Second gen pedal ratios

[Skip Fix]

So got some measurements for you all on an old pedal and ratios using upper hole for manual(where brake light switch bolts) makes it 6.5:1. Regular power brake hole for single diaphragm booster (11"?) 3.53:1. 79-81 4WD dual diaphragm booster(9"?) 2.8:1

When I first swapped the '81 4WD system on my '78 I kept the same "regular" power brake pedal on the new booster. Stopped good not much feel until I bought the correct 4WD pedal with less ratio.

[badazz81z28]

Interesting, did the 4 wheel disc on 3 sec gens with the stock pedal and no complaints. Works fine.

[Skip Fix]

maybe Tobin can comment on the assist of a single 11" vs a dual 9". I can tell you that dual 9" would work fine unless idle vacuum got below 9"

[badazz81z28]

I'll have to try it and see how it feels.

[Skip Fix]

When I did the swap it stopped great but hardly much feel as it had a little more ratio as well as more assist. Not terrible but not as good as the lower ratio.

Now my daughter's Kia Soul if you blow on the pedal it sends you through the windshield!

[kevs79]

So are you saying it I decided to go with a manual set up the I would use the upper hole for the rod? Where do you put the brake switch then? Someday I am going to be putting a TA disc brake rear end and have been reading up on it and alot of people like that set up with manual brakes instead of power.

[NOT A TA]

So are you saying it I decided to go with a manual set up the I would use the upper hole for the rod? Where do you put the brake switch then? Someday I am going to be putting a TA disc brake rear end and have been reading up on it and alot of people like that set up with manual brakes instead of power.

You want to have the correct ratio to work with the rest of the system. Which isn't necessarily the upper hole (although that hole might be correct or close for your particular application). In my case I'm installing twin manual master cylinders and needed to position a new hole where it would interfere with the brake switch bracket hole. I drilled a couple new holes for some small bolts to hold the bracket (Why GM used such a thick bolt I have no idea). Modified the bracket. Welded a piece of threaded rod to fill the old upper hole and drilled a new hole for the clevis mount press in stud.

https://static1.pt-content.com/images/pt/2016/02/020_zpsy2pmsuvx-1.jpg (http://s240.photobucket.com/user/NOTATA/media/The%2014%20Car%20Performance%20Therapy/020_zpsy2pmsuvx.jpg.html)

https://static1.pt-content.com/images/pt/2016/02/003_zps2i4v1uye-1.jpg (http://s240.photobucket.com/user/NOTATA/media/The%2014%20Car%20Performance%20Therapy/003_zps2i4v1uye.jpg.html)

https://static1.pt-content.com/images/pt/2016/02/002_zpsxhwfost0-1.jpg (http://s240.photobucket.com/user/NOTATA/media/The%2014%20Car%20Performance%20Therapy/002_zpsxhwfost0.jpg.html)

[Skip Fix]

I kept the switch bracket on the upper hole and clevis I used on my '79 Camaro project.

[bkbkbk]

So got some measurements for you all on an old pedal and ratios using upper hole for manual(where brake light switch bolts) makes it 6.5:1. Regular power brake hole for single diaphragm booster (11"?) 3.53:1. 79-81 4WD dual diaphragm booster(9"?) 2.8:1

I know this is an old post but I'm corresponding with Wilwood right now and just took pedal measurements on my 73 firebird equipped with manual brakes from factory. My pedal seems to have different measurements from the one above so maybe they changed over time. Being a manual brake car, my pedal is already set up with the push rod in the upper of the two holes position and here are my measurements if it helps anyone:



A. Distance from pivot point to middle of push / pull point (center of brake pedal pad) = 11.5 inch
B. Distance from pivot to point of push on master cylinder = 2.0 inch
Pedal ratio = 5.75 inch
Note: The factory cast iron master cylinder appears to be a 1.0 inch bore according to my calipers.
For instructions on calculating ratios: https://shop.wilwood.com/blogs/news/pedal-ratios-and-how-to-find-them