4 to 6 conversion: rear brakes Options

[Steve]

Compare the pad size of a 914-4 rear caliper to a 914-6 rear caliper. Not much of a difference. For me the 914-6 rear caliper is not worth the money. I used to have 911S aluminum front calipers and 914-6 rear calipers. I am now running Boxster brakes front and rear with a 911 parking brake. Much better braking for the money.

[technicalninja]

Rear rotor centers from the backside on the outer edge of the hub. There is a bit of rotational play but once the wheel is torqued down nothing moves. Not an issue.

The -6 vs -4 rotor diameter is 3mm diameter, or 1.5mm radius. Someone anal made the claim that it needs machined and it's been parroted since. Plenty of clearance on the -4 caliper for a -6 rotor.

-6 rotor securing bolts (available at Lowes etc) don't line up with -4 hubs.

Thanks for that bit of info. Best to try fitting it FIRST before machining it.
You might have saved me some time here.

Also, good to know the rotor centers on the hub flange. I was not aware of this and this negates the need for the centering screws.

Sorry If I posted mis-information. I was going off other vehicles and not 914s for that bit of knowledge.

[mepstein]

Rear rotor centers from the backside on the outer edge of the hub. There is a bit of rotational play but once the wheel is torqued down nothing moves. Not an issue.

The -6 vs -4 rotor diameter is 3mm diameter, or 1.5mm radius. Someone anal made the claim that it needs machined and it's been parroted since. Plenty of clearance on the -4 caliper for a -6 rotor.

-6 rotor securing bolts (available at Lowes etc) don't line up with -4 hubs.

Chris - I wish it was true that it always fit but that wasn’t the case for mine. It locked up until I asked my local mechanic to take off a couple mm. One he did, the rotor spun fine. So now I recommend people turn it down right off the bat.

Steve - you are really buying the six calipers for the bigger pistons to match the fronts. M calipers have small pads so they don’t last as long as later, larger calipers, but it’s the pistons that are doing the stopping. Once upon a time, six calipers weren’t so much money and since they were a bolt on upgrade, the swap made sense. Boxster calipers are great but by the time you buy the adaptors, calipers, 911 ebrakes and all the hardware, it’s not an inexpensive upgrade either.

But it’s good we have options.

[zoomCat]

Compare the pad size of a 914-4 rear caliper to a 914-6 rear caliper. Not much of a difference. For me the 914-6 rear caliper is not worth the money. I used to have 911S aluminum front calipers and 914-6 rear calipers. I am now running Boxster brakes front and rear with a 911 parking brake. Much better braking for the money.

The /6 rear calipers have about 30% less more piston area than the /4, which results in a similar reduction increase in clamping force and braking effort. Most braking is done by the fronts, so it may not be noticeable in most situations. Porsche did a pretty good job balancing brake components, care should be taken when changing out individual bits. The 911 could be a good match for the /6 rears, and the Boxster stuff should balance appropriately since they were designed for a similar weight distribution. Big honking front calipers and stock /4 rears will be somewhat unbalanced, particularly on cold, wet pavement.

And then there’s the pressure regulator, but that’s another story.

[Superhawk996]

Compare the pad size of a 914-4 rear caliper to a 914-6 rear caliper. Not much of a difference. For me the 914-6 rear caliper is not worth the money. I used to have 911S aluminum front calipers and 914-6 rear calipers. I am now running Boxster brakes front and rear with a 911 parking brake. Much better braking for the money.

The /6 rear calipers have about 30% less piston area, which results in a similar reduction in clamping force and braking effort. Most braking is done by the fronts, so it may not be noticeable in most situations. Porsche did a pretty good job balancing brake components, care should be taken when changing out individual bits. The 911 could be a good match for the /6 rears, and the Boxster stuff should balance appropriately since they were designed for a similar weight distribution. Big honking front calipers and stock /4 rears will be somewhat unbalanced, particularly on cold, wet pavement.

And then there’s the pressure regulator, but that’s another story.

Not sure what you’re comparing to.

914/6 has larger rear pistons - 38mm vs 33mm on /4

914/6 has more rear lining area 106 cm^2 vs 80 cm^2 for /4

[zoomCat]

Compare the pad size of a 914-4 rear caliper to a 914-6 rear caliper. Not much of a difference. For me the 914-6 rear caliper is not worth the money. I used to have 911S aluminum front calipers and 914-6 rear calipers. I am now running Boxster brakes front and rear with a 911 parking brake. Much better braking for the money.

The /6 rear calipers have about 30% less piston area, which results in a similar reduction in clamping force and braking effort. Most braking is done by the fronts, so it may not be noticeable in most situations. Porsche did a pretty good job balancing brake components, care should be taken when changing out individual bits. The 911 could be a good match for the /6 rears, and the Boxster stuff should balance appropriately since they were designed for a similar weight distribution. Big honking front calipers and stock /4 rears will be somewhat unbalanced, particularly on cold, wet pavement.

And then there’s the pressure regulator, but that’s another story.

Not sure what you’re comparing to.

914/6 has larger rear pistons - 38mm vs 33mm on /4

914/6 has more rear lining area 106 cm^2 vs 80 cm^2 for /4

Sorry, I misspoke and got that backwards. The /6 has roughly 32% more piston area in the rear than the /4 (14.4 cm sq vs 10.9 cm sq)

[mepstein]

Compare the pad size of a 914-4 rear caliper to a 914-6 rear caliper. Not much of a difference. For me the 914-6 rear caliper is not worth the money. I used to have 911S aluminum front calipers and 914-6 rear calipers. I am now running Boxster brakes front and rear with a 911 parking brake. Much better braking for the money.

