tom1283

I bought my first RX7 a month ago. It's an 84 GSL that was raced in ITA. I have been pulling it apart the last few weeks to check over everything and I have been making a list of things to change. One of the projects for the car will be to go to a dual master cylinder setup with a balance bar. I don't know much of the history of the car so I can only assume it is still running the stock GSL calipers, which leads me to my questions.

1.
Are the calipers in the pictures below from a GSL?
2.
What are the front and back piston diameters?

I need the piston bore diameters to size the master cylinders.

RX-7 Chris

iTrader: (19)

the pics are not working

tom1283

ok I attached the images. Hopefully this works.

RX-7 Chris

iTrader: (19)

worked. Yes that is stock.

not sure of the piston sizes though

RX-7 Chris

iTrader: (19)

this is what rock auto has for the front: 1.71" Overall Length / 1.996" Piston Diameter
Rear: Bore Size=1.37"

tom1283

Thanks. I didn't know where to start looking.
I'll keep this updated as I'm going through the mod. I should have enough time to get an excel sheet going tonight.

j9fd3s

iTrader: (3)

piston size is in the FSM too.

trochoid

iTrader: (6)

What master cylinder setup are you looking at? I'm always curious about different mods, Pics or a link would be helpful.

tom1283

I don't want to spend tons of money on this so I'm just going to go with the fairly cheap and easy to get a hold of wilwood parts. I haven't decided yet if I want to run a remote adjuster to the dash.

jinxed4dub

iTrader: (7)

^^umm do it. That would be a worthwhile mod. adjust for diffrent weather and driving conditions/styles on the fly without having to do any real work. IMHO that would be a sweet mod.

mustanghammer

iTrader: (3)

Contact Charlie Clark at www.kcraceware.com. They use dual master cylinder setups on all of the E Production RX-7's they build. While they run run GSL-SE calipers front and rear and he can probably tell you the master cylinder diameters you need.

Actually what KC Raceware does in hang a two pedal assembly with tripple master cylinders - two for the Bakes and one for the Clutch. There is a plate they weld in to the fire wall that supports the pedal assy - probably attaches to the cage too. If you go this route - the remote adjuster is worth doing - not that expensive.

The interesting thing is that I have seen allot of very successful E Production cars that use the stock power master cylinder with the GSL-SE brakes. One even won the runoffs in 2007 with power brakes - Thomas Thrash. So I am not 100% convinced that that the dual masters are necessary. You can control brake bias to a certain extent with an adjustable proportioning valve on the stock system. But you do get more adjustment with dual cylinders.

Do you have a specific braking issue you are trying to correct? I have to use stock GSL brakes and the stock pwr master cylinder on my race car. I really have no issues with the setup. The trick is to make sure that you control weight transfer under braking so you don't unload the rear tires.

tom1283

I had no real problems with the stock brakes. I'm just changing them cause I like the adjustability and feel you can get from the setup. But the biggest reason is because I can. It keeps me busy and gives me a chance to brush up on my math.

I plan on reusing the stock pedal box and brake pedal with some modifications. The prefab pedal assemblies tend to be pretty expensive and I would have to make some mounts for them.

I'm mostly done with my spreadsheet and it looks like a .625 front and .7 rear will work well. I'll check back over everything tomorrow to make sure I didn't make any mistakes.

blackdeath647

iTrader: (2)

make sure you document and take pics please. i'm curious as to how this will turn out. never heard of a dual break booster set up before...maybe i'm too much of a noob? lol

elwood

iTrader: (1)

First, I agree with everything mustanghammer said. I did a balance bar setup on a FWD racecar, and I liked it, but I never did and A to B comparison so I really can't vouch for the improvement. As for the benefits, there are two that come to mind:

1. Fine adjustability
2. Elimination of hysteresis in the rear brake system

(With a rear prop valve, if you lock up the rear brakes, you have to release the pressure drastically to unlock them. Not so with a balance bar setup.)

Glad to see you're using some math skills. That's often avoided. Pat youself on the back

I'd check the diameters with the company that mustanghammer mentioned. I screwed up my calcs the first time and had to make some changes. Pay special attention to the lever ratio and what the resultant pedal force requirement will be. I like fairly high pedal force . . . YMMV.

tom1283

After playing around with the numbers some more I think I'm going to go with a .7 front and .625 rear.

I have attached my spreadsheet in case anyone else wants to play around with the numbers. It's quick and dirty so it might be hard to follow. The left column is all calculations to find a theoretical master cylinder size required. It uses a very simplified weight transfer equation to guess at dive. I didn't have real numbers for some of the inputs so they are guesses. The right column you can put in what size master cylinder, pedal ratio and a few other things. It gives out the resultant torque at the wheel. You can compare these numbers to the theory side for a sanity check.

I threw this together quickly so there may be some math errors.

RX7_Brake_Calcs.zip

elwood

iTrader: (1)

Nice work! I checked your math on the front rotor effective diameter and a few other calcs -- everything looks good to me. One other thing I ran into when I did this was excessive dash panel flex. Once you put 120 lbs of force on the pedal, it might not feel as stiff as you would like. I solved it by bracing the master cylinders directly to the strut tower bar.

tom1283

I finally got most of the work done this last weekend (it sucks only being able to work on the car on weekends). I attached some pictures of the progress. I just need to reroute the brake lines. Unfortunately it'll be a while before the car will be running as you can probably tell from the pics. So I won't know how well the setup works for a while.

tom1283

more pics