Tom Pultz

[quote]Originally posted by mmmbeer:
I am talking about the stock 968 front calipers (not the M030 calipers), pistons are 33mm and 37mm<hr></blockquote>Like I said in my private response to your email, this is not true. No Porsche caliper that I'm aware of has used an odd number piston size. They all use something like 28, 30, 34, 36, 38, 40, 44, etc. The stock front calipers on the 968 use 36 and 40 mm pistons, not 33 and 37 mm. These are functionally equivalent to those on the '87 and on USA standard 944 Turbo and non M030 S2.

[quote]The pistons doesen't stop their travel when the pad/rotors are worn, it will surely travel the extra 2mm per side<hr></blockquote>Yes, it will travel the extra 2 mm, but the back end of the piston may also come very close to the sealing ring and that won't give you much margin for holding pressure. Without having the exact dimensions of all the components I can't say it WILL be a problem, just that it MIGHT be. In short, I still think it is a bad idea to run a mismatched caliper/rotor combination. Yeah, you can do it, but why? There are other options if you want more rear braking.

[quote]I have read that even when using stock 968 brakes you could upgrade to the 33-5 valve.<hr></blockquote>This is probably true, but I would have started with the 45 bar, not the 55. I guess it's just a different philosophy. I prefer to approach the limit from the lower end and you went right to the limit (other than removing it altogether). Maybe what you really need is a manually adjustable valve, so you can fine tune the braking.
[quote]Remember that i am using 13" front rotors with a LARGE calipper and pads. Without more force to the rear brakes, they would'nt be doing any of the braking.<hr></blockquote>If the weight transfer is the same as it was before (it won't be, but assume it is) they'll still do just as much work as they did before... it will just be a smaller proportion of the total.
[quote]For street driving you must know the surface, tires,weather and more factors + be an excellent driver with a LOT of braking practice to out brake the ABS system.<hr></blockquote>That is true. I did not imply that you can stop faster in a car without ABS than one with ABS (although that can be the case for very experienced racing drivers). What I meant was, if the ABS is constantly engaging when you hit the brakes hard, that means you have too much force in the rear and you will actually stop slower because the ABS is limiting the braking to prevent lockup. If you limited the force with a 45 bar valve, so the ABS only came on under rare occasions I think you would stop faster. Remember, any time the ABS is activated, you are not getting full braking, but are getting an ON-OFF action from the ABS as the unit limits pressure.

Let us know how it works when you get it running.

mmmbeer

I know what the sizes of my own calipers are!

I moved the front stock 968 (first picture, 33mm & 37mm pistons) calipers rear and sold the rear brakes along with the 968M030 front brakes I was suposed to use, because i got a good deal on a set of GT3 front brakes.

Ok, i agree that 33mm and 37mm sound weird, so I can admit to +/- 1mm when i measured them. But I would know if they were as much as 36mm & 40mm.

The m030 brakes that I sold (second picture) had 36mm & 40mm pistons. You cant mean that the stock 968 and M030 (s4) caliper use the same sized pistons!?

The next level is 36mm & 44mm used in the 993turbo and 996GT3 calipers (i have measured the 996GT3 calipers myself)

With the GT3 brake's larger rotors, you get more braking torque because the calipers are mounted further away from the hub. This mean that I must give the rear brakes more pressure to keep up.

Tom Pultz

[quote]Originally posted by mmmbeer:
I know what the sizes of my own calipers are!<hr></blockquote>Unfortunately, you don't. Would someone else please straighten out this thick-skulled Norwegian. I'm giving up <img src="graemlins/cussing.gif" border="0" alt="[grrrrrrr]" />

[quote]Ok, i agree that 33mm and 37mm sound weird, so I can admit to +/- 1mm when i measured them.<hr></blockquote>What tool did you use to measure them, a "yard stick?" If you can't measure closer than +/- 1 mm there's something drastically wrong.

[quote]But I would know if they were as much as 36mm & 40mm.<hr></blockquote>Evidently not.

[quote]The m030 brakes that I sold (second picture) had 36mm & 40mm pistons.<hr></blockquote>Nope.
[quote]You cant mean that the stock 968 and M030 (s4) caliper use the same sized pistons!?<hr></blockquote>Finally you got something correct. No I don't mean that. The stock 968 has 36/40 pistons and the M030 has 36/44. You are wrong, wrong, wrong. But you will never admit it, so we'll just move on...

Tom Pultz

This ought to settle the 968 caliper piston size debate...

tecart

i got a 1986 944 turbo, should i change the brake bias thingy? if it went bad does porsche automatically upgrade it to the next size? is this a bulletin? what size do i goto? i got stock brakes and stainless stell lines? what is good or leave it alone? i could always break better right?

