My C2 has the two-pot rear calipers. I can get hold of some of the later four-pot type is it worth doing this upgrade? <img src="confused.gif" border="0">

 

My C4 has four-pots all around and sometimes I wonder: What was the point? As evidenced by the disproportionate amount of brake dust on my front wheels vs the rears, it seems my car's brake bias doesn't allow for full utilization of the four-pot rears.

In my search for answers, I noticed a tech article on <a href="http://www.porsche964.co.uk" target="_blank">John Miles' </a>website written by Ken Tubbs about brake pressure limiters. In it, he recounts his experiments with various limiters for track outings. It made me wonder if the LIMITER was actually the key to increasing rear braking power? I was concerned, however, about the possible downsides to altering the factory bias, so I haven't made any changes just yet.

I think an understanding of the brake pressure limiter, Steve, may help you decide if the four-pot upgrade is worthwhile ... that is, if we could obtain some expert advice. Anyone?

 

I've performed the conversion on my '91...it was worth it due to the fact that I was wearing out a set of $120 performance pads (Mintex C-Tech) in one weekend at the track! I ran the car for two events with the factory 45bar bias valve to the rear brakes and it was evident that the calipers were getting HOT!

Upon returning home, from my one month journey to the Midwest tracks in 2001, I promptly changed the bias valve out to a 33bar...it obviously made the front brakes work harder as I now wore out a set of pads on the front calipers in one weekend.

So, where's the "fine line"...I dunno. I would suggest to people that don't track their cars often to just leave the two-piston calipers alone...although I must admit the upgrade to four-piston does look cooler.

Also, if you're not an insanely late braker, as I am...or at least I've been told, or as evident by the nail marks in my dashboard... <img src="graemlins/roflmao.gif" border="0" alt="[hiha]" /> your factory 45bar bias should work okay once you upgrade to original rear four-piston calipers that came on the 1992 and above C2s.

Now I have Big Red calipers on the front and use a 60bar bias valve to the rear (thanks to "Cupcar") and that combination seems to work great...although Cupcar has a similar setup and uses NO bias valve on the rear...I may try that soon, as my front pads are wearing considerably faster than the rear after a good weekend at the track.

Good news! My Big Reds seem as if they'll pay for themselves as I went through $200 worth of brake pad in one weekend on the front with the original four-piston calipers. <img src="graemlins/cussing.gif" border="0" alt="[grrrrrrr]" />

Now that I have two track weekends on my Big Red setup, there's still some pad left...so I've already saved $200! ...now multiply that by four and come up with $800 - being that it's apparent I'll get 3 track weekends out of a set of pads on the front and I do 12-14 events a year.

I still use the smaller four-piston calipers, from a 1992 964 on the rear...I have heard of people putting the original front calipers on the rear once they upgrade to HUGE calipers on the front. This is not good for two reasons:

1) You will have TOO MUCH braking in the rear as that is at least a 60% increase in piston to brake pad area...you would have to reduce your bias valve to at least the 33, maybe 22 bar model from the factory 45 or your car would be utilizing mostly the rear brakes...causing your ABS to kick in quite often.

***SO, what is the point? Leave your 1992 and above four piston calipers back there...I did.

2) The front calipers are for a 32mm thick rotor, NOT a 28mm like the rear four-piston calipers on 1992 and above C2s. While I don't think that's a big issue, if your pads wear down too close to the backing plates - this could cause undue wear on your pistons/seals as they will "cantor" under load and basically "egg-out" the passage for the piston.

Just some food for thought...

...let's review - just kidding!

In summary, I would suggest upgrading to the rear four piston calipers only if you plan on doing track events, not for more braking power...but to save on pads, as the pads for the four piston calipers are sometimes cheaper than the performance ones (and there aren't many options out there) for a two-piston caliper.

 

Thanks for such a comprehensive reply <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

 

Dear Joey,
Nobody has ever really asked why the C4s had four pots onthe back and the first C2s only 2 pots. Your comments are valid however the reason the C4 was built with four pots on the back was because of braking requirements based on the weight of the car with one brake circuit out. The limiter does not function as a limiter if the front brake circuit is disconnected or vice versa of course.
In the event of an emergency and you feel the brakes fading, stand on them with all your strength and hold it. Hand on emergency brake is also a good idea. By the way this only works if you have a failure in either circuit, front or rear. If you have a catastrophic central failure, use the handbrake without thinking about it. This is indicated by the pedal going hard and you cannot depress it.
If you also read Kens article he talks about ABS problems as well. On a C2 this is manageable but beware the C4. The C2 was vacuum assisted boost. The C4 has up to 180bar hydraulic pressure all of the time.
Dear Steve,
I also recommend you replace the 2 pot rears on a C2 with four pot. Saves a lot of hassle including purchasing the incorrect brake pads and also getting things like squealing rear brakes fixed.
Ciao,
Adrian
911C4

 

When's the book coming out? I can't get enough!

