[therealdb1]

Let's not too technical on a technical forum, eh?

Don't confuse force with pressure. Actually the bigger master cylinder piston will generate less pressure. Simple physics tells us that pressure is force per unit area so if the force is equal but we make the area bigger the pressure drops.

However, as has been mentioned, the available braking effort at the disc is about displacement of the fluid acting on the caliper piston. If the caliper pistons did not need to move the small master cylinder might be ok but you need to move the caliper pistons themselves to take up the running clearance before you start to apply force. Consequently, a small master cylinder piston will need to travel further to provide the same volume of fluid to give the same action of the caliper piston. Therefore, by definition you will need to push the brake pedal further to get the same braking effort at the road wheel. Ok if you like long pedals I guess!

Forget the servo that is only to reduce the effort in pushing the pedal but for safety's sake, and to avoid running out of pedal travel, the master cylinder and calipers really should be matched. Bigger calipers need bigger master cylinders to provide the appropriate displacement of fluid and keep pedal travel to the intended movement.

BTW The rear brakes are a red herring too as I think the hydraulic circuits are split and the master cylinder piston that operates the rear is the same size regardless of model.

[david in germany]

Originally Posted by GeorgeKLT

So coding the car with +spbr would compensate for the weaker master cylinder on my 318d?

Coding will adjust the brake bias as well as increase the brake response.
Remember, BMW designed the car, BMW allowed an option for larger brakes, and BMW experts decided that only coding was needed to allow safe operation of the car after a brake upgrade.

[therealdb1]

Originally Posted by david in germany

Coding will adjust the brake bias as well as increase the brake response.
Remember, BMW designed the car, BMW allowed an option for larger brakes, and BMW experts decided that only coding was needed to allow safe operation of the car after a brake upgrade.

In that case why did BMW factory fit larger master cylinders to their cars with larger brakes?

Forget all the ABS, DSC and stuff, the brakes are still a hydromechanical system and not a computer system.

You cannot overcome the laws of physics with coding!

[david in germany]

Originally Posted by therealdb1

In that case why did BMW factory fit larger master cylinders to their cars with larger brakes?

ask the engineers this question. Potentially, larger /higher performance cars will go faster than the lower model cars be heavier therefor require more braking potential creating more heat. Heat kills brake fluid, more fluid needs to move to dissipate the heat to ensure fluid life.

Originally Posted by therealdb1

Forget all the ABS, DSC and stuff, the brakes are still a hydromechanical system and not a computer system.

you can’t forget these VERY important variables. Coding changes these variables including fluid transfer.

Originally Posted by therealdb1

You cannot overcome the laws of physics with coding!

We are not trying to, coding again modifies electronic variables such as controlling fluid flow. If you change out the master cylinder but maintain the same electronically controlled fluid flow you have now retarded the capabilities of the system.
Stop thinking of only hydraulic pressure/flow that is only ONE of the variable. BMW engineering verified all of these variables and approved JUST the brake caliper, pads and rotors as being a SAFE and effective upgrade to the cars WITH coding.

I can assure you, my wife’s 328i with only 335i front brakes and coding (no other mods) will out brake my factory 335i brakes.
This is fact and not opinion or hypothesis.

[david in germany]

Originally Posted by Andystobbs

Not trying to rock the boat at all. I was very grateful for the advice given to me when I was asking about the nuances of adding bigger brakes.

David in germany was a responder in the thread I posted in and advised I code SPBR. At the time I had the wrong DSC module - electrics only, not hydraulics - after bricking my original during software update. I sourced a module with correct part number but the software was different and consequently NCS wouldn't access it. Fearful of another failed programming session, and another bricked module I never coded SPBR and still maintain the car is fine.

Some people say the addition of SPBR adds feel to the brakes. Personally I'd rather not have them so sharp that I'm in danger of going through the windscreen.


I have since successfully programmed my DSC but still left off the SPBR option.

I'm about to retrofit (around Christmas time hopefully) a 6 cylinder DSC complete - electrics and hydraulics - in readiness for active cruise control. So we'll see if that changes anything. Technically shouldn't as donor car is a 330d.

I did run my wife’s car about a week prior to coding. In the case of my wife’s 328i it turned out to be a dangerous decision. The 328i 2011 model ) would jerk side to side in medium to hard braking and you would feel barely in control of the car. Once I coded it, it is perfect. So for safety reasons I recommend coding no matter what.
Others have said they didn’t have this issue, good for them. They got lucky.

[Silver Streek]

Interesting info about brake master cylinder and how different sizes effect the baking:

History of Hydraulics
The term Hydraulics comes from two separate Greek words referring to water and pipe. As far back as the 6th century BC, the behavior of water and its flow through pipes has been studied.

The father of modern Hydraulics is French mathematician Blaise Pascal (1623 - 1662). Pascal established Pascal's Law and is the namesake of the SI unit for pressure, the Pascal (Pa).

Pascal's law states 'a change in pressure at any point in an enclosed fluid at rest is transmitted undiminished to all points in the fluid.' In Pascal's 1646 barrel experiment, he fitted the top of a barrel with a long vertical pipe. While this barrel would have no problem containing the pressure of the fluid it would take to fill it, the additional pressure from the weight of the fluid in the tube attached to the barrel would cause it to eventually leak due to the increase in pressure.

