Suggestions on brake master cylinder sizing.

BBBrad · Post by **BBBrad** » Thu Jan 03, 2013 9:43 pm

Hi everyone, been awhile. I'm building a McRae Porsche 550 at the mo. I'm setting it up for race. I've put some good brakes on it and am in the process of sorting master cylinder sizes. Obviously the car is quite lightweight, approx 650kgs, and its mid engined. Brake specs are...
Front rotors are 295mm, wilwood 4 pot calipers with 43mm pistons.
Rear rotors are 275mm, wilwood 4 pot calipers with 43mm pistons.
It's setup with a wilwood dual cylinder pedal box and adjustable bias bar. Pedal ratio is 5:1 and I will be able to get the full 1.1 inch stroke out of the standard wilwood master cylinders.
So I need to know what size cylinders to put on, for front and for rear?
I have been searching on the net for answers but I think asking might be more helpful.
Thanks in advance.
Brad

Grrrrrrr! · Post by **Grrrrrrr!** » Thu Jan 03, 2013 11:28 pm

What's the front/rear weight balance with full fluids and driver onboard?

BBBrad · Post by **BBBrad** » Fri Jan 04, 2013 7:45 am

Hi, not quite sure yet as its not complete. But I'm guessing at close to 50/50. Maybe slightly heavier at the rear.

DexGT · Post by **DexGT** » Fri Jan 04, 2013 9:20 am

Sounds like a cool project ,
I have fitted wilwoods all round and a pedal box and I got some books on the subject.
I found some spread sheets on line that let you put in the weight, piston sizes etc and you can play with cylinder sizes , I have had a quick look and can't find the one I liked best but if you want I can find it.
I think you probably have too much braking on the rear but you may be ok with a big (1" or 1-1/8") for the back and around 3/4" front.
I have used Les Hunter http://www.leshunterauto.com/contact.htm for parts and he would be a good person to ask for suggestions , he has been doing race brakes for a long time now.
I hope this has helped

BBBrad · Post by **BBBrad** » Fri Jan 04, 2013 9:44 am

Yup, sounds close to what I have come up with. Be great if you could find that info. Cheers

nz_climber · Post by **nz_climber** » Sat Jan 05, 2013 2:24 pm

Sounds like a awesome project, keen to come check it out sometime and catch up (I am down in Bulls)

I think 1 inch might be a bit big unboosted, probably want to look at 7/8 - 5/8 range (I run 11/16 front and rear on my Lotus 7 replica - similar weight)

But yeah talk to Les, he has heaps of knowledge on everything brakes, and helped me tons when building my car, including giving me heaps of advice of setting up the master cylinder and push rod length to ensure it all works correctly together.

DexGT · Post by **DexGT** » Sun Jan 06, 2013 8:09 am

Here is a link to the spreadsheet I used http://wascb.org/?p=407 , it allows you to play with different caliper piston sizes/ master cylinder sizes etc and see the effect.