Brake Master Cylinder Sizing
 

I have a set of Wilwood GNIII 6 pot calipers on the front of the car and standard FD calipers on the rear (for now). The stock master cylinder is just not doing the job (long peddle) so I`m going to upgrade the whole pedal box and run seperate master cylinders for front and rear.

My question is what diameter Master cylinders will I need?

The piston diameters on the GNIII`s are 1.75" 1.38" 1.38" and the brake pedal ratio will be 6:1 and manual (no booster)

Also forgot to ask, whats the diameter of the stock clutch master cylinder?

Is this an FD? Have you considered a 929 MC yet?

Sorry yeah its an FD.

I have a 929 M/C fitted but its not a Mazda Genuine part. The 929 was never released in the UK so I had to find somewhere online in the US that would ship one and unfortunatley I couldnt find anyone who would send a genuine one. I did ask on here but no one got back to me.

What size rotors?

Go here

http://www.tiltonracing.com/content.php?page=faq&view=9

and fill it all in. They will get back to you with what you need.

13" diameter 1.25" width


Thanks I saw that site a while back and its the one I`ve been looking for :)

You'll want to size the MCs to avoid too much front bias. Something like 3/4" front and 5/8" rear should be a good starting point.

BTW, I'd appreciate any info you have on your front setup. Is it custom?

Yeah its a totally custom setup.

The rotors are from a Chevy WTCC (world touring car championship) car. I had custom bells made up for them by a company here in the UK. The Calipers are Wilwood GNIII`s from a NASCAR. The caliper mounts I made myself from 15mm steel. My inspiration was from this thread on here

https://www.rx7club.com/showthread.p...ght=big+brakes

The kit fully fabbed and fitted by me came to around £500 ($1000 approx) so a big saving on a real kit. The only problem I have is getting hold of pads as I have to buy them from over there in the US also they only do a racing compound which, for a road car, causes a few problems when they are cold (lock up easily, no abs) but this is managable if your sensible.

I`m on a tight budget so the parts are mainly 2nd hand bought from ebay

Theres no reason why anyone couldn`t build themselves a similar kit

I agree with biasing the MCs but I dont think 3/4" will be enough on the front. The 929 MC is 1" so it must be at least this which looking at a few sites gives me 2 choices 1" and 1-1/8" MC

The only reason I havn`t gone with the 1" already is that I do have a 929 MC fitted and the pedal is still long but I dont know if the bore really is 1" as its not a genuine Mazda one. Also speaking to some people with the 929 fitted their overall caliper piston bores are smaller than mine so I can`t make a direct comparisson.

There's a BIG difference between a tandem MC (like stock) and separate MCs (like your racing setup). With dual MCs, your pedal effort is split between the two pistons, so you need either more leg or smaller MC pistons to produce the same line pressure. How much can you leg press?

That said, the area difference between your GNIIIs and a 3/4" MC is >24x. Could you bottom the MC before generating enough braking? 1" of MC stroke would be about .040" of travel in the caliper pistons, so that's a potential issue, I suppose. Perhaps this is another reason that pad knockback becomes such a problem with BBKs.

Please let us know what Tilton and Wilwood recommend.

But running a smaller MC for the rear will give it a higher line pressure (same force over smaller area = more psi), if you use a larger MC in the rear you get less line pressure and less braking force in the rear. Maybe with that brake setup having more pressure in the rear is required, but the above is true.

Yes, to avoid too much front we increase pressure to the rear. This compensates for the increases in front piston area and rotor diameter.

I have those calipers and a similair sized rotors on a late model stock car and use the 3/4" front MC and a 7/8" rear MC. All of the different diameters will operate the brakes. larger diameter MC will give a shorter pedal stroke but higher input pressure(from leg). smaller diameter MC will give longer stroke(softer pedal feel) and less input pressure(leg power). all the charts show what is recommended for a set input force. I would buy a 5/8", 3/4", and 7/8" and do some testing to see what feels best for your driving style.

Right, my bad, but the whole small piston = larger psi is still true.