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Discussion Starter · #1 ·
I searched thru all the usual threads about the e350 MC swap in various Jeeps. I couldn't find anything definitive that specifically addressed my question. If you know of a good thread that answers it, please link up!

I'm swapping mine into an 88 XJ, and I want to know what is it about the e350 master cylinder that makes it better than the stock stuff. I'm guessing, just by initial comparisons, that the 350 MC has more fluid capacity, longer piston stroke, and a bigger bore. So the combo of these traits would mean that the pedal in the XJ now moves MORE fluid to actuate the vehicle's brake system, and consequently is able to opperate larger brake components (ie bigger calipers) and stop heavier loads by providing more hydraulic force. Is this correct? What else can you tell me?

I'm trying to get my "facts" straight for a magazine story.
 

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So when you put it on you didn't know why the first time :flipoff2:

BTW - Sounds like you answered your own question :D
 

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Pushes more fluid to operate bigger calipers yes. In easy language, you use less stroke to operate the vehicle but it'll take more foot pressure to get the vehicle to stop.
 

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Discussion Starter · #4 ·
Todd W said:
BTW - Sounds like you answered your own question :D

That's what I was thinking/hoping... but I wanted the reassurance of someone else KNOWING it.
 

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Larger bore does not equal more hydraulic pressure, it equals less.

Think of it this way:

Your foot exerts 50 lbs. of forceon the pedal.
The pedal has a 6 to 1 ratio, to the pedal exerts 300 lbs. of force on the MC.
Now let's say the MC has a 3/4" bore. You need to conver this to area with the formula A = (D/2)^2 * PI. That's the radius squared times pi. For a 3/4" bore, that is 0.44 in^2.

To get the hydraulic pressure generated, you divide the force by the area (hence psi, or pounds per square inch).

So with 300 lbs of force and 0.44 in^2 of piston surface area, you generate about 681 psi.

Now say you had a larger piston bore, 1 inch. The area is now 0.78 in^2. That means the hydraulic pressure generated is now 384 psi. That 1/4" bore increase with the same force on the piston leads to almost half the resulting psi!

Getting a MC requires balancing the new for more fluid volume for larger bore caliper pistons with the loss of hydraulic pressure resulting from a larger bore MC.

I have been searching for info on properly sizing a MC for certain calipers for quite some time, but I haven't found anything yet that gives good rules for calculating the necessary fluid volume based on caliper piston size, and what level of psi you should shoot for.

Also, keep in mind that larger caliper pistons equals more braking force (multiply the psi times the surface area of the brake pistons to get the clamping force on the rotor), so it does equal out, but only if the surface area increase in the caliper pistons is the same as the increase in area of the MC piston.

I hope that helps clear up some of the voodoo around braking systems.

- Eliot -
 

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Yep what he said, this is why it is usually a good idea to upgrade the brake booster when you use a MC with a larger bore, and calipers that require a larger amount of fluid.
 
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