5/8 or 9/16" wheel cylinders on GLH (220mm drums)

[4 l-bodies]

Okay guys, thought I definitely knew the answer to this but Rock Auto and Mopar parts catalog are making me second guess myself.

85/86 GLH's come with 220mm rear drum brakes. Normal N/A Omni's and other non high-performance vehicles had 200mm rear drums. There are two different size wheel cylinders to go along with the two different drums sizes (5/8" & 9/16" bore).
I always thought the larger drums would take the larger wheel cylinders. I just pulled a wheel cylinder out of one of my lower mile GLH's and it was indeed the larger 5/8" bore. Original? Can't be 100% sure of that, but vehicle has only 60K on it. I know I've never changed it the past 15 years of ownership.

Rock Auto and Mopar parts catalog seem to suggest that the non high performance 200mm (smaller drums) received the larger 5/8" bore wheel cylinders, while the GLH/SC and other vehicles with 220mm larger drums, received the smaller 9/16" wheel cylinders? HUH? The funny/sad part of this is I've probably replaced 20 wheel cylinders in GLH's over the years and always have put in the larger 5/8" bore cylinders.

Anyone have a definite answer or can shed any light on this?
Todd

[135sohc]

The smaller cylinder will result in more applying pressure to the shoes.

[4 l-bodies]

The smaller cylinder will result in more applying pressure to the shoes.

Thanks for the reply. Yes I agree, but with that perhaps less travel (fluid volume) to actuate the shoes? These wheel cylinders are cheap enough that maybe I should buy some of each and experiment. I've never really paid much attention to the rear wheels and brakes in FWD's. What's that sayin, on a FWD they just keep the gas tank from draggin on the ground (lol).
Todd

[GLHS069]

Todd:
I put the minivan wheel cylinders in the rear of my S car and they seem to be working pretty good. I don't remember what size or part # they are though.

[anarchyjet]

The smaller cylinder will result in more applying pressure to the shoes.

Nope...the larger one provides more force. F=PA holding pressure constant means increased area equals increased force. Incase anyone forgot A=pi *r^2

[wallace]

Nope...the larger one provides more force. F=PA holding pressure constant means increased area equals increased force. Incase anyone forgot A=pi *r^2

Negative. The smaller diameter cylinder provides greater pressure for a given amount of force. Formula P=F/A

[shackwrrr]

Negative. The smaller diameter cylinder provides greater pressure for a given amount of force. Formula P=F/A

That goes for the master cylinder. If you apply 100lbs to a 1in cylinder and the fluid then the fluid pushes on a 1 in cylinder it applies 100 lbs of force. If you reduce the second cylinder to 1/2in then the pressure drops to 50 lbs. If it increases to 2 in you have 200lbs of force. Works exactly like gear reduction:

Big in little out you'll have less force but it'll move fast.

Little in big out you'll have lots of force but it'll move slow.

The way you quoted the equation was wrong, you create the pressure with the master cylinder so that is a constant, force is what is applied to the brake shoes.

(P)brake line pressure
(F)force out of wheel cylinder
(A) area of cyl

So it is F=PA of the wheel cylinder/F=PA of the master cylinder=force multiplication.

[wallace]

This formula is incorrect? http://en.wikipedia.org/wiki/Pressure Using that formula for given force (mc pressure) the smaller the bore the greater the force applied.

P=F/A P=pressure output at wheel cylinder. F=MC pressure. A=area of wheel cylinder. If you don't change the bore of the mc but just the wheel cylinder the smaller bore will create more pressure.

[4 l-bodies]

Todd:
I put the minivan wheel cylinders in the rear of my S car and they seem to be working pretty good. I don't remember what size or part # they are though.

Steve,
I believe Mopar FWD vehicles also had some 3/4" wheel cylinders. Did you put those in? Parts guy at Napa told me he had a listing for 86 Omni SE trim that used 3/4" wheel cylinders! Both wheel cylinders in my 86 GLH are K/H wheel cylinders and both are 5/8". Date coding on cylinders suggests they are originals.
Todd

[shackwrrr]

This formula is incorrect? http://en.wikipedia.org/wiki/Pressure Using that formula for given force (mc pressure) the smaller the bore the greater the force applied.

P=F/A P=pressure output at wheel cylinder. F=MC pressure. A=area of wheel cylinder. If you don't change the bore of the mc but just the wheel cylinder the smaller bore will create more pressure.

you are getting the Pressure confused with the Force. Think of it like gears, levers, sprockets, pullies, and other mechanical devices. If the input stays the same size, the smaller the output the less strength the larger the output the more strength

Push a 1" cylinder at 100lbs creates 100psi by using P=F/A Now using that 100psi in F=A*P with a 3/4" bore gives us 75lbs of force. If you use the 100psi in F=A*P with a 1.5 in bore you get 150lbs of force.

[wallace]

I don't understand why you are using multiplication in the calculation? Read this article: http://www.hotrodheaven.com/tech/brakes/

Taken from the article (see below) is how you calculate Pressure output of a MC. Using the same logic at the wheel cylinder you can easily see that a smaller diameter wheel cylinder produces a higher pressure for a given input Force. Plug the numbers he has provided into the formula P=F/A and you will see they add up.

Formula for Master Cylinder Pressure

I have been asked hundreds of times how do you determine the pressures output of the master cylinder. The following information will help you determine the proper size master cylinder:

To figure how much pressure your master cylinder is putting out:
C = pedal ratio
D = pounds of pressure apply by your foot
E = area of you master cylinder
F = pounds of pressure out of the master cylinder
C X D /(divided by) E = F

Example: If you have a 1" master cylinder the area equals 1/2" x 1/2" x 3.14 = 0.785 Square Inches. So, 100 pounds (of applied foot pressure) X 6 (pedal ratio) divided by 0.785 = 764 pounds of pressure.
If you have a 1-1/8" master cylinder, 100 psi X 6 (pedal ratio) divided by 0.9935 = 604 pounds of pressure.

Here is some info on master cylinder with "constant" of 6 to 1 pedal ratio and 100 psi being applied.
3/4" master cylinder = 1359 psi
13/16" master cylinder = 1158 psi
7/8" master cylinder = 998 psi
15/16" master cylinder = 870 psi
1" master cylinder = 764 psi
1-1/8" master cylinder = 603 psi

[shackwrrr]

When calculating the psi output of the Mc you know force and area so you use P=F/A. When finding force output of the wheel cylinder you know pressure and area so you use f=p*a, you have to reverse the equation. Yes what you posted about master cylinder output is right now you know the psi in the brake line, now you need to find what that psi does to the wheel cylinder.

[shackwrrr]

http://science.howstuffworks.com/tra...hydraulic1.htm

[wallace]

I stand corrected Shack you are right.

[shackwrrr]

I stand corrected Shack you are right.

Didn't mean to go crazy, all this crap in my head, sometimes I gotta let it out or I can't sleep at night. I drive my coworkers nuts.

[wallace]

No worries I was wrong!

[GLHS069]

Steve,
I believe Mopar FWD vehicles also had some 3/4" wheel cylinders. Did you put those in? Parts guy at Napa told me he had a listing for 86 Omni SE trim that used 3/4" wheel cylinders! Both wheel cylinders in my 86 GLH are K/H wheel cylinders and both are 5/8". Date coding on cylinders suggests they are originals.
Todd

I'm not sure which ones they are, but I could go through my receipts and double check if you want me to.