What is our pedal ratio? It's raining here and I have no garage or I'd go ahead and measure myself. I just figured somebody on this board would know already so I didn't have to do any dirty work .

 

Which pedal

 

He's refering to the brake pedal.

However, the brake pedal is not what is measured, its the mastercylinder that has a specific measured bore and stroke.

I don't know what the stock stroke is on any of the MC units that are found in 3rd gens. bores are I believe mostly 13/16", with the ecception of the larger and harder to press 7/8" bore MC found on a few factory performance package cars (they were often complained about because extra effort was required to apply brakes- though you had a better feel from them for road racing with this MC)

 

Actually I need pedal ratio to fine tune (re-engineer) my from brakes. I know effective master cylinder pressure and that in turn tells me what my input force requires. From that there is mechanical leverage at the pedal and I need that ratio to calculate pedal force into brake force. This is so I can decide which master cylinder and what bore pistons to run on the front and rear calipers. Having 13" brakes there's a LOT of mechanical advantage for a street car, I just don't want to loose any feel so I'm trying to figure all of this out. I'm also looking at the brake pad compounds (wilwood polymatrix) to figure which pads will be a good balance between feel and stopping power. For the street I'm going to go with T pads (the lowest Cf but sqeek free) and I'm looking at either C or H pads for the track.
I have to balance everything carefully because this car will see rain and snow. I don't want to lock em up with just a big toe on the pedal! My mom's new Volvo has this problem but I think it's too much power assist and the squishy pedal.
The other reason I need the ratio is to see if it's possible to go without the power assist.

 

So, you're looking for measurements on the pedal arm; from the pivot point of the arm to the pivot for the pushrod, and from the pivot point again to the approximate center of the pad?

 

vsixtoy, actually, stock "non performance" was a little bit larger- the J50 m/c was 24 mm, and the J65 (82-88 rear disc, the one that the weaklings complained about) was 25.4 mm (1 inch). I've always seen the J50 referred to as "24mm", but the J65 as "1 inch"... go figure. I don't know if GM couldn't make up their minds (metric vs SAE) or if that was the aftermarket's doing!

 

JPrevost, Are you intending on welding in new brackets for an aftermarket pedal assembly? Thats alot of work for a designated street car. Do you intend to put a balancebar assembly also?

If you plan on just retaining the stock pedal assembly, might I suggest also to first put the binders together, throw in a bias valve, and then see what feel you have after the bias is set with 1/2tank of fuel. If the pedal is not sensitive enoughat that point, THEN change the MC to a larger bore.

 

Yeah.

vsixtoy, I have the ability to modify the pedal ratio to some extent. There are a lot of things I'm trying to take into consideration (cough~ mechanical engineer blowing things out of proportion).
I have a Tilton prop valve that'll be esential to getting good feel. I'm also going to first try a metering valve to first engauge the rear brakes. Besides, the rear is still drum, still looking for an LS1 rear brake setup in the junkyards that I visit once in a blue moon.
The issue really lies in the calipers. They WERE going to be the Billet Superlites with differential bores (1.75 and 1.38) but now that I'm getting the Forged Superlites it's either 1.75 or 1.38. I think I'll go with the standard master cylinder and 1.38 calipers if I keep power assist. If I go manual I'll go with the 1.75 calipers and the same small master cylinder. To get even more feel I could go with the larger master clyinder and 1.38 .... too many options , I like options!!!

 

Bringing back from the dead, incase anybody wanted to know this information for their own brake design;
Pedal ratio is 3.3:1 and can be easily made to go all the way to 3.8:1 by just drilling through the bracket and bracket to relocate the pivot lower. With some basic welding you can decrease the ratio to give you a better angle on the pushrod but increased pedal effort (if nothing else changed).
I've got my bracket modified to so I can use either the stock 3.3 or the 3.8 ratio. Since my goal is to minimize the pedal travel slop I've left it at stock for right now.
The other downside to increasing your pedal ratio is the master cylinder travel. You MIGHT bottom out the pedal before you get optimal master cylinder stroke. With the pedal assembly out I didn't measure the pedal travel but when it goes back in I'll post up the data so people can get the correct stroke master cylinder for their application.
Oh, and another thing, I'm trying out a Hydrobooster inplace of the vacuum assist. With the relatively large cam I'm running my idle assist will be very minute and I never did like the noticable delay in assist. Suposedly the Hydroboost is a much firmer pedal with linear assist depending on input force... and I think I'll like that. Only one way to tell, try it out .