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Twin Piston Rear Calipers

 
Old 07-13-2001, 06:14 AM
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Richard H
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Has anybody changed the single piston rear calipers on a 90 C2 to Twin piston calipers?
Is there noticiable improvement in braking?
Is it just a caliper change or any other mods required?
Is it possible!?
Just curious.

Richard.
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Old 07-13-2001, 06:14 PM
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GeoC2cab
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Hey Rich,
I did a brake upgrade on my 1991 C2, i used big blacks on the front, and moved the front dual piston calipers to the rear(installed new cross drilled rotors, factory pads and sensors)with no problem at all.,took 'em off the front and put 'em on the rear(cleaned 'em up nice).....it feels great, the bk end never lifts under hard braking,on the dime with 9 cents change. They came stock on model year '92,thats what prompted me to try it.
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Old 07-14-2001, 05:52 PM
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Cheers Geo, food for thought.....

Richard
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Old 07-14-2001, 07:24 PM
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Roland Kunz
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Hello

All C4 and 92 on 964 ave a four piston caliper on the rear.

As far I know the 2 piston caliper is NLA.

If you drive normal you will not find a difference. If you drive real hard on the track you will find the difference but will move to bigger brakes anyhow.

GrŘsse
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Old 07-16-2001, 12:19 AM
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Jeff Curtis
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Richard, I am about to try the conversion or upgrade you speak of. I just got a pair of four piston calipers from a '93 964 sent to me from a parts place in Colorado.

I actually had the guy trial bolt them onto a '90 chassis he had in the bone yard. He said they bolted up perfectly and had the correct spacing as to center the rotor.

According to UPS tracking, the calipers should be here tomorrow, I already have a good set of pads waiting to go on as well.

My main concern is the brake bias to the rear now. I have a 45 bar pressure limiter (factory installed on a '91) installed in the rear brake lines right after the master cylinder/abs unit.

I'm making this upgrade due to the fact that I wore out a very expensive set of performance pads in the rear during one track weekend! I have NEVER experienced this before I bought my 964.

So, my thought is that the rears will work less and actually provide more clamping (braking) power w/less pad wear...it will be interesting to see if I have to change my brake bias valve (pressure limiter).

I'll certainly get back to you on the subject...also, will be taking some digital pics during the upgrade - you're more than welcome to personally email me for a set of those. Cheers!
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Old 07-16-2001, 03:57 AM
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Dear Jeff,
I have read a few things about this upgrade in the past. I think you need to replace the bias valve with one from a 964 Turbo or a 1992 C2. I will try and find the details. I think I read it in the C2 Carrera Supplement in the PMM,
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Old 07-16-2001, 04:03 AM
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Dear Jeff
Just for info, I forgot to put this in my last post. I know a couple of people who are into Touring car Racing. They are telling me that they hardly use the rear brakes on the track anymore and they are fitting aerodynamic covers to the rear wheels to improve the airflow. The rear brakes are almost biased right out on most tracks. They only bias them back in on really short twisty tracks but keeping them cool is no longer required.
If you watch the European Super Touring Car Championship or the British Touring Car Championship you will see the competitors with what look like solid rear wheels. Obviously you will set up your 911 to what you are comfortable with but maybe you could experiment and see what happens. You may not need to change your bias or you may reduce it further. I would tend to think you will end up reducing your rear brake bias,
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Adrian
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PS: If you check out www.eurosport.com you might be able to find some photos of these BMWs, Hondas etc.
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Old 07-16-2001, 11:07 AM
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Jeff, I won't tell you I told you so about those weenie rear brakes! Glad to see you changing. Remember that at auto zone, if you ask for performance friction pads for a 944 S-2 or turbo, they will sell you the right pads for this set up. BTW what is the name of the dismantler you used on CO? I would like to find a used 21mm sway bar. I think that that is what Adrian said was stock on a 91 C-2. Adrian is that correct???
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Old 07-17-2001, 07:43 AM
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1990 C2 Horst for the 21mm rear sway bar.
In 1991 they installed 20mm.
Ciao,
Adrian
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Old 07-18-2001, 12:39 AM
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Horst, the name of the boneyard is Dart Auto, the guys name is Dave if I remember correctly. www.dartauto.com is their url if I remember correctly again! Aaaah, I'm getting old!

FYI, (Horst) I will be in St. Louis on the 8th of August if you're interested in getting together for some P-Car talk and suds. I will have truck/trailer and "Track Rocket" in tow. Making my way to Heartland Park for a DE, then up to Omaha, Ne. to see Mom, then to Road America in Wisconsin for another DE...wheeeeeeeee! If interested, email me w/your #...I'd like to see this "Concours winning Turbo"!

