Manual Brake System Trouble Shooting: Expected Line Pressure?

[vstol]

Originally Posted by Ron Sutton

I find the challenge is ... when someone goes to one of the aftermarket 6-piston caliper brake set-ups looking for bad ass brakes ... they are occasionally disappointed with the braking result ... because the pistons are so small, the total piston "area" is insufficient ... and they didn't account for that in the total system

Most car guys don't know the piston area formulas ... so when you glance at the piston sizes written on paper ... and there are 6 of them ... it seems they should add up to more brake caliper clamping force ... but often they do not.

The problem is compounded if we make any one (or more) of these changes:
A. Utilize brake calipers designed to have a strong power booster ... now running without one.

B. Utilize brake calipers designed to run .50 to .65 CoF racing brake pads ... and now have street pads in the .35-.42 CoF Range.

C. Utilize brake calipers designed around systems typically with a 6-1 or higher pedal ... but some factory pedals have less.

D. Utilize brake calipers designed for a 2700# car ... now on a 3500# car.

The charts below, use the standard brake formulas all brake engineers use for manual brake systems. It accounts for every aspect of braking except the tire. This graph shows several front brake calipers ... ALL using the same pedal ratio, same brake pads & same master cylinder size.
*The pedal ratio, brake pads & master cylinder size do not represent what comes from the factory. I made these 3 items a constant only for comparison.

The rotor sizes are different ... to better represent how they're being sold & used. Regardless ... look at the braking force numbers at the bottom. The 3 common GM factory brake calipers are for Gen 2 F-body, G-Body & Impalas are highlighted with blue columns.

Go down to the 2nd line from the bottom ... marked "Brake Torque" ... and compare the factory numbers with aftermarket options. When you wonder why some don't stop stronger ... simply go to the line marked "Caliper Piston Area x2" ... and you'll see important differences.

There is nothing wrong with any of these brake calipers. But if we are building a better brake system for our PT cars ... we need to better understand the total picture. We can't put a caliper on our cars with significantly less piston area ... without running the proper:
1. Pedal ratio
2. Master cylinder size
3. Brake pad CoF
4. Rotor Size
Even then ... we still can't run too small of piston area for our weight of car & expect it to work well.

The numbers can be kind of confusing. All most of us want to know is how much do I need?
In my experience, for 3500# cars, here's a GUIDELINE that is based on total braking force including the front & rear brake systems.

GUIDELINE:
2500# = Average passenger car
3000# = Performance production car
3500# = Good street & track braking system
4000# = Track braking system with good tires
4500-5500# = Full race brake systems

The very bottom line of the chart shows total braking force with that combination if you were looking for 70% front braking, as I often do in race cars.

If anyone is putting together a system & has questions on one part or another, I'll be happy to calculate things and/or advise you. I have spread sheets to work out front & rear systems & compare options. I can also change inputs in this chart to show you how they all look with a different brake pad, pedal ratio, master cylinder, etc.

Ron I have a C3 Corvette with a custom frame and currently C4 12 rotors and calibers and may upgrade to 13 inch with C5 calibers. The car is 3200" build is in the below site. I have not had a manual master cylinder in my car since 76 but I am leaning twds it in this build. Any help in steering me to what manual master I need would be great or if you think I should stay with vacuum assisted as well. Many thanks, oh the car will be manly on the street, some auto cross events and one road race course a year.

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