Help with Custom Hydroboost Brake System

[ccgjag31]

Good day all, I have been beating my head against a wall over a custom brake job and I've come to you all for help. As a mechanic for the military, we typically only replace OEM with OEM. So knowing how to do things like pedal ratio and which master cylinder bore size is appropriate, etc..not my forte. I have a 67 Chevy Impala that I swapped to wilwood disc brakes up front, and now i'm trying to swap a hydroboost in it as well. I've read articles and other forums talking about how "easy" it is, but I'm just not finding it so. I've got roughly 3 questions that hopefully won't end up leading to more. 1) measuring my stock pedal ratio, I've got a 14" pedal from pivot point to middle of pedal pad. I've got 5" from pivot point to pushrod. 14/5=2.8 wait what? 2.8:1 pedal ratio?! Does that sound right? Shouldn't it be 4:1 or 5:1? 2) The wilwood kit I bought has 4 piston calipers that contain a piston area of 4.80^2 and I have 2 master cylinders, one which is 15/16 and the other is 1" bore. The 1" bore would give me a .78^2 piston area. I found on the interwebs that by dividing the caliper piston area by the master cylinder piston area you get what you'll need to have as the proper pedal ratio. so 4.8/.78 = 6.15. So 6:1 pedal ratio would be appropriate? Most articles I've seen on hydroboost state that 4:1 is ideal. Thoughts?3) The 15/16 master cylinder is a wilwood which, of course, is stunning aesthetically. The 1" is a standard GM. Is there anyway I can make the 15/16 work for my hydroboost setup? Also, there's mention on the wilwood that 1200 PSI is maximum. So I'm pretty sure that makes it a no-go, but figured I'd ask. I believe I covered everything, but if not please feel free to ask any questions you may have.

[toddz69]

A 2.8 pedal ratio is in the ballpark for an OEM power brake setup - many of them are in the 3:1 ratio area (like yours). I've never seen anything that says 4:1 pedal ratio is ideal for hydroboost. My F250 has a pedal ratio of 6:1 with its hydroboost and my Bronco has a 6:1 pedal ratio and I converted it to hydroboost as well.

Personally, I'd run at least the 1" bore master with your setup.

Todd Z.

[stab6902]

1) That pedal ratio sounds a little low to me too, but it could be. Based on your description it sounds like you're measuring it correctly; you can double check based on the images here: https://techtalk.mpbrakes.com/how-to...ng-pedal-ratio

2) There's a lot more to brake system sizing than whatever you found on the interwebs. Things like brake pad friction coefficient, rotor effective radius, tire rolling radius, etc all need to be taken into account too. I'd start off using your factory power brake pedal ratio and not mess with that part of the system unless absolutely necessary. You should be able to dial in your setup with the master cylinder size.

3) I would contact Wilwood to give you recommendations on master cylinder sizing since you're running their wheel brakes. Or others with similar setups might be able to chime in with their experience. You'll likely need an adjustable proportioning valve and some testing to get your front/rear bias dialed in too.