Another irritating master cylinder post

[John Larkin]

Hehe! Ok, did some searching and found that a guy had used an '85 Station Wagon master cylinder with 1.25" bore on a 12" brake conversion. I'm not interested in the conversion, but the master cylinder is worth thinking about. Apparently, the G-body style master cylinder (vacuum system) has a bore of 0.8". Obviously less pedal pressure would be needed with the bigger bore to push an equal amount of fluid since pedal leverage remains equal. So, the theory is when needed, the larger bore would be advantageous to hold boost, right? The part number is Cardone 10-1863; I was quoted $18 + core. Sounds pretty schweet if it'll fit my booster.

 

[Worthog]

The smaller the bore, the less pressure you'll get with the same amount of pedal travel, but the easier the pedal will be to press for a given pressure. The guy converting to 12" brakes went with a bigger bore because he now needs to move more fluid to get the same amount of braking power (assuming the calipers are also internall larger). If you want more pressure to the calipers, you can go to a larger bore than stock, but it the pedal will be that much harder to push.

Now, if you're not compressing the cylinder all the way right now because you're running out of foot power, you'll actually get more pressure if you move to a smaller bore as you'll be able to get more pressure will less effort, even though the total possible pressure will go down.

 

[John Larkin]

Well, that's the plan: to increase more possible pressure. I don't have a problem with pushing harder if the line pressure will be multipied somewhat by doing the swap. I'm basically looking for the highest line pressure possible without blowing out the system, which I'm sure would be hard to do. As long as the system performs satisfactorily on the street, then I'm for doing whatever it takes at track time. I hear of full-on race TRs using truck master cylinders for weight savings. I'm sure the truck cylinder is used for basically the same performance reasons that I am looking for, only with the vacuum booster behind it for streetability..

 

[Eric Stage I]

Well actually, as strange as it sounds, the smaller bore will give you more pounds per square inch. If your foot presses with 100lbs of force, the .8" bore will produce 199psi. The 1.25" bore will produce only 81psi! However, you will have to push a little further with the smaller bore because it will move less volume.

By the way, I didn't count pedal leverage ratio in the calculation which will increase the pressure greatly.

Eric

 

[TurboJim]

I think you have it backwards.

Hydraulic leverage is what automatic transmissions are based upon. You will apply more force by applying the same pressure to a LARGER piston than a smaller one. It may TAKE 81psi (vs 199) of PRESSURE to generate the SAME FORCE however....

 

[Eric Stage I]

In the transmission, the pump is creating psi, which multiplied across a larger surface will create more force, I agree. However, here, your foot is only creating pounds of force (not psi). When you divide the pounds of force across a larger surface area, the psi created goes down. In braking systems, if you need more psi, you use a smaller bore master cylinder.
It definitely sounds back a$$words though

Eric

 

[salvageV6]

Hey Eric, I used the DPS switch today at the track with my older LED display.

It worked flawlessly recording my boost from 22-26 pound range in 1 psi. increments.

I verified calibration at 22psi. with a very expensive lab. Omega gauge. Now I have to calibrate it at 24 psi. since the alky. is working so well.

I need one with 10 psi. range!

No way can I look at the boost gauge while trying to drive.

If you have any more lying around......

 

[John Larkin]

Well, I caved in and stuck the Regal MC on there. Hood clearance may be an issue with an old style MC so I'll run this one for a while and see what happens. Hopefully I'll be pleasantly surprised.

 

[Louie L.]

Guys I have a question kinda relating and using what has been stated in the above posts.

In reference to manual brake boosters obviously one with a larger bore will take more pedal presure than one with a smaller bore.... correct?
Taking that into consideration what would be the acceptable bore to buy for such a set up?

Ive driven cars with manual brakes before. But i felt the pedal on a Strip GN with manual brakes(1.25 bore) In comparison they were hard as hell.Would a smaller bore soften up the pedal and still get the job done?

 

[Eric Stage I]

salvage, I'm glad the DPS is working well for you. I'm testing one now that has tps input for alchy injection.

Louie, see the web page listed below, #4 on that page in particular. For a manual brake car, you probably would want a smaller bore than 1.25" so you could get more pressure and more pedal travel (less hard).

John, I found the calculation for brake pressure, this is from the Outlaw brake company(http://www.outlawdiscbrakes.com/faq.html) :

Calculating Master Cylinder Line Pressure
Pressure = (Leg force on the pedal) x (Pedal Ratio) divided by (Master Cylinder Piston Area).

Example:
Calculate the line pressure of a 1" bore master cylinder using a 7:1 ratio pedal.
Leg Force (Effort) = 150 lbs.
Pedal Ratio = 7:1
Square Inches of Area (1" Bore Master Cylinder) = .785
150 x 7 divided by .785 = 1,337 psi


Eric