The /6 rear calipers have about 30% less piston area, which results in a similar reduction in clamping force and braking effort. Most braking is done by the fronts, so it may not be noticeable in most situations. Porsche did a pretty good job balancing brake components, care should be taken when changing out individual bits. The 911 could be a good match for the /6 rears, and the Boxster stuff should balance appropriately since they were designed for a similar weight distribution. Big honking front calipers and stock /4 rears will be somewhat unbalanced, particularly on cold, wet pavement.

And then there’s the pressure regulator, but that’s another story.

Not sure what you’re comparing to.

914/6 has larger rear pistons - 38mm vs 33mm on /4

914/6 has more rear lining area 106 cm^2 vs 80 cm^2 for /4

Sorry, I misspoke and got that backwards. The /6 has roughly 32% more piston area in the rear than the /4 (14.4 cm sq vs 10.9 cm sq)

Yes. 914-6 rear calipers are basically an M caliper with a handbrake.

[zoomCat]

Yes. 914-6 rear calipers are basically an M caliper with a handbrake.

I believe the M caliper has a 48mm bore and was used on the front of the /6, rather than the 38mm bore of the stock /6 rear. A previous owner of my car had changed out the rears for M calipers, and then added an aftermarket brake 'proportioning' valve to keep it from swapping ends.

[mb911]

One note for those thinking about doing this. I also upgraded to 108mm CVs and stubs and also 911 ebrakes. This has been one of the more challenging jobs that I have done recently as absolutely nothing is straightforward. Be careful going overboard. For my 6 there was no real reason to upgrade rear brakes and in hindsight I don’t think I would do it again. Just would do the CVs and hubs and still use 4 cylinder calipers

[hencar]

I agree with Ben and admittedly haven't read all of the posts. Here is a simple solution for your consideration. I have set of front struts with hubs drilled for the favored 5 x 130 mm bolt pattern by PMB Performance. Use your stock front calipers. The struts need shock inserts but otherwise they are complete with dust guards.

I also have a set of new rear wheel hubs from URO with the correct 5 x 130 bolt pattern. They are perfect for your stock stub axles, half shafts and rear calipers.

All for less than $750.00 not including shipping and handling fees. If you are interested send me a PM with your email address for details.

Thanks in advance for your interest

Henry

[Montreal914]

Actually, rear rotors are different in thickness. (IMG:style_emoticons/default/smile.gif)

/4 rear rotors thickness: 9.4mm
/6 rear rotors thickness: 10.4mm

[mepstein]

Actually, rear rotors are different in thickness. (IMG:style_emoticons/default/smile.gif)

/4 rear rotors thickness: 9.4mm
/6 rear rotors thickness: 10.4mm

I didn’t realize this.
I wonder if it matters - since we can adjust venting clearance. I have 914-4 calipers with 911 solid rotors on my 3.2 conversion car. Everything works fine.

Thinking about it more, I’m sure we gain or loose more than one mm when we switch out old brake pads for new.

[dr914@autoatlanta.com]

we can use stock six rotors with the aftermarket 914-4 five lug hub (five lug with splines that fit the four lug stub axle) and the four rear caliper, bolts right in and works just fine

Collecting the bits for my project, (4 to 6) rear brake question...
Are the rear 4 rotors the same thickness as 6?
If so, can the 4 rear calipers (with their parking brake) be used?
I am doing the 5-lug.

Thanks in advance

[dflesburg]

Here is how I made it work

911 rear brakes - e brake was shoes pulled by lever

[Eric_Shea]

Or if you are not in need of the parking brake, your front calipers can move to the rear with a 6mm spacer. Works great with SC front calipers.

Extremely bad idea.

[Eric_Shea]

Yes. 914-6 rear calipers are basically an M caliper with a handbrake.

I believe the M caliper has a 48mm bore and was used on the front of the /6, rather than the 38mm bore of the stock /6 rear. A previous owner of my car had changed out the rears for M calipers, and then added an aftermarket brake 'proportioning' valve to keep it from swapping ends.

"Rear" M-Caliper is what Mark's referring to. 914-6 calipers actually share the same nose section casting and they are basically the same 38mm piston size. A proportioning valve would not be necessary (and would only limit rear engagement) with a 48mm front and 38mm rear. I would keep a stock pressure regulator with that setup and ditch the P-Valve.

[JmuRiz]

Eric, is it possible to space out a stock 914/4 rear caliper to use vented rear disks?

[Eric_Shea]

Actually, rear rotors are different in thickness. (IMG:style_emoticons/default/smile.gif)

/4 rear rotors thickness: 9.4mm
/6 rear rotors thickness: 10.4mm

Correct (IMG:style_emoticons/default/wink.gif)

[Eric_Shea]

Eric, is it possible to space out a stock 914/4 rear caliper to use vented rear disks?

Yes, but "why?" is the question.

A stock 914 rear caliper will never heat up a solid rotor enough to require a vented rear. We had them made in the past but when you look into it, it just doesn't make sense.

BMW 3.0CSi rear calipers have the spacers and the fasteners required for this conversion. Hens teeth (IMG:style_emoticons/default/wink.gif)

[mgphoto]

See photo
<———-

I’m using the vented rear disk cut by 3 mm with the /4 caliper using the V kit, the spacer for the brake shield has to be placed on the outer side of the caliper instead of between the caliper and trailing arm for the correct spacing.