Cupcar

Question for Bill Verburg-

Will the 23.81 master cylinder from a normal vacuum boosted 993 bolt into a 964?

This would solve the problem you point out. <img border="0" alt="[typing]" title="" src="graemlins/yltype.gif" />

Konstantin

just in case you want to buy the brakes I have the 928 S4 front calipers $420
the Gt3 front and rear calipers and the Gt2 yellow six pistons calipers you decide. Rotors from 304 to 380 mm and pads are also available.
<a href="http://www.968turbo.de/partsforsale.html" target="_blank">http://www.968turbo.de/partsforsale.html</a>

Konstantin

Cupcar#12

Just an FYI...
if you are upgrading your brakes to the Medium or Large Options you Need to upgrade the M/C and the Brake Bias. Recently a friend of mine completely locked up the fronts of his C2 due to these parts not being installed, flat spotting the tires and punting the car off the track. He had only upgraded the brakes themselves, nothing else. you really want ABS to be there when you need it. I his situation i believe he pushed though the ABS system due to a spongy pedal and locked them up (BTW this happened twice at different tracks). The factory made the modifications for a reason and i for one would not go without. I will be upgrading my car to Bob's specs soon.

Bill Verburg

Bob, I do not know. A casual look at them suggests that probably yes, but as always the devil is in the details someone needs to try it and let us all know.

That is the same situation w/ the transverse mount S4 size 993C4 rear calipers and their 322x28mm rear rotor, a casual glance at them says maybe yes, but I haven't done that either, and don't know anyone that has. It would be a nice rear option.

Bill Verburg

It's the combination of 44/36 frt pistons in either medium or large calipers with 34/34 rear pistons in either a medium or a small caliper that causes the problem.

A slave to master ratio of 25.241 is known from experience to be insufficient

A stock 964RSA is 21.502
a '92 Euro 964RS is 16.393
a 993 w/ 23.8mm mc is 18.655

a 964 C2 20.64mm m/c w/ 44/36 and 30/34 slaves is 24.827 as I said uncomfortably close to in sufficient. An insufficiently large m/c will not be able to move enough fluid volume to properly service all of the pistons, What ends up happening through a combination of tolerences is one or two wheels will lock first.

The basic bias is set by the f/r piston ratio, modified slightly by rotor radius, pad material & pad area.

Bill Gregory

I have 993 calipers on front and the stock C2 92- rear calipers with the Turbo 60 bar bias valve with the stock C2 master cylinder. Having had it on the track for several days, I can't say I noticed a mushy pedal. Ditto experience by a friend that just installed big reds front and back with stock C2 master cylinder. Ditto experience with another C2 that installed S4's in front and larger calipers in back. Installing the larger 993 C2 master cylinder would certainly firm things up if a mushy pedal is a problem. But I don't know if we can say everyone will experience a mushy pedal. And maybe we're really talking shades of a definition here, ie, what's mushy to me may not be to someone else.

I'll be installing 993 rears to better balance things with the front S4's. After driving with that combination, I'll consider if I want/need to pull out the 60 bar bias valve.

Perhaps most importantly, I'll likely route air hoses from the front valance to the center rotor hubs to aid in cooling, having developed heat cracks in the front, non-frozen, slotted rotors after 3 days on the track. The 993 rears plus, if need be, removing the bias valve, will reduce some of the front braking demand. That, plus air, should hopefully, better!, gain a longer rotor life. Also returning to slotted frozen rotors, which I usually use, and have been pleased with in other sizes.

ThomasC2

I agree with you Bill. I have the same setup, except for the red 3.6 Turbo rear calipers, and my brakepedal feels harder (read better) than with the stock setup. And there has been no problem at all with locking up. The ABS also works as it is supposed to do.