Great info from you, too, Jeff.

Thanks! <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

 

Thought I'd add $.02.

The 4 pot calipers actually have less stopping (clamping force) power than the 2 pot calipers. The clamping force is a function of the surface area of the pistons in the caliper. The C4's 4 pot caliper has 2 X 28mm pistons + 2 X 30mm pistons for a total area of 2643 square mm. The 2 pot caliper has 2 X 22mm pistons for a total area of 3039 square mm. So, the two pot can develop approximately 15% more clamping force than a C4 4 pot caliper. To compensate for this change Porsche raised the crossover treshold from 45 bar with the 2 pot caliper to 55 bar in the 4 pot caliper. The advantage as I see it with the 4 pot is more even pad wear. Just changing the caliper from 2 pot to 4 pot and not changing the proportioning valve will decrease the effective braking available at the rear of the car and probably increase stopping distances during all out braking as well as increase front pad wear relative to the rear.

I think Porsche deliberately endows the 911 with less rear bias than maximum stopping power would dictate in order to guarantee stability in all situations given the rearward weigh bias of the chassis. I have the factory Carrera Cup racing bypass line that eliminates the proportioning valve altogether installed and my front brakes still lock up before the rear with my antilock switched off. Remove at your own risk however. <img src="graemlins/yltype.gif" border="0" alt="[typing]" /> <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />

 

Cup car is correct but the total bias needs to be considered, all of the p/v shift bias to the front in varying degrees during hard braking, the f/r hydraulic bias ratio for a few Porsche models are

• 964 2 piston rear 1.832
• 963 4 piston rear 1.720
• pre 83 911 1.596
• 78-89 930 1.604
• 95 993 1.572

The 964 2 piston rear having slightly more front bias

 

Bill-

How did you calculate the bias ratios? I come up with a static bias ratio (below the proportioning valve set point) of 1.5 for the 2 pot and 1.72 for the 4 pot. This means the 4 pot has the bias toward the front, not the 2 pot. I understand this changes when you factor in the proportioning valve set point but the bias ratio then varies from that point as a function of line pressure. This characteristic is a nice touch with a proporting valve as it follows the weight shift toward the front that occurrs with increasing braking G by increasing braking bias toward the front. <img src="graemlins/yltype.gif" border="0" alt="[typing]" />

 

964 front 4x40 + 4x36

964 rear 4x30 +4x28 HBR = 1.720

or

964 rear 4x44 HBR = 1.496

My bad, I was looking at some experimental #s in my spreadsheet, didn't notice the piston sizes I quoted, should have looked at my website

The 2 piston rear is a little more rear biased than the pre 83 911

The 4 piston a little more front biased plus ABS covers a lot of sins.

The #s quoted in my previous post would be for 964 front calipers w 84-89 Carrera rear calipers

 

I'm trying to follow this discussion and, yes, you're going way over my head, but I do have a question. Why is the clamping force a function of the pistons and not the pad area?

FYI, I have the factory manuals, as well as the BA and Puhn books, so if anyone wants to send me home to do my homework, that's not a problem.

 

Pad area is not an important issue since as pad area increases the surface loading in pounds per square inch reduces. Double the pad area and cut the contact force in pounds per square inch in half. The initial contact force-clamping force-is key.

 

[quote]<strong> your factory 45bar bias should work okay once you upgrade to original rear four-piston calipers that came on the 1992 and above C2s.
</strong><hr></blockquote>

Although my 93 came stock with a 55 bar bias valve , I found that heavy track braking was a bit unstable. Swapped in the Turbo 60 bar valve, and, although only a 5 bar difference, that improved stability.

 

JB, clamping force is simple hydraulics. The ratio of piston areas is everything.

The pad area, is only a tertiary issue, after piston area and rotor diameter.

The larger pad area just distributes the forces over a wider area, minimizing deformation and thermal damage. As an analogy, if your wife is anything like mine, pads under the sofa legs are critical issues Why? because the weight(force) is distributed over a larger area, causing the carpet to be spared permanant deformation(hopefully). Calipers and rotors are similar in this regard.

 

The way I look at it the tires stop the car and the brakes are a friction device to stop the tires. The calipers apply a clamping force to the disc which produces a counter-torque to stop the disc and wheel from rotating thereby stopping the car.
Factors that increase braking torque:

Longer brake pedal arm
Smaller master cylinder piston
Larger caliper pistons
Larger disc diameter
High coefficient of friction brake pads

Factors that decrease braking torque:

Shorter brake pedal arm
Larger master cylinder
Smaller caliper pistons
Smaller disc diameter
Low coefficient of friction brake pads.

The trick is to get this counter-torque adjusted by manipulating the above so all 4 wheels are stopping at their maximum utilization of tire grip given the load transferrs that occurr once the car starts to decelerate.