Pacha

I thought converting from power to manual brakes was a good idea but I guess it is not!!, unless you have a really heavy left foot.
I think it is better to take some time finding a way to mount a slave cylinder for the clutch than ditching the power brake booster.


Manual brakes
Master cylinder 1"
Pedal ratio 6.25/1

Power brakes
Master cylinder 1 1/8"
Power brake booster diaphragm diameter 9"
Vacuum 17 "Hg
Pedal Ratio 3.5/1

Force at pedal Lb......Line PSI Power brakes*........Line PSI Manual Brakes
20.................................655.. ........................................ .179
40.................................734.. ........................................ 358
60.................................813.. ........................................ 538
80.................................892.. ........................................ .717
100...............................972... ........................................ 896
120............................. 1051.................................... .....1075
140............................. 1130.................................... .....1254

* Assuming the use of 100% booster capacity


Pacha


[Modified by Pacha, 12:59 PM 4/14/2004]

stingry

I just changed from manual to power brakes and the difference is phenominal!

In the process of fooling around I installed a Power Master Cylinder without the booster. It was like pushing againest a brick wall and still not stopping.

If you do go from power to manual it will be a longer push to get the same results and you need to change the Master Cylinder as well as the Pedal Ratio. The Pedal Ratio will change for you as the manual M/C connects lower on the pedal giving more leverage.

427V8

When designing brake systems you typically want to be able to lock the wheels with 100 lbs force on the pedal.

This calculation requires the coef for the pads, tires, leverage of the rotor, clamping pressure on the pad, hydralic leverage, and pedal leverage. :crazy:

but if the numbers you show are right ( they don't looks right, the power booster side seems high ) why do you say you need a heavy foot?

I think your pedal ratios are wrong for the Vette...

GrandSportC3

I don't know about your brake math.. but I have manual brakes.. and I can lock up my brakes at 70 mph if I want to.. A 5'1 100 lbs woman probably couldn't do that.. but it really doesn't take too much effort to do it...

Pacha

Pedals ratio are ok, at least for a '76 but I think all Vettes are the same
Power booster force = pressure (differential) x area (17Hg x 9" Diam).... just like a big suction cap
The force I show is the maximum the booster can give, in real life you modulate trough pedal travel.

The conclusion I get is that the 2 systems can generate the same braking power at panic stops but manuals are heavier at normal braking situations.
Also higher pedal ratio (manual brakes) means longer pedal travel.

Pacha

Pacha

I totally agree, braking capacity is the same but on manuals you have to press a "litle" harder.

I think i´ll change to manual brakes and see if I can get used to them.
I would like a tandem masters and balance bar setup.

Pacha

James

This was a problem I ran into also after building a new big block motor with tall valve covers for my 69. The car had factory power brakes and I found out the hard way that the booster would not clear the valve covers so I decided to convert to manual brakes. I have manual brakes with drums all around in my 68 Camaro and I can lock all four up with no problem so I thought a Corvette with 4 wheel disk would not be a problem at all. Man was I wrong!!!! I think the key thing here that you guys have mentioned is the pedal ratio. Since my car originally had power brakes the hole on the brake pedal arm is about an inch lower than what a car that came with manual brakes would of had. According to Pacha's spec's above its amazing how much more leverage you get by messing with that pedal ratio!!!! I finally decided to get away from the manual brakes and go with a Hydraboost setup from Paul at Hydratech brakes. I'm just in the process of finishing up the install and should know by this weekend how it works.

LemansBlue68

They taught me in driver's ed to never be a left foot braker. Consequently, the right foot is used for the accelerator AND the brake. Using the increased resistance of the brake pedal to build your right leg muscles enables you to exert similar forces on the accelerator!!!!! :auto: :jester

I installed power brakes on my big block '68. I believe that cars that can accelerate quickly should have the ability to slow down quickly as well. Power brakes cost nothing with respect to engine power although radical cams can limit the vacuum that is available to the brake booster if no auxilary resevoir is used.

Interesting calculations! Thanks.

Brettmc

I've got power brakes in both my daily driver trucks (original hydroboost in the diesel pickup). There is no problem whatsoever with exerting proper pedal force in my vette with manual brakes. I've had to make some panic stops and never wanted for more braking power.

Brett :thumbs:

joe73vette

Besides using the correct hole in the pedal, you have to use the correct non-power master cylinder. If you don't, all the theory in the world is useless.
Joe
PS, My brother's 73 had manual brakes and it wasn't that hard to stop. My 73 has power and it's good too. My 69 had power brakes, but kept getting air in the rears so they were awful.