Mathematically, Pascal found the amount of pressure resulting from this column of fluid would be a function of the height of the column and the density of the fluid. Later experiments with U-tubes filled with water with varying size pistons and tube diameter’s led to the invention of the Hydraulic press.

Brake System Hydraulics
The hydraulic brake system on a vehicle starts at the master cylinder and ends at the calipers. The master cylinder is responsible for establishing the amount of pressure in the brake lines and calipers. A mechanical lever, known as the brake pedal, accepts a mechanical force input from the driver’s foot.

The force causes a push-rod to push the piston assembly within the master cylinder. In turn, the master cylinder increases the pressure in the hydraulic brake system. This increase in pressure exerts additional force on the calipers to squeeze the brake pads to the rotors.

Any portion of the brake system that carries the hydraulic brake fluid is part of brakes hydraulic system. This includes master cylinders, hard brake lines, proportional valve, flexible brake lines, race car bias pedal box and calipers.

Race Car Hydraulic System Performance
If you have ever changed a master cylinder, brake line or caliper on a vehicle, you are probably quite familiar with how air trapped in the brake's hydraulic system affects its performance. A spongy pedal or brakes that simply don’t work are usually the result of air getting into the hydraulic system. Proper bleeding of the system to remove the air from the system restores the functionality of the hydraulics.

While brake fluid and its incompressible nature do an excellent job of transferring a pressure increase across the entire system, air simply compresses and fails to transmit the effort put in at the pedal.

Since most of today’s vehicles incorporate an ABS system into the hydraulics, its important to follow the vehicle manufacturers recommendation’s for bleeding the system. In most cases, bleeding is started at the caliper furthest from the master cylinder. Using a suction type bleeder can be helpful in removing stubborn air pockets from the brake system.

Hydraulic Ratios
On a factory vehicle, a group of engineers determine the exact amount of pedal travel and pedal resistance they desire for the driver. By varying the bore size of the master cylinder, the pedal ratio and the total piston area of the caliper’s pistons, engineers establish total pedal travel and pedal effort.

The amount of travel that OEM’s set generally pretty long with ABS system activation occurring a couple inches off the floor in a properly working system. A long pedal travel minimises effort while maximising modulation (the ability to vary the degree of braking applied).

For a racing application, there may be a desire to shorten the pedal travel to allow full braking power to come sooner. One way to accomplish this is to change the master cylinder to replacement featuring a large piston and bore area. While this will reduce the pedal travel, it will come at the cost of reduced modulation and increased effort.

Of course braking systems feature some type of assist (vacuum or electric), may not reveal the increase in pedal effort in full. Conversely changing to a master cylinder with a smaller bore will result in increased pedal travel, decreased effort and improved modulation.



Motorsport Tech Session - TOP TIPS
Pedal effort is directly proportional to the amount of pressure created in the hydraulic braking system peak pressure will reach between 600 and 1200 psi.

A stroke of about 25 - 35 mm at the master cylinder should be needed to reach the peak pressure.

The bore diameter of the master cylinder, the pedal ratio, the total area of all the calipers pistons and the pressure in the brake system all influence the pedal effort.

These also influence the amount of travel required to reach the peak pressure.

TO INCREASE PEDAL EFFORT:
Increase Master Cylinder Diameter Choose Lower Pedal Ratio

Remove or Decrease Brake Assist

TO DECREASE PEDAL EFFORT:
Decrease Master Cylinder Diameter Choose Higher Pedal Ratio

Add or Increase Brake Assist.

Three factors influence the line pressure in the brake system: the pedal force, the pedal ratio and the master cylinder bore sizes.

About 100 to 150 pounds of force from the leg to the pedal is a good working range.

The pedal ratio multiplies this effort.

For example a 5.2:1 pedal ratio would multiply a 100-pound pedal force to 520 pounds force going into the master cylinder. With a 13/16 bore (area=0.5185) the pressure in the system would be 520/0.5185 or 1003 psi.

IF YOU INCREASE MASTER CYLINDER BORE DIAMETER:
Increase Pedal Effort

Decreases Total Pedal Travel

Reduced Push-rod Stroke to Master Cylinder Decreases Line Pressure for a Given Pedal Force

IF YOU DECREASE MASTER CYLINDER BORE DIAMETER:
Decrease Pedal Effort

Increases Total Pedal Travel

Increases Push-rod Stroke to Master Cylinder Increases Line Pressure for a Given Pedal Force

[GeorgeKLT]

I was able to finally install a new 335d brake master cylinder.

The braking has changed for the better.

The pedal was very soft before and now it is firmer. Before when I was pushing the brake pedal regardless of how hard I was doing it, the braking was linear.

After the brake master cylinder upgrade, the initial bite is very strong.

It was worth it, so I strongly recommend this upgrade to others as well.

To complete the process I will code the car with +spbr in the near future.

Thank you all for your help.

[HoraceMihai]

GeorgeKLT : Does your BMW accelerate the same after the big brakes upgrade? I am asking you because the front 348 mm brake discs for the 335i E90 weigh 11.3 kg, which is 3 kg more than the stock 300 mm brake discs. I am curious if you have some acceleration time worsening given the higher unsprung mass of the big discs?

Same question is for you guys too: Does bigger and heavier brake discs affect acceleration? And did anyone of you notice better stopping distance with the 335i brakes as compared to the stock 300 mm brake discs? Thankx