Robby

Erik,

I'm not sure about the odd numbered piston thing, although I've never heard of Porsche doing it. That doesn't necessarily mean that they don't make any w/odd #'s, just, I haven't ever seen it. I DO know that the 928S4 (944 Turbo S) calipers have just as much piston size, IF NOT MORE, than the Big Reds & 928GTS. Everywhere I have read has said that the piston bore of the BR's/GTS are 36/44, while the S4/Turbo S are 38/44. The BR/GTS DO have a larger pad though- it is 10mm taller. However, I have heard several people on this list contend that the piston sizes are the EXACT same. KVR is one of SEVERAL sources that I have gathered info from, and they contend that the BR's were made for heavy rear-engined cars (ie: 993TT's) while the S4's are supposed to work better for slightly lighter front-engined cars. Of course the 928 WAS a (VERY HEAVY) front-engine car and it had the BR's (basically) for the GTS. Anyway, one advantage the S4 calipers have is that they are welded closer to the center point, which makes them deflect less than the others, although I don't know how big of a deal this is to begin with...


You had said that the 993 and GT3's were the next step up and that you had measured the GT3's yourself- you didn't mean that they (993TT & GT3) were the same size did you? I've seen listings for them that show that not only are the GT3 mono-bloc, which pretty much alleviates the deflection problem altogether, but they also have larger pads- not sure about the piston size though...

I'm not positive about the S4/Turbo S pistons being larger than BR/GTS, but I have seen and heard SEVERAL different places that claim this, but they are at least AS LARGE. Remember, piston size is only 1 factor in the overall braking equation. As Tom pointed out, you CAN end up w/too much piston size. I wanted to put original 951/968 front calipers on the rear of my Turbo S, and either install BR/GTS or just keep my stock S4's up front, and Chris Cervelli talked me out of it by saying that that was MUCH too large proportionally, and that the pedal travel would be RIDICULOUS. At that point I decided I would just use a proportioning valve to fix the rears, regardless of whether or not I go to BR/GTS. BTW, is it possible that your measurements and Porsche's measurements are taken in different ways or at different points? I have no idea, but was thinking that it could be possible(?)...

BTW, how difficult is it to install the ABS kill switch? That's something I've always thought would be cool, even though I would almost always keep the ABS on- Audi had ABS kill switches back in the late '80's when ABS was first getting popular. I've seen your site, and you are doing some really cool things to your car- actually, I think I e-mailed you about it awhile back...

Tecart,

Since your '86 doesn't have ABS, I wouldn't go higher than the 5/33. You may want to call someone like Chris Cervelli at Technodyne- he sells them for ~$65. If you do get a bias valve, you should make sure all of the equipment you have is in good condition. For instance- you might want to know what type of pads (and rotors) you have, and have the same all around, or, more aggresive up front. You never know what a PO might have stuck on there to save a few bucks, or whatever, and it wouldn't pay to find out that you somehow had a more than agressive rear set-up w/an average, or sub-par front. In any event, if you do the bias valve, you can test the car by driving around some parking lots, both dry & in rain, and really experiment w/it to make sure the rears aren't locking up first. I'm about to do the 5/33 for my Turbo S- I plan to post my results when I do...

Robby

BTW, here is a decent link to a brake upgrade site for Stealth's and 3000GT's- all of the upgrades are Porsche/Brembo calipers, etc. If you read what KVR and some of the others have to say about the stuff, it's fairly interesting- at least to me. If you have a few minutes, hit some of the links that are provided on that page- I went through a few of them one night, and found some really cool listings of various caliper, piston, pad, rotor, sizes, etc for our cars, but am not sure which one(s) will get you there...

<a href="http://www.stealth-3000gt.st/FAQbrakeupgr.htm" target="_blank">http://www.stealth-3000gt.st/FAQbrakeupgr.htm</a>

Tom Pultz

[quote]Originally posted by Robby:
I DO know that the 928S4 (944 Turbo S) calipers have just as much piston size, IF NOT MORE, than the Big Reds & 928GTS. Everywhere I have read has said that the piston bore of the BR's/GTS are 36/44, while the S4/Turbo S are 38/44.<hr></blockquote>The S4 and Turbo S use 36/44 mm pistons, not 38/44. Look at the PET image I posted for the 968, it has 36/44 pistons in the M030 caliper, which is the same as the S4/Turbo S. Brembo may manufacture a caliper with 38/44 pistons, but Porsche does not use one with these size pistons.

[quote]However, I have heard several people on this list contend that the piston sizes are the EXACT same.<hr></blockquote>This is true.
[quote]KVR is one of SEVERAL sources that I have gathered info from, and they contend that the BR's were made for heavy rear-engined cars (ie: 993TT's) while the S4's are supposed to work better for slightly lighter front-engined cars.<hr></blockquote>By this logic Porsche should not be using the 330 mm rotors, etc. (or larger) in their light weight GT3 race cars. In some ways, what KVR contends does make sense... the S4 caliper is lighter, and that improves handling. But stock Turbo S rotors do not hold up well on the track compared to the ventilated versions most people run with the big reds.