Adrian, thanks for the info. on the touring cars not using rear brakes and all...but I think I will stick w/my rear brakes!

The 45 bar pressure limiting bias that was factory for my rear brakes should be fine from what I have researched...although I have had a few suggestions to look up a 30 bar or so limiter...any part# that you have handy would be awesome.

As of now I have one front and one rear caliper...error on the boneyard's part...they emailed me right after shipping them out to inform me. They seem to be a great and knowledgable group of guys, guess they just "f'd" up!? They have sent another rear on the honor system, 2nd day air to remedy the situation...so guess I will start the upgrade tonight?? (7/18) Cheers!
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Old 07-19-2001, 12:41 AM
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Okay, for all who cares, I have completed the four piston caliper upgrade on my '91 which previously had two piston calipers.

The calipers I used are off the rear of a '93 964 C2, confirmed by part#, also the same as a 944 Turbo as Horst suggested in his post.

It was a VERY straightforward conversion w/the brake line matching up and all, no modifications necessary.

I pressure bled the system and bolted the rear wheels back on just in time for it to rain...so haven't tested them out just yet.

Still investigating the brake line bias valve and if I need to change it out.

I took pictures along the way while performing the upgrade and plan to write up a procedure...anyone interested? Let me know by email - Cheers!
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Old 07-19-2001, 04:28 AM
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Dear Jeff,
I will be very interested in an academic way in how the ABS handles these new brakes.
Please let me know. When I get a chance I will check out the bias valves part numbers.
Ciao,
Adrian.

PS: Yes I do have my reasons. The ABS computer changed part numbers a few times. I have no idea if the programming was changed or needed to be changed to go from the two pot to four pot calipers. A quick lock up check would probably sort this out. The bias valve may also play a role here as well.
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Old 07-19-2001, 06:51 AM
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Actually Adrian, that's exactly what I had in mind to "test it out" is a lock-up check. Again, it was raining upon completion of the "upgrade" so I will get back tonight w/the results as I have a few errands to run today...hopefully it is dry!

As I type this, it is not.

From all the research I have done, it doesn't seem as if the ABS computer plays a different role...I just think it may "kick in" sooner for the rear due to the bigger pads, etc. I plan to stop by a friend's house today that has a '93...I will "persuade" him to pull back the carpet in front to check out the bar rating on his bias valve...from what I have come up with - it will be the same, 45bar.

A lot of people have been moving the front calipers (bigger pistons), which I am to understand is a direct change over, to the rear and putting Monoblock (Boxster) calipers or "Big Reds" on the front, that way they have both the front and the rear upgraded!

If I had known that before I started the upgrade...I would have sunk the $350 I have in the upgrade into the front calipers!! ...damn! Hindsight is always 20/20.
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Old 07-19-2001, 11:17 AM
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High,
I can confirm the upgrade of using "big
reds" on the front and moving your front 4 piston calipers to the rear:"it works awsome"
I have been running this way for over a year(only w/ blacks) and still cannot believe the way the car stops; smoother,faster w/less pedal effort.
Just a question, wouldnt the bias valve be ok if big reds were on front and the now "smaller" 4 pot calipers were in the rear considering the stock set up had larger calipers in the front and smaller in the rear? any advice? and if you were gonna change it shouldnt you go less bar in the rear??? My set up feels like its working fine so I was gonna leave it alone.
Jeff, hind sight is sometimes less expensive, not only would you have needed the big reds,rotors,a set of 17" rims(if u dont have em yet)and a correct spare(to fit over that new set up), mo money!
peace, Geo.

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Old 07-20-2001, 05:05 AM
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Jeff Curtis
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Okay, during my night time travels at work (in-between offices)...I had a chance to do the "lock-up" test a few times.

Results?? The front seem to be the only ones that do the short lock-ups w/the rear feeling quite a bit more stable. Could it be that the extra bite in the rear is helping the car stay straight? I tried a few stops from 60mph, then 80mph...okay, I did a few more "tests" at higher speeds...but let's not talk about that!

All in all the car actually feels more stable when hard braking from high speeds! Yeah!

That's exactly what I wanted for the track, as the car had a tendency to "waggle" at the end of the straights during hard/threshold braking.

Can we do a survey here? Could some of you guys w/'92 and above >>C2<< 964s check your bias valve and tell me what the number is on it? It should be a 45 or possibly lower. The valve is found inline for your rear brakes, beside the ABS plumbing that is forward of your master cylinder in the front compartment, left side...at least for the U.S. cars. Please pull back your carpet and the valve will be easily seen right behind your battery and almost touching your spare.

You may have to take out your spare tire to read what's on the valve.

I would appreciate this as I work nights and am having a hard time getting to my friends Porsche facility and/or friend's house who has a '93 to verify!

Thanks and have a great weekend!
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