Correct Pedal Ratio for Power to Manual Swap?

[hardline_42]

I'm in the process of swapping out the stock power brake booster and MC in my '81 camaro in favor of a manual setup using a Mopar style 1.125" bore MC and a custom adjustable push rod. This is planned only as a temporary setup until I get my C5 setup going but my engine won't make enough vacuum to run the stock power setup in the meantime. Since I'm keeping the same bore diameter and brake setup (disc/drum), the only change I need to worry about is making up the lost mechanical advantage by altering my pedal ratio, right? What is the target pedal ratio for this swap (I've heard 6.5:1)? Can I use my stock pedal and redrill the pushrod hole in the correct location or will I need a different length pedal? Thanks.

[astroracer]

Do a search, there is a ton of info out there. This is a link to Wilwoods info...
http://www.wilwood.com/Products/005-.../pedaltech.asp
I really doubt you will have to change anything but it is a good idea to verify what yuo have.
Mark

[paul67]

9-to-1 non servo

[hardline_42]

9-to-1 non servo

9:1? I have no reason to doubt it but it seems kind of high. That's like 1.5" down from the pedal pivot on a 14" pedal. I'm looking for about 80 lbf at max braking. Wouldn't 9:1 make it excessively soft and too long of travel?

[70 LS1]

The stock C3 corvettes used a ratio of 5:1 for manual and 3.5:1 for power. Most power setups are in the 3.5:1 range and most manual setups are in the 6-7:1 range. Depends on master cylinder size also. Smaller is required for manual. Again I can speak for the vettes, but the manual master is 1" and the power is 1.125".

You can use the stock pedal and redrill, but you will also have to move the master cylinder up higher so the pushrod is level with the connection on the pedal.

[Madspeed]

I think the power booster hes removing is angled upwards and points the rod down.
when he removes the booster and mounts the master directly to the firewall it will be straight.
And If im not mistaken the hole is allready there for this.

It is if its a 2nd gen or at least I would think so =)

[funcars]

1.125 bore sounds pretty good sized for manual brakes. It may be a bit more effort than you expect unless you get the ratio up to at least 6/1. More may be better, depending on your brakes, pads, etc.

I've put manual brakes with balance bars in several cars and found the ideal master size was smaller (3/4) for my applications with about a 6/1 pedal ratio in both cases.

Good luck

[paul67]

looks like we might be at cross purposes , what do you mean by pedal ratio , I mean from foot pad to pin 9" from pin up 1", also the dia of master and size has to be big enough to move correct amount of oil, you need to know calipers piston dia and movement, in relation to master bore and stroke.

[hardline_42]

I think the power booster hes removing is angled upwards and points the rod down.
when he removes the booster and mounts the master directly to the firewall it will be straight.
And If im not mistaken the hole is allready there for this.

It is if its a 2nd gen or at least I would think so =)

The power booster and current MC both angle down and the pushrod is on the lower hole in the pedal. Most people who have done the swap have simply moved the pushrod to the upper hole. However, these swaps usually involve a 1" bore MC. I wanted the weight savings of the aluminum MC and plastic reservoir but the Mopar MC only comes in a 1 1/8" bore. Logic tells me that to move the 1.125" piston I would need more mechanical advantage and that it's possible that the stock upper hole location wouldn't give me enough pedal ratio, but I didn't just want to drill randomly. Would the stock upper hole work for the amount of pedal force I'm looking for?

[hardline_42]

looks like we might be at cross purposes , what do you mean by pedal ratio , I mean from foot pad to pin 9" from pin up 1", also the dia of master and size has to be big enough to move correct amount of oil, you need to know calipers piston dia and movement, in relation to master bore and stroke.

Calipers are stock. The original MC is 1.125" bore so that relationship doesn't change. The only variable I'm not sure of is MC piston stroke but I'm sure it'll be moving the same amount of fluid as the other MC, right? So the only other thing would be pedal ratio to make up for the loss of the booster.

[Madspeed]

Calipers are stock. The original MC is 1.125" bore so that relationship doesn't change. The only variable I'm not sure of is MC piston stroke but I'm sure it'll be moving the same amount of fluid as the other MC, right? So the only other thing would be pedal ratio to make up for the loss of the booster.

With a larger bore M/C you are going to move more fluid at less pressure with more effort.

So yes you either gonna need a bigger leg or a different ratio
you will need to move the rod closer to the pivot pin.
by how much ?
I am dumb with math =) actually very lazy

[hardline_42]

With a larger bore M/C you are going to move more fluid at less pressure with more effort.

So yes you either gonna need a bigger leg or a different ratio
you will need to move the rod closer to the pivot pin.
by how much ?
I am dumb with math =) actually very lazy

Let's assume I'm not gonna be hitting the gym anytime soon (also very lazy). Can anyone give me a formula to figure it out at least?

[hardline_42]

bump

[Skip Fix]

I don't know that you can really make a general statement on ratios. I swapped my 78 Trans Am from the original booster to the 81 Trans Am dual diaphrgam booster 4WD setup and originally kept the same pedal. It had too much leverage. Stopped great no pedal feel. I had to get the correct pedal from GM that moves the rod down.Neither of those pedals has two holes for a brake rod.So power brakes on these took two different pedal ratios.

FYI my roller cammed 455 made plenty of vacuum for the dual diaphragm booster to work and ran 10.90s @ 123.

I'm making a similar swap for an all out race 500ci Pontiac motor in a 79 Camaro(yes different). Looking at that Mopar/Strange M/C also, but what rod do they take? MP has a bracket to bolt a manual MC to the power booster studs.

[hardline_42]

.

I'm making a similar swap for an all out race 500ci Pontiac motor in a 79 Camaro(yes different). Looking at that Mopar/Strange M/C also, but what rod do they take? MP has a bracket to bolt a manual MC to the power booster studs.

I made my own adjustable rod from a rod end and a long bolt threaded into it. When I finalize the placement of the MC and the pivot, I'll measure and cut down the bolt to fit. The Mopar MC has a piston hole depth of about an inch or so (I can check it and post it later) which IMO is enough to keep the pushrod from falling out. If it seems too shallow, I can fab up a spacer/bracket to give it some extra depth and mount the MC in the right place. The Mopar MC has a 3.2" mounting hole spacing while the GM MC has a 3.4" spacing so an adapter piece might be required anyway (or I could elongate the holes on the MC).