6 Piston Front Master Cylinder sizing

I'm looking for some guidance on my master cylinder sizing. Dual circuit system. On the front, I'm running 6 piston calipers with 27mm / 31.8mm / 38.1mm cylinders.

Pedal ratio is around 5.5 : 1

I can't find my notes on working it back to the correct master cylinder size.
A little calculation shows that piston configuration is the equivalent to 4 x 40mm. Fairly small, so I estimate that 5/8" or 11/16" master would be adequate. That, of course, is also dependent on the rear master cylinder size.
So you're running dual MC's with a balance bar? In other words, you should be able to balance front/rear so long as you're in the right general range for sizing of both front and rear MC's.....

Bob's estimate of sizing looks right to me - might lean a bit in the direction of the 11/16.

Howard Jones

GT40s Supporter
I have a GT40 with an effective peddle ratio of 6.5 to 1. The front calipers are 4 X 1.75 pistons and the rear are 4 X 1.38 pistons. My masters are .625 front and rear. So calculated piston area F= 9.6 sq ins total R= 5.9 sq ins total. This setup was arrived at after two caliper changes and many pad selection changes.

You will have a lot less effective peddle pressure due to the much shorter peddle ratio, 5.5 to 1. Your front total piston area is 6.67 sq ins. You can see that even if you run the same master cylinder sized as I did you will have a LOT less brake clamping pressure than I do with the same leg effort. Really a lot less! This is going to be a very stiff peddle.

So run the smallest master cylinders you can find. 5/8 is the most common smallest size . If you can find race rear pivot Alcons and Bremos,, they go down to .59 or 15mm.

Read up on pad coefficient of friction. You may need to go with a very high friction pad material to try and reduce the stiff high effort peddle. "Normal" street friction numbers are in the .4 -.45 range and race pads can go up to .7 and above. There are a lot of trade offs here so knowledge is you friend .

What do you have for the rear? Maybe you could use the ones you mention for the rear and get some larger piston sizes up front. Wilwood makes a 1.62,1.12,1.12, 6 piston I believe. That would put you at total 8.08 for the front and your 6.67 total for the rears. That seams like something that can be balanced out with the balance bar.

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