Mike_964

I have recently upgraded from 2 pistons calipers to 4 pistons in the rear of my 91 C2. I changed the bias valve in order to compensate for the larger caliper, but....
Last year when I tested the brakes I got the following figures 1.8 rear and 2.3 front, but when testing after the brake upgrade I got 2.3 both front and rear.
So what is wrong? If there is anything wrong? The test is done in a brake tester that tests one axle at a time, and then maximum pressure is applied. Can I conclude anything from such a test?
I have been on the track without any problems, except from when I drove of at the end of the straight because I couldn't brake in time, but then again it started to rain, so I thought that it was just lack of traction, but now after the test I started wondering if that was the only problem...
Advice is appreciated...

Best regards,
Michael

Bill Verburg

what are you measuring?

line pressure?

Mike_964

Braking force on a test stand...
And no, this is not my highly modified Porsche

Steven C.

I have never seen one of these? Must be a new EU requirement

warmfuzzies

hmm

My MOT tester always does lbs pressure tests front and rear,,,, not like that though

kevin

Bill Verburg

Not sure what that is measuring

Do you still have the stock 964 front calipers and rotors?

There are several different small body 4 piston calipers. You want the 28/30.

W/ the stock front and correct 28/30 rears and at low line pressure(<55bar) the f/r ratio will be ~1.7:1

boxsey911

There's some great info in this thread:

https://rennlist.com/forums/showthre...ighlight=brake

which will save Bill from going through it all again

Mike_964

Thanks for your input. Yes I had read the thread, before doing the upgrade ;-)
I have the stock front calipers and rotors.
On the rear the rotors are stock.
The new upgraded rear calipers are the 28/30 ones, and the bias valve should be the 55 bar. Below are what I ordered from Porsche when doing the upgrade.
1: Bias valve (55 bar): 964.355.305.10
4: Damper 30mm: 964.352.096.00
4: Damper 28mm: 964.352.096.03 replaced by 996.352.086.01

But I might not be able to conclude anything from the brake test I have done..
Just out of curiosity, how do you test if the right and left side of the car brakes equally, if not by testing on the tires themselves? In my perspective this is the only way to make sure that the applied force is the same on both side. It ensures that brake line pressure, calipers, brakepads, rotors and tires, all matches up to the same brake force in real life.

Best regards,
Michael

Geoffrey

It isn't a bias valve, it is a pressure limiting valve and the two are much different.

Mike_964

That’s funny. I have read a lot of posts where it was called a bias valve, but when you mention it I also recall some calling it a p/v... But being a Dane living in Sweden and trying to express technical matters in English isn't always easy.

Geoffrey

The key is that the pressure ratio between the front and rear will remain constant until the rear line pressure hits the 55bar pressure limiter and at that point, the more front pressure you have, the more front bias you have.

Bill Verburg

What Geoffrey means is that the line pressure is the same f/r <55bar. The brake torque ratio due to diffferential piston sizes f/r is ~1.7, and the brake torque ratio f/r due to differential rotor diameters is also ~1(same f/r)

The net effective braking ratio is variable and depends on all the above + suspension, speed, aero package and so much more.