4 piston calipers

4 PISTON CALIPERS

BMW Models	E34, E31, E32, E28, E24
Difficulty	AVERAGE
Price estimate	400 - 500

Purpose:

I am working on a set of brackets to install wilwood type calipers 4 pistons on the front of the car.

Front rotor size will be: 324x30. The rotor size 334x32 will need different brackets and might not fit all the 17" rims.
Rear rotors size will be: 328x20, but I have some issues locating the caliper.

Front setup:

Rotors: OEM BMW 324x30 p/n: 34-11-6-756-087 (21.5lb) or 34-11-1-159-895 (22.9lb)
Calipers: Wilwood Superlite 1.75" pistons will create a tad more braking power.
Other calipers will work as long as they have the same pad location and mounting points.
Bracket: Racingking.ca design, steel.

Calculations: Piston area= 2Ap for single piston and 4Ap for 4 pistons

Master cylinder diameter = 25mm
The target is a brake ratio of 70:30

Ft Piston diam	Piston Area	Rr Piston diam	Piston area	Brake Ratio
1x60mm (850i)	5,655 sqmm	1x40mm (M5)	2,513 sqmm	69 : 31
1.75" wilwood x4	6,207 sqmm	1.38" wilwood x4	3,848 sqmm	61 : 38
1.62" wilwood x4	5,319 sqmm	1.00" wilwood x4	2,027 sqmm	68 : 32
1.75" x2 - 1.62" x2	5,763 sqmm	1.12" wilwood x4	2,533 sqmm	69.5 : 30.5
Corvette 2x40mm	5,027 sqmm	Corvette 1x1.75"	3,104 sqmm	62 : 38

Finding the right caliper piston size:

1- A bigger master cylinder means less pressure, and therefore higher pedal effort.
2- A bigger master cylinder means less pedal movement.
3- Sliding calipers act like they have twice their actual piston area.
4- Bigger pistons in the calipers mean more force and, therefore, less pedal effort.
5- Bigger pistons in the calipers mean more pedal movement.
6- Bigger rotors generate more brake torque, which makes the pedal effort feel lighter without affecting pedal travel.

Calculations:

Area: PI x Radius^2
Master cylinder stroke= Caliper piston stroke/(Master Area/Caliper piston area)
The E34 and upgraded E28/E24 have a 25mm master cylinder, the idea is to keep the stock master cylinder (at least on the E34 and upgrade to the E32 one for the E28/E24)

My setup right now is 1.38" pistons at the front and 40mm 540i calipers for the rear, the brake pedal feel is same as stock 635csi brakes.
I still want to install some 4 pot calipers at the back, and will try different front piston diameter calipers to get a similar pedal feel.

Brake booster for the E28 and E24:

If you have a 535i E28 or a 635csi, and like me you are tired of the old brake booster setup with the brake bomb, the valve and the hydraulic booster, you can replace all that with the following:
- Brake booster from an E34 525i Wagon (with master cylinder, vacuum hose and reservoir)
- Power steering hoses, power steering cooler and reservoir from an E34 535i.

I decided to install the power steering cooler considering that the volume of ATF in the system is greatly reduced compared to the old system.
The brake lines going to the ABS pump need to be bent down a little bit to make enough room for the new brake booster.
So far the brakes are running great with this new setup but I need to test them more on the race track.

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