KaiB

When I can, I'll do up a video of the car at the limit and again at full lock up.

Below is a pic of the car exiting the infamous Bitch at Hallett; braking zone is angled, short, fast, downhill from 4th to 2d gear...any problems with the system would be obvious here:

Bill Verburg

I'm sure your brakes work fine, I just believe that you are misinformed as to what calipers are being used.

moving 964 fronts to the back just does not work for the reasons I mentioned. The fact that yours do work tells us that you do not have 964 fronts in back.

964RS used a version of the 928S4, 36/44 pistons, 132x48mm pads caliper that you have on 322x32 front rotors and 30/34, 172x98mm pads on 299x24mm rotors. Cups used the same brakes w/ the p/v removed.

You do not need to know much to see that using 964 fronts w/ 36/40mm pistons 172x98mm pads moves the bias way to the back., way too far to the back to work well.

My 993RS brakes are as far rear biased as can be used and even then the rest of the car has to be set up for them.

KaiB

I just phoned Gamroth - despite the fact that I had no need to, it's my car.

The 964 fronts are indeed hung on the back. You do not want to hear what Jeff says about the fact that they "can not work properly".

Bill Verburg

Well once again lotsa luck

alexjc4

Again just for my understanding, would the presence of a pv turn a bias of 1.0 into more like 1.5, and therefore within normal range? Or is it not that simple?

Bill Verburg

p/v do move bias to the front all Porsche p/v reduce line pressure to the rear by ~50%, the differences are in the knee or pressure at which the p/v starts to take effect, for a 964 w/ 2 piston rear the knee is 45bar, for a 4 piston rear it's 55 bar

so for a stock 964 w/ 36/40 front 28/30 rear the static hyd bias is 1.72, net mechanical bias is due to rotors is 1.734 add the effect of the p/v at line pressures above 55bar(moderately heavy braking) results in 3.467

for a 964RS it's 1.572, 1.713, 3.4, 964 Cups are the same but w/o a p/v so 1.713 is the net bias ratio w/ the same pads f/r

for a 964 w/ RS front and stock 964s in back it's 1.116, 1.216, 2.43

Frank 993 C4S

Why "full lock up"? Did you deactivate the ABS?

race911

Friend's GTL class (yes, torsion bar chassis) is being built with 930 rear calipers in all four positions. Mat Lowrance build, ex-IMSA racer and routinely finishes first overall in every PRC race we have out here in a 3.8L torsion bar chassis car on bias ply slicks (de facto spec for the GT classes locally). And those first overall often include beating Grand Am GT3 Cup entries.

I'll get some time in it next month when it makes its debut and report back.................

(Oh, and back in the Fred Flintstone days we'd run front calipers in all four positions, or even a proportioning valve to limit pressure to the front.)

boxsey911

Back on topic for Clinchy regarding options for brakes...

I've only done the rear brake upgrade i.e. the 4 pots used on C4s (and the late C2s) plus the high pressure (55 bar) proportioning valve. With uprated pads (Hawk blues) and uprated fluid (Castrol response super dot 4). I've found the brakes plenty good enough for track days. With this set up I get no fade over a 20 minute session.

The thing about this set up is that the rotors and pads are relatively cheap. When you consider the cost of the upgrade to big reds, etc and the much higher cost of the rotors and larger pads (on the front), you would have to go through a heck of a lot of standard rotors and standard size pads to make the upgrade economic.

Bigger brakes look better and allow you to brake slightly later. These would be the reasons I would upgrade if I had the cash to spare.

KaiB

Nah. I've had a glitch since end June that I just can't find (or take the time to find). If it rains, I won't go on track without it tho - I have locked the brakes up on a wet street at 60mph no problems at all.

clinchy993

Cheers Steve

It is a job for the future and just updating the rears is the way to go I think
I have a few weeks of fun ahead fitting and re- fitting the car
I have just ordered RS rear seat carpetset from Dave @ Southbound

What with the KW V3s /Powerflex bushes/engine mounts/RS fog ducts /front strut-brace /Speedline copies + tyres /G-pipe/ momo s/wheel /new front wing and rear quarter + paint and fit

The piggy bank is getting empty LOL

Thank god my 993 is all up to date !



I will be at the RS day Snetterton as a guest of some GT3 Buddies are you going

bigunit_271078

whats the best approach to sourcing the "the 4 pots used on C4s (and the late C2s) plus the high pressure (55 bar) proportioning valve" - through OPC/design 911 etc or second hand? How did you source yours Boxsey?

This is on my to do list at some pt as well. I have the RS upgrade on the front (it does look the nuts) but only the old 2 pots on the back (1990 C2)

Cheers

clinchy993

Forgot to say thanks to all for the responses

ThomasC2

Remove the proportioning valve instead of buying the 55 bar version as they did on Cup cars. I did it 3 years ago without any downsides. the car is much more stable under heavy braking and the pad wear is more even.

It's easy to do, just remove the valve and connect the two pipes with each other. You can bend the pipes gently with your hands to make them line up.

Thomas

boxsey911

Andrew, as you already have the RS brake upgrade at the front you want the larger piston 993 rears to balance them. These have 34/30 pistons. They are the same as 964RS rears. They bolt straight on to your existing rear rotors.

My own rears came off a 928 S4 and I got them off ebay for £150 but they needed a lot of work to refurb them. I also had to swap the bleeds and feeds so that they were oriented correctly for a 964. The p/v was bought new from Porsche (about £60).

Clinchy - no, I can't make Snetterton but will be at Oulton Park next Wednesday with some local 964RS lads