Bill Verburg

I did not say that the stock 964 vacuum boosted m/c would be too small. Only that it was uncomfortably close to the value where issues will occur. A slave to master ratio of 25.241 feels fine and it is perfectly acceptable 99.99% of the time but that 0.01%( i.e. a panic stop where the brakes are jammed on suddenly w/ full adrenilin pumping) will cause a lock up of the smaller piston caliper(the back). Not e that the stock 964 m/c w/ 44/36 & 30/34 has a slave /master ratio of 24.827, slightly onthe good side. Also note that the factory ratios are between ~16 and ~19 for their RS models

The physics of the situation is such that a bigger slave/master ratio will always entail longer travel and more force multiplication than a smaller ratio. Remember that we are talking about a very small travel under all conditions.

Ergonomicly the human foot can modulate a higher force over a narrow range of positions better than a lower force over a wider range of positions. This is why a larger m/c will feel better to the vast majority of users. You relly need to drive the same car back to back to fully appreciate the difference

I agree and have stared so many times that the 44/36 front(either large or medium) & 34/30(either medium or small) rear is the most desireable for 911/964/993

Just throwing out something else to chew on and be aware of.

tonytaylor

My C2 has " big red" 992tt front calipers 965 front discs and 3.6 turbo rear calipers with 951 "964RS" rear discs and steel braided hoses.
I would agree with Bill Verburg that the pedal feel is noticably " mushier" than the stock set up (and mushier than any other performance car I have driven - until their brakes started to fade!)
Since I have had issues with ny ABS since the upgrade I would also agree that the discrepancy in master cylinder size may be affecting the ABS

Bill Gregory

</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"><strong> a panic stop where the brakes are jammed on suddenly w/ full adrenilin pumping) will cause a lock up of the smaller piston caliper(the back). </strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">Why would the rears lock up with the 45-55 bar bias valves in place that our 964's have? Isn't that why Porsche put the bias valves on, starting in 1984, to prevent rear wheel lock up?

tonytaylor

Bill Gregory,

When I first had my brake upgrade fitted to my C2 a full "crash" brake would cause the rear wheels to lock ( I think I have posted my experiences with this on a previous thread), In my case the problem resolved itself when I upgraded the suspension from the original factory to H+R coilovers and RS ride height. Perhaps the front end dive under this "crash" braking was unloading the rear sufficiently to cause the rear tyres to lock - not something you would expect in 911.

Bill Verburg

Bill, the car I was referring to had neither a p/v(it shouldn't be needed w/ the 36/44 & 30/34 combo anyway) nor ABS. I doubt that would have helped, but one never knows.

What happens is even if the the hyd. bias is correct that only tells what happens once the pads are in full solid contact w/ the rotor. Prior to that there is a period of time(small but potentially critical) where all of the hysteresis in the system is working itself out.

The rate of fluid transfer from the m/c is directly proportional to the speed of the actuating shaft, the smaller rear pistons will move further & faster than the front, contacting the rear rotors first, there is a short window of time where the rears are in full contact and the fronts are not, the smaller the m/c the larger this widow and the more potential for rear lock up. The bigger the m/c the shorter the window and the fewer issues. Once sliding the rears can remain locked due to the lower coefficient of sliding friction.

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

Cupcar

In Tony's situation of rear wheel lock up under heavy braking, why doesn't the ABS fire and unlock the rear wheels? The 964 has 3 channel ABS and should be able to operate on the rear axel only eliminating lock up there.

Hard to imagine wheels are so unloaded the ABS can't work, could there be a problem with the ABS system?

Am I missing something? <img border="0" alt="[typing]" title="" src="graemlins/yltype.gif" />

Bill Verburg

Bob, When it comes to ABS I am over my head. I was involed in converting an RS America to the 993 front & rear calipers w/ larger sport rotors. We had feed back from a contact in Weisach that the entire ABS unit needed to be changed. It was dutifully and expensively changed, but what the specifics were was never explained.

My personal opinion was that that a bigger m/c was needed because that always worked in the non ABS 911 world.

tonytaylor

I was pre-warned that the larger brakes could over-come the ABS, so I assume that this was a known problem.
The ABS unit was tested prior to the upgrade (because of locking - make of this what you will) and found to be sound.A sensor was found to have failed.
The ABS system does work, albeit only above 50-60mph, when the distinctive pedal "pulse" can be felt, but it still locks the (now - see previous posts) front tyres at lower speeds.
Oddly the ABS is better in the wet.