Piston size for the 993 TT, 996 TT, and GT3 are all 36/44.

[quote]I'm not positive about the S4/Turbo S pistons being larger than BR/GTS, but I have seen and heard SEVERAL different places that claim this<hr></blockquote>Hmmm, these sound like places to avoid

drsimon

Thanks to all who posted replies and carried on a spirited discussion after my initial question of what the stock brake proportioning valve of my 1988 Turbo S is. There seems to be conflicting opinions on whether it is the 18- or 33- bar. I suppose the surest way is to remove mine and see the part no. But I read that it is a PITA to access it and I want to do it only once ie when I replace it. I had intended to purchase the 33-bar valve but am now having second thoughts - I would hate to find that valve already installed as stock when I or rather my mechanic goes in to replace it! Perhaps the safest thing to do is to replace the stock with the 45-bar valve which, we all know for sure, is definitely not stock. Are there any potential problems with using the 45-bar valve in place of the 18-bar? Can the brake system, which was initially designed for the 82% front/18% rear brake forces, handle this drastic redistribution of braking force well?

eugene

drsimon.

The 88 TurboS came with a 5/18 bar bias valve.
In fact ALL 944Turbo/Turbo S came with the 5/18 bar unit from the factory. If the car is used strictly for the street only, I would leave it as is. If you do some track driving( Very hard to max braking) replace it with the 5/33 bar unit. To get the full benefit of the 944TS/S4 brakes, you will need more rear bias and the 5/33 bar unit
is just right. The bias valves do not have the Porsche part # on them. You will see the 5/18, 5/33 etc, pressed on it. 5/45 bar would be a bit too much rear bias for the TS brakes. You would need the 5/45 bar unit only if you upgrade the front brakes to Big Reds. My car is a 100% track only car and I have done all of the items that I have mentioned. Also, a side item, I lived in Malaysia from 1966 to 1972 including 3 years in KL. Please let me know if I can provide more detail. BTW, changing the bias valve on the TS(ABS) is a bit of a PITA but not that bad.
NO special tools needed.

Robby

Simon,

Sorry about all the excess stuff on your thread- I thought it was understood that the stock bias valve was a 5/33. At any rate- I have always thought and been told that it is. Unfortunately, there is a TON of conflicting info on our cars braking systems. As far as whether or not there is really any extra piston size or not, well, I think the extra 2mm, if in fact it really does exist, is pretty insignificant, and the extra pad, in my mind, would probably outweigh that by itself. I've looked at the PET scan of Tom's, and I've also e-mailed him back and forth several times, and know that he is very knowledgeable about these cars, but it still makes me wonder when I've spoken w/so many others that contradict the piston size thing. There are DEFINATELY some inconsistencies here...

I have a 1988 Turbo S myself, and am about to replace all rotors, pads, lines, and fluid, and yes, I am going to the 5/33 bias valve too. My car is a street car that MAY eventually see SOME track use, and I believe the 5/33 will really benefit straight-line braking. From what I have been told, the car seems to dive a little less under normal braking, and straight-line stopping distances are supposedly reduced w/the 5/33. When my ABS is invoked, it really only feels like it's the fronts that are being activated. If that is the case, then I am definately not getting the maximum amount of stopping power. The idea behind ABS is to get all 4 wheels to be at their maximum threshold. As a side note, most modern day systems are 4-channel, whereas our cars are 3-channel. In other words, our ABS controls the rear wheels as a pair- this was supposedly to help keep the car more stable in cornering manuevers. If you're worried about it, you can get an adjustable bias valve, but I'm not sure how difficult it would be to adjust after it's installed- it might be worth asking about, but I think you'll be fine w/a 5/33...

Good luck,
Robby

tecart

guys im still not sure you answered if the 5/33 upgrade on my 300+ hp 1986 944 turbo will help me or not, i don track it but want stronger braking power and i dont want lock ups in the rear, and my parts are all stock now, so what ya think? do it or no? also i dont got abs

Ahmet

Thought I'd add another question:

My rear brakes lock up WAY too early, the car won't slow down all that much.

I'm putting new rotors/race pads in next week, if that doesn't solve the problem, is it possible that I have a bad proportioning valve? Very firm pedal btw, but I'm putting in SS lines anyway.

This is on an 86 turbo.
Thanks
Ahmet

Mike 89 NA

Why is the valvae mounted in the fender well on ABS cars? From the descripton here, it goes between the ABS unit and the master cylinder, if so it seems that it could be mounted at the master cylinder. What am I missing here? Thanks,

Mike

Ahmet

I believe it's because of the hydrolic split of the left front/right rear and right front/left rear brakes being on the same circuit, and the fact that BOTH the rear brakes must operate at the given pressure (thus the pressure switch must be made after the ABS pump).
Ahmet

EDIT: I haven't slept in a couple of days, and reading over this it doesn't sound right... I will post again if I think of anyhting.