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Old 09-24-2022, 10:42 AM
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eschaider eschaider is offline
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Location: Gilroy, CA
Cobra Make, Engine: SPF 2291, Whipple Blown & Injected 4.6
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Quote:
Originally Posted by Argess View Post
Pedal Ratio is 5:1. No room for a booster. Those hydraulic pressure increase units don't appear to offer any more than replacing my front master cylinder with a smaller diameter unit.
You want to be careful not to estimate what your brake system is or is not doing. Your 4.5:1 pedal ratio for a pedal with a 6:1 ratio is an excellent example of this. Estimation errors in assessing your current system can be a recipe for failure and braking system failures are more often than not, disastrous. You don't need to go there.

As you begin whatever modifications you determine are necessary, measure for certainty before moving forward. It will prevent disasters.



Quote:
Originally Posted by Argess View Post
Anyway, my plans are (if no-one has any different ideas):

1/ deglaze the pads and rotors, or instead install EBC pads as they have a deglazing top coating.
This is not a bad idea but has absolutely no effect on the line pressure your braking system sees at a given pedal pressure. Your current system, at a 500 or 600 psi operating pressure is woefully inadequate.



Quote:
Originally Posted by Argess View Post
2/ play with the balance bar adjustment. The pedal arm is more or less centered between the two cylinders now. I hanv't made any measurements yet, but if the distance between centers is 2" and I move the center of applied force 1/4" towards the front cylinder, I can expect 625 psi vs. 500 for the front brakes. The rear brakes will suffer, however as the front discs are so much larger, I may get an overall improvement.
The balance bar is quite useful in, not surprisingly, balancing the braking capacity between your front and rear wheels. You do not want the rears to lock up before the fronts — bad things happen. This bar is the adjustment you use to prevent that.

More to the point, this bar will not increase line pressure from your current levels to where it ought to be. It will allow you to, as its name implies, balance the car's braking capacity front to rear.



Quote:
Originally Posted by Argess View Post
3/ Adjust the pedal height so the force is at 90 egrees to the cylinders and hope it's ergonomically OK.
Unless you have a very significant misalignment between your master cylinder (M/C) bore and the brake M/C pushrod that applies pressure to the M/C piston, this amounts to little more than rearranging the deck chairs on the Titanic. Here is a graphical representation of the possible change in applied pressure. The graph on the left is the equivalent of the pedal force applied and the various pushrod offsets. The right graph is the change in force seen at the M/C piston for a zero, 5˚, 7.5˚ and 10˚ misalignment.



Although the misalignment units are metric the message is clear, small changes in axial alignment offer only small changes in applied pressure and therefore line pressure. It is improbable you have a significant misalignment.



Quote:
Originally Posted by Argess View Post
4/ Install a 3/4" front master cylinder instead of the original 7/8" one. No money wasted as if it doesn't work out, it can be a replacement for the rear brake cylinder when needed.
THIS will change your braking system line pressure. Again you want to proceed with caution. As Bob pointed out he is doing his own testing to look for situations where the smaller diameter M/C may underperform or fail. You need to do the same thing.

An example of potential braking system failure would occur as the pads and discs wear and the pistons need to protrude further from the caliper to clamp the disc with sufficient force to stop the car.

If the volume of brake fluid used in this scenario exceeds the capacity of the ¾ inch bore in the M/C, braking pressure will be inadequate and the car will not stop as anticipated.



Quote:
Originally Posted by Argess View Post
A couple of odd things:

One member did actually post his front caliper pressure and got 760. Not sure how that was posiible.
His pedal ratio and M/C bore along with how hard he presses on the brake pedal determine this number.



Quote:
Originally Posted by Argess View Post
Also, another member once complained one of his front wheels would lock up before the other one. I say this is odd as I can't lock up any wheels at all. Not that one would want that, but it's an indication the brakes can work to the maximum.
When a single front or rear wheel locks up before it's twin, the braking system is telling you it needs maintenance.



Quote:
Originally Posted by Argess View Post
After all this, I must admit my brakes are more or less satisfactory for normal driving. It's a panic stop situation I worry about.
The challenge you are dealing with is similar to attempting to explain to someone who has been blind from birth what the color blue looks like. The blind person simply has no frame of reference.

Same thing when attempting to assess the efficacy of your brake system from the seat of your pants while driving the car. It is very difficult, if possible at all. Because you have no frame of reference (and other reasons) is why the industry settles on standards for the myriad of systems like brake system line pressure that we have in these cars.

The standard for hi-po street driven cars brake system line pressure is the 750 to 1000 psi number Tom Wells was speaking about. If your line pressure is correct, your friction material (brake pads) are correct and your disc diameter is adequate your car's stopping experience will be as it should be and very likely much different than it is right now.

If you want to absolutely nail it then the there is the racer approach Morris shared with you using Tempilaq® temperature indicating liquids. You have multiple paths to a happy ending available to you. All that remains is for you to choose the one you wish to use.

You already have the right discs and calipers and very likely the right or close enough to right brake pads. The only thing you are missing is the proper line pressure.

As the secretary in the mission impossible series always informs Mr. Hunt, "You mission Mr. Hunt, should you choose to accept it ..." In your situation, your mission, and you have no good alternative other than to accept it, is to get your brake system line pressure up to normal levels.
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Last edited by eschaider; 09-25-2022 at 05:16 PM.. Reason: Spelling and Grammar
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