Does anyone have any experience fitting a larger than stock master cylinder to a OEM Brembo car? Any recommended products?The goal is to improve the pedal feel for track use without going to aftermarket calipers.

 

More feel with less braking power ?

I'm assuming you already have SS brake lines ?

You might have better luck going with a single stage brake booster instead of playing with the master cylinder itself.

 

I have SS lines.

Having to push a bit harder doesn't mean you have lost braking power. I prefer a firmer pedal I can modulate with pressure rather than with excessive pedal travel. I personally find the stock set up pretty mushy, even with regular bleeding.

 

Aaah, if you change the master cylinder section to a larger one, you will loose some clamping force on the calipers. The force on the pedal is divided by the surface area of the master then multiplied by the section of the caliper pistons to generate force. This is how a hydrolic system works.

The Brembo equiped Z have a 2 stage brake booster. These boosters typically have longer travel. If what you want is less travel, try and find a single stage booster that can supply enough power. The "regular" booster might do the trick, but I'm not sure.

 

Kolia, thanks for your input.

OK, I'm not sure about the physics here.. If pressure = force/area, and the area is increased, can't I create exactly the same pressure with more force?

In other words, yes, I have to apply a greater force to the brake pedal, but I can end up with the same line pressure and therefore clamping force at the caliper?

The bigger the master cylinder bore, don't I need less stroke, but more force, to reach a given pressure? So as long as I don't go so large as to be unable physically to generate enough force for a given pressure, I should still have the same clamping force but with a shorter stroke?

I'm not sure how the booster comes into this equation other than as a force mutiplier, so maybe this is where I am going wrong?

From experience I know that on race cars brake pedal forces are generally higher with unboosted dual master cylnder setup, but you have a much harder pedal with less travel and it is much more accurate and easy to modualte.

I don't know exactly how to get that on a boosted road car system, but I want to get closer to it than the stock set up without changing calipers. I will investigate the single vs 2stage booster idea.

 

This is what I mean about loosing power. The force multiplication won't be the same, thus requiring more leg power for a given decelleration.

As it is, I'm literally standing on the brake pedal while braking at the race track. I'm pushing as hard has I can, using my little 200lb of self as leverage. And still, I'm not locking the wheels. So in my case, I would not be able to push harder on the pedal to compensate for a larger master cylinder, I'm already maxed out.

On another note, biomechanicaly speaking, the more force a muscle exerts, the less fine and precise it becomes. So it’s questionable as to the gain in feel will offset the lost finesse.

You’re right about the booster less feel. A twin master cylinder setup with no booster will have the best feel you can get. Pedal stroke will be pretty large as you need a longer pedal to give you some mechanical advantage, but it will be solid. To do that, you’ll have to loose all the electronics, including ABS. It’s a compromise most people aren’t ready to make on a street car.

There are also master cylinder brackets that help in holding the brake pedal – master assembly in place. You get to a point where the firewall is actually flexing. These brackets are there to help.

Let us know if you find a solution that works.

 

overall pedal feel has as much to do with pads as it does anything else.

 

I agree with this. My pad progression was OEM, Axxis ULT, HP+ and now PFC01 Front/PFC97 rear's. The PFC pads have excellent bite and feel and I do have a much better pedal overall than with stock, plus pad fade has been eliminated.

It still has a long way to go to approach the feel of a Formula car pedal, though

 

ss lines, motul RBF600 fluid... your set

 

I have the stoptech stage II setup on my OEM brembos and it still doesn't feel as firm as my friend's Evo 8. I really like how firm the brakes are on that car.

 

The OP already has SS lines and good fluid.
And he is unwilling to change the calipers.

 

Yes I have all that, RBF600, SS lines, brake ducts, good pads, etc.

I'd say I am reluctant to change the calipers rather than unwilling. They have enough torque to lock my semislicks, so it's not about ultimate stopping power. The only reason to change calipers is to accomodate a larger rotor if needed for more heat control. I might still need to do it, but am currently content to try to optimise the current system with the original calipers.

It's about confidence in the ability of the sytem to stop over and over again in hard tracking and in the accuracy of the pedal to modulate braking force. The first is an issue of controlling heat to avoid fluid fade, and the second is more subjective. My car stops fine until the brakes overheat, I just don't like the long travel to initial bite and the slightly spongy feeling you get from the power assist.