Does anyone know if the 740 manual pedal box is more compact than the 240's?

Ya, I know about that page . I'm off to the JY to see if I can find a small 7" booster and pedal assembly to put together an underdash system for the 122.

In answer to your question: nobody knows.

I do have threecount'emthree 740 pedal assemblies for 740s. 2 with hydraulic clutch pedals and 1 with a cable pedal. The 740 has a plastic clutch pedal which may or may not be good for you, and the 740 has a more integrated assembly than the 240's modular style. The pedals are pretty long in both cars but then again they're probably longer in the 122. That box is NARROW though.

If you're thinking what I think you're thinking, it would look sweet to ditch those pedal boxes altogether and smooth the firewall. Thought of a floor mount, hot rod style? And you've decided you need a booster, and can't get the appropriate combination of piston sizes to get the travel/force you can live with?

Greg helped me pull a 240 pedal cluster yesterday...$20 sure beats $200 right now. It's a nice compact (ish) set-up that could be narrowed without moving the pedals closer together. It'll work or at least I'll make it work. Now to get back to our regularly scheduled programming...cue the elevator muzic.

Maybe not...doesn't look like a booster is going to fit with the steering column. A reverse mount race setup is so compact it's almost silly to NOT just buy one of them and get it done.

So I'm not leaning towards a Tilton 600 series three pedal cluster. Ban for the first person that points out that I don't really "need" this.

Next problem is that the Tilton master cylinders (clutch = 3/4" - front's = 0.7" and rear = 0.875") are almost twice the price of the Wilwood parts. Don't know why they are different.

Hmmmm. And Wilwood doesn't make a 3 pedal cluster right?

They do but it doesn't seem as compact and the reviews of the Tilton balance bar are more favourable.

The big problem with that setup (aside from price, which isn't REALLY that bad) is that the master cylinders are more than 2x the area of your current one. The pedal ratio doesn't look as high either. You're going to need some much bigger caliper pistons if you plan to run anything reasonably racey like this unit. That's not a problem you're going to get away from if you go with an unboosted dual master (or any dual circuit, more likely) system, though, so don't blame the cluster.

I think that's money well spent, really. It'll be sexy, it'll clean up the firewall and will use underhood space that you're not using now, it'll give you a balanced brake system, it'll give you a throttle pedal ready for a cable (I assume?), and it'll let you adjust the pedals to exactly where you want them.

It's been a really, really long day...I know that I'm adding a MC, but my understanding (foggy though it may be) is that if I can get things sized correctly, the dual curcuit dual MC system should be less pedal effort. I would size the front MC a little smaller than what's there now (7/8") so that there is more pressure (less pedal effort). As the MC's are a split system (one for the front and one for the rear) for any given amount of pedal travel, the force applied by the pedal is split between the MC's. I haven't measured the stock pedal ratio's in a while, but 6:1 rings a bell for the system (I'd normally run out and check...but not tonight).

For a given pressure at the calipers the amount of pressure exerted at the MC is fixed by P=F/A (sorry, I've got to work this out) - so if I have a single MC and I exert 100 lbs of force through a 6:1 pedal, I've got 600 lbs of force at the rod, yielding 996.43 PSI for a 7/8" MC. Now if I put two MC's in there, then I split the force and would likewise split the pressure.

I need more pressure applied to the front, so the balance bar can reduce the pressure to the rear (weight transfer and all that stuff), so I should be able to adjust the bias so that the force (combined) is about equal, but the braking would be used more effeciently.

Now, my plan was to decrease the size of the front MC to .7 for my same data as above, you get 1556.93 PSI vs 996.43 PSI (56% increase). Then you have built in bias for any given pedal force when it is simply split and even more effective braking when you do the rest of the math and balance it out.

This is how I understand it...

Add to this the fact that I've never driven a car with a balance bar, but I do know that I would also have to move the same amount of fluid either way with the same calipers (not yet set in stone). I was happy with how it all worked before, so I don't think I'll change things all that much, but have to run the numbers with the different front and rear set-ups.

Oh, you've got a 7/8" MC in there now? Dale's got a 3/4", which is why I thought. His pedal effort was nice, firm but nice.

You're nearly doubling the master cylinder area, which means it takes twice as much force on the MC rod(s) to develop the same pressure. The pressure is converted back to force in the calipers, and since you haven't yet doubled their area, they need the same pressure to develop the same clamping force as they did in the car with the single MC. Putting a yoke on the horse doesn’t mean the horse can suddenly pull two plows.

