The Jensenator originally had a booster, as part of my complete molestation of it I went with Miata front brakes, RX7 rear brakes and a 2nd gen RX7 master cylinder. I removed the booster entirely for various reasons (mostly clearance and parts availability issues), I figured the pedal would take more effort and hell yes it does. 
I want to move the push rod pin position for a more manual brake friendly ratio.
So what's the best pedal ratio for this? If it matters, the pivot pin for the pedal is above the push rod pin position.
Carroll Smith suggests a pedal ratio of 4:1 to 9:1. I have a Mathcad sheet setup up to calculate the pedal force required based on a number of different variables. I can send it to you if you have Mathcad.
4:1 to 9:1 is a pretty big spread. Sorry, don't have Mathcad. If I get you the measurements, can you crank it through?
The typical Wilwood / Tiltons are 5.5:1 or 6.2:1. I have the 6.2:1 in my E30 and its just about perfectly weighted. It goes without saying that there was a lot of time getting the sizes of everything from pistons to masters correct... but assuming you have done most of that already that would be a good starting ratio to work from.
Instead of the Miata setup, why not just go with a Tilton pedal setup? It is already engineered for what you are trying to do - and its all standard so you can use off-the-shelf masters, rebuild kits... and so forth. I think mine was $100 from a circle track place (out of a wreck). It came with 3 masters that I was able to use two of (1" & 5/8" for the clutch). I bought another... for a total outlay, not counting fittings of something like $157. Having them be separate and adjustable with a balance bar is awesome so you can swap sizes and fine tune that last little bit (or change setups for rain from inside the car).
You can kinda see what I'm talking about in my gratuitous hotlink of my racecar below. if you aren't familiar... these are overhung style but they also make straight ones so the masters are on the other side of the firewall.
6:1 for manual is what most race cars use.
44
J-man, help me help you. To give you a worthwhile answer, I need to know caliper piston diameters, rotor diameters, corner weights, master cylinder bore, and maybe a couple other things i'm too tired to remember right now, like CG height, wheelbase, tire sizes, etc.
problem with increasing pedal ratio is you increase pedal travel, and at some point your pedal hits the floor before building sufficient pressure to pull the decel you're looking for.
june 2008 GRM. read it. know it. live it.
Woohoo. That's a lotta measurin'. 
I'll get some numbers for ya. TIA.
You can take a bit of a shortcut if you're basing the brake system on another car. For example, my MGB is going to have Miata parts at the wheels. In order to keep a similar pedal feel and braking effect to a known good car (the Miata), I'm matching the master cylinder to the pedal ratio so I end up with the same overall ratio of hydraulic pressure to pedal pressure.
You also need the coefficient of friction of the pad.
In reply to Keith: The whole setup is actually very similar to the Abomination, which also used Miata everything except the master cylinder, which was a 13/16" RX7 piece. When I set up the brake system for this car, I used the Abomination's setup as a guide, since it brakes REALLY well. The one major difference between the two is the pedal ratio, since the Triumph did not have a booster and the J-H did.
The car stops well, you just gotta be a gorilla to be able to get enough pedal pressure. My co driver at the last event was downright scared of them (of course, he's a 'stick boy'. 
) I want to keep the 'hard' pedal feel, which would probably go away with a bigger M/C.
So you've got a you've got a known good setup. Multiply the pedal ratio by the area of the master cylinder on that car. Now adjust your master and pedal on the new car to come up with a similar number. But the booster screws with things, I don't know how to deal with the booster. I think you multiply the number by the boost ratio.
If you're at the point where your driver can't generate enough braking power, chances are you won't soften the pedal considerably by putting in a smaller booster or increasing the pedal ratio. I had this happen with my Locost - the initial pedal setup was super-hard (woo!) but took a tremendous shove. I put in one step larger masters and the pedal still feels like a rock, but mortals can now lock wheels if desired.
Toyman01 wrote:
Tell aunt boy to man up. 
Those bony lil' legs need some help, don't they?
Okay Angry, here's my measurements.
Total weight with fat daddy in the saddle ~2100 pounds. Accurate CG height and corner weights not available, these cars were 49% front/51% rear when new and I'm going to have to say it's probably closer to 50/50 now. I'm guesstimating the CG height at 16".
Front rotor diameter 9.30, rear same.
Don't know the coefficient of friction, but I'm using stock Miata semi metallic pads front/stock RX7 semi metallic pads rear.
Front caliper piston diameter stock Miata 2.01, rear stock 1st gen RX7 1.375. (Stock Miata rear calipers are 1.250 bore.)
Master cylinder bore 13/16.
Adjustable proportioning valve for the rear.
Wheelbase 94".
Tire OD 22", width 10.50".
Current pedal measurements: from center of upper pivot to center of pad is 13.750, center of M/C pushrod pin to center of pedal pad is 10.750. The M/C pushrod pin is below the top pivot. If I'm doing the math right, that's a ~ 4.58-1 pedal ratio and the pedal feel is like hitting a 2x4 nailed to the firewall; all that happens is I take up the free play and the pedal just does not go any further after that. It stops but man does it ever take pedal effort. Everything in the brake system is free and moving easily.
From what I've seen somewhere around 6:1 is supposed to be about right. That seems to mean the M/C pin should be around 2.29 inches from the center of the top pivot.
This pedal box is a cast iron bitch to remove and I'd ideally like to do this only once. So I genuflect to your vast knowledge of things anchor related.
In reply to Keith: I'm not running a booster and I think that's my problem: a power brake pedal ratio being used with non-power assist brakes. I considered going with a larger M/C but that can get expensive if I get it wrong.
No smaller bore on the master not bigger you need to up the ratio.
But better to drill new hole in the pedal its okay if the rod runs at some angle but make sure it don't hit in the bore at full stroke.
44
I figured I could drill more than 1 hole; moving the pivot point toward the fulcrum .500" would give me a 5.5-1 ratio, another .200 would make it 6:1.
OK J-man, sorry it took me so long to get back to you. If you change pedal ratio from 4.5:1 up to 6:1, you will reduce the pedal force required to generate a particular pressure by 33%. The downside is that you will increase the pedal travel by the same percentage. Certainly can't hurt to try it. Unless the pedal hits the floor before you've built enough pressure to stop the car in time, and then it truly could hurt to try it.
On my V8 944, pedal was redrilled to give almost 2x the ratio, which meant almost 2x the travel. Long travel hurts driver confidence, regardless of the actual deceleration the car can generate.
33% more pedal travel wouldn't be a bad thing. The pedal is literally rock hard as soon as the free play in the pedal pushrod is taken up. Another 33% of travel would only be around 1/2" - 3/4".
I'm going to drill new holes for 5:5-1 and 6:-1 ratios and redrill my master cylinder mount plate to scoot the M/C up about 1/2" (the plate is made of 3/4" thick aluminum) so the pushrod won't hit the bore.
Thanks to all!
