolerances are critical or you get piston binding, sticking, etc.

The seal groove geometry is even more critical. It is not a just simple recess to hold the piston seal. The shape of the groove has everything to do with piston roll back that you need to "pull" the piston back when the brakes are released.

Too much rollback = very poor pedal feel
Too little rollback = dragging brakes


Not saying it can't be done. Just not as simple as it might seem.

Gotcha.

@mepstein

Purely anecdotal and somewhat unrelated to the original post, but when i switched from my Carrera calipers and rotors to Boxster calipers and the same Carrera rotors, the improvement of stopping power and better heat management was significant!

I ran the Carrera calipers for many years and was never satisfied with their performance, both in terms of stopping power and brake fade due to heat.
I tried many things to remedy that but nothing really worked, which is what lead me to switch to the Boxster calipers.
Night and day difference, simple as that.



PS: @campbellcj The Boxster calipers fit under my 15" cookie cutter wheels ...

I know this isn't what you want to hear, but if you go to the last page of the thread i linked earlier, you can see Luke's 911 e-brake setup with Boxster calipers and honestly that doesn't require that much work at all.

Thankyou for all the responces. The simple conclusion is that there appears to be no rear caliper available with >38mm pistons that has been adapted to work on the 914 rear arms in terms of caliper position and handbrake. Something with say a 42-45mm rear piston would open up a range of front alloy caliper uprate options that would be low cost to install.
The work to remove a perfectly set up pair of arms and strip to modify and weld to take a full set of 911 e brake parts and then repaint before refitting with new bearings, new discs, calipers and pads is a big job if you have to pay someone to do it. Probably around £3.5-4k for all the calipers (used), discs, pads, ebrake conversion kit, complete ebrake parts, bearings, painting, and labour including geometry set up.

I have decided to compromise and just fit 3.2 Carrera front discs with refurbished 3.2 calipers with Ti pistons, heat shields and air ducts up front with just the std 914/6GT set up at the rear (refurbished) other than the uprated pads.

Time will tell if I've done enough, but no point in uprating the front any further unless I can balance it with a matching uprate at the rear.

Thankyou for all the advice
Andrew

When you say "porterfield pads and race fluid" can you tell me which compound pad and which fluid (brand / type).

I've used Hawk DTC pads on the street just having to remember the first stop in the morning drag the brakes to get some heat in them. In a light car like a 914 you'll never cook them.

If you're cooking pads on the track them you're going to have to give up some street drivability to achieve your goals.

As for fluid I run Motul RBF600 in everything I drive from my sports cars to my SUV. I hate ATE blue as it doesn't hold up nearly as well as the Motul.

If you want to get crazy Stoptech does have a kit for early 911 which you could try.

You could ask Chris Foley but I bet the trailing arms could be modified while on the car. Remove the shocks and drop them down. Some small tack welds to hold the e brake adapters in place and a little touch up paint. An angle grinder to modify the e brake area. $150 for adaptors, 250 for 911 ebrakes, 100 bearings, 600 for disks, calipers and pads. So $1100 for parts. Probably 500-600 labor. Sell your 914-6 calipers for $1600 and it’s almost a wash. I’m just not seeing 3-4K.

On my part, I'm gonna start planning and gathering parts for the 911 setup shortly. My car needs trans/clutch work and rear wheel bearings soon anyway, so this should work out nicely timing-wise though not $ wise.

Appreciate the info in this thread and will stay subscribed but I'll post my updates in my build thread (eventually).

Cheers,