If you want to do the maths, I'll make a few assumptions:
- Existing MC = 7/8" diameter
- Existing pedal ratio = 6:1
- New MC = 3/4" front & 7/8" rear
- New pedal ratio = 5.75:1 (the highest that 603 pedal box gives you)

So a 100# pedal effort in the old car gave you 600# of push at the rod, and with 0.601 square inches the old MC makes about 1000 psi in the line. To get the same pressure, the new front MC requires 442# of force at the rod, and the rear MC requires 600# of force at the rod. The force required at the balance bar pivot is 1042#, and with a pedal ratio of 5.75 that means 181# pedal effort. Said another way, if you put 100# force on the new pedal and keep the balance bar in the middle, each MC will get 287.5# force at the pushrod and the front MC will produce 651 psi while the rear MC will produce 478 psi.

Going further, I’ll need to make more assumptions:
- Your old rear caliper piston was 1.5” and the new rear caliper is 1.75” IIRC.
- You like the existing pedal effort
- You’re okay with buying larger front calipers
- We’ll keep rotor diameter and pad material out of this
- Given the above, clamping force must remain the same for a given pedal effort.

1000 psi in each 240 rear caliper produced 1767# clamping force. To get the same clamping force from the new pedal box and Supra caliper, you need 735 psi in the rear lines. With the balance bar in the middle and putting 100# of leg strength into the pedal you’re only making 478 psi, so we need to wind in more rear brake.

How much more: Hmm… 575# force at the bar pivot, using the front MC as a fulcrum and the lever is 1:2 ratio, you’re developing 287.5# at the rear MC. You need to make (287.5x735/478=) 442# at the rear MC, so the lever ratio must go to 1.54:1, meaning it will be a bit more than 75% of the way towards the rear MC, giving you the remaining 25% of the force towards the front MC. That leaves (575x0.25=) 144# pushing on the front MC.

144# pushing on the front MC develops 325 psi in the front lines. Your old brake system applied 1000 psi on two 1.38” pistons per side, developing 2991# of clamping force. To get the same clamping force out of 325 psi, you need to bump the front calipers to a whopping 2.42” 4 piston unit, which is 9.2 square inches. The biggest caliper on Wilwood’s site is the TC6 at 6.9 square inches… You might need to factor in some bigger rotors and stickier pad material!

If your current front rotors are 11.75” (I can’t remember…) your brake radius is about 4.875” (assuming a pad width of about 2”). If you go with a 13” rotor (easily done on a 17” wheel) you will have about 5.5” radius, or 13% more torque, so now instead of 9.2 square inches you only need 8.1 square inches, still too much. I think you’ve got Poly-Q compound in the front, right? If you go with BP-30 you’ll have 38% more friction from 100*F and 28% more friction at 500*F.

Lots to think about…

Good Lord!

How long did that take??

Thanks Matt - I've worked through this all once before for this car and I'm not too worried. We'll get it worked out. All stock 122's are 7/8" brake MC and 3/4" clutch MC. My pedal was harder than I'd like, so I was going to go with a .7" or even a .625" front and was toying with either a 7/8" or 3/4" mc rear respectively.

I think the idea of the balance bar dual set-up initially was to allow moderate pedal effort but by splitting the system (horses to the plow or whatever) you can dial in the pressure needed for the front and back. I alluded to this in my previous post. It takes less to lock up the rears under heavy braking as the weight transfer is to the front. So it no longer takes as much force to get equivalent braking. While my car is pretty good, we know that it will lock up the rears before the fronts so dialing back that pressure via a BB and MC sizing should make it work even better.

I'm not set on the Supra sliding calipers at the rear. I haven't moved over to this project just yet, but I was thinking that sizing them like the 240 but with a Wilwood caliper would work. I've got to get some rotors (not the drilled kind ) and get some measurements done.

The pedals are a done deal (just not ordered - I need more time to work out the rest of the parts).

How is the balance bar supposed to reduce pedal effort, or am I misunderstanding you? I know you know all this so I must be missing something you're saying or not saying, but the bottom line is that when going from a single circuit MC to a dual circuit, you need to keep the total MC area to total caliper piston area ratio the same, otherwise effort will change.

If you bump up the rear brake effectiveness, whichever way you choose, it will make it much easier to deal with the front. You can dial the balance towards the front and increase the pressure in the front lines, which means you can go with a smaller caliper. Seems counterintuitive, but it's the troof.

But yeah, we'll get this figured out.