Brake Booster Size: Dual 8" vs Single 11"

[argonaut]

I recently installed 4-wheel disk brakes onto my 64 with a kit from CPP. They supplied me with a dual-diaphragm, 8" booster. However I was wondering if I might actually get more power assist from a single 11" booster. I don't have clearance issues since I'm running a small block. Does anyone have any experience or thoughts?
Thanks.

[nicke]

No, you won't get more power assist from a single 11" vs. a dual 8" booster. Its all about surface area, which increases exponentially with the dual. The 8" dual will generate more line pressure with the least amount of pedal effort. Also, a consideration besides clearance issues, is that the dual's are used when people want the power booster setup but have low vacuum caused by a big, long duration cam.
My vote, stick with the dual 8".

[mbgmike]

Good advice. You will like the dual diaphragm better.

[lakeroadster]

In reality they are almost the same.

Numbers don't lie.... Calculating diaphragm areas you get the following:

• dual 8" = 100 sq. inches
• single 11" = 95 sq. inches

So..... yes the dual 8" is better, but by a negligible amount, about 5%. (In reality, by the time you remove the area for the actuator rod in the center, which the dual gets a double deduction for, the areas are probably identical.)

No doubt that is why the 8" diameter was selected for the dual-diaphragm configuration. It gives about the same vacuum area as the single 11", but does it in a smaller package that better clears the valve covers of big block and LS motors.

Hope that helps.

John

p.s.: I personally like the 11" better from an appearance standpoint, looks more "factory" correct, for whatever that's worth.

[nicke]

Originally Posted by lakeroadster

In reality they are almost the same.

Numbers don't lie.... Calculating the areas you get the following:

• dual 8" = 100 sq. inches
• single 11" = 95 sq. inches

So..... yes the dual 8" is better, but by a negligible amount, about 5%.

I am sure that is why the 8" diameter was selected for the dual-diaphragm configuration. It gives about the same vacuum area as the single 11".

Hope that helps.

John

Yep, that is correct. Not sure why my quick calculations had the 8" way ahead. Good call.

[mbgmike]

Actually its about more just the surface area's. Any air that may get past the first seal is usually stopped by the second and the force is not lost. A single only has one chance.

[lakeroadster]

Originally Posted by mbgmike

Actually its about more just the surface area's. Any air that may get past the first seal is usually stopped by the second and the force is not lost. A single only has one chance.

It is purely a math thing in regards to the amount of pedal pressure, based on:

Force = Pressure x Area

Areas, per my above post, are the same
Pressure = engine vacuum, which is also the same

Therefore the force is the same.

Originally Posted by argonaut

I was wondering if I might actually get more power assist from a single 11" booster.

It doesn't matter which set-up is used, dual 8" or single 11", the power assist will be the same.

[nicke]

Originally Posted by lakeroadster

Surface area's are the same, that's what the above data I posted shows.

It's purely a math thing. In regards to the amount of pedal pressure its is based on:

Force = Pressure / Area

Areas, per my above post, are the same
Pressure = engine vacuum, which is also the same

Therefore the force is the same and therefore it doesn't matter which set-up is used, the result will be the same .

Is my statement about a dual working better with low vac true? You have me second guessing myself :-/

[lakeroadster]

Originally Posted by nicke

Is my statement about a dual working better with low vac true? You have me second guessing myself :-/

A dual 8" will be no better than a single 11". Now if the dual diaphragms are larger than 8" then it will provide more power assist than the 11".

Again, it is simple math; calculate the areas and compare them. More area = more force.

[mbgmike]

not if they leak

[nicke]

Originally Posted by lakeroadster

A dual 8" will be no better than a single 11". Now if the dual diaphragms are larger than 8" then it will provide more power assist than the 11".

Again, it is simple math; calculate the areas and compare them. More area = more force.

Interesting... I reached out to these guys a while back with the same question, as they said specifically that for all wheel discs, a dual booster is the only way to go. Admittedly, in the FAQ section when comparing they use a 7" single as comparison to the dual 8", but when I asked, they stated that the dual 8" was better than the single 11".

[lakeroadster]

Originally Posted by mbgmike

not if they leak

All bets are off if the system has leaks.... You can't design the required capacity of a system if right off the bat you assume it leaks unless you define how much it leaks.

Originally Posted by nicke

Interesting... I reached out to these guys a while back with the same question, as they said specifically that for all wheel discs, a dual booster is the only way to go. Admittedly, in the FAQ section when comparing they use a 7" single as comparison to the dual 8", but when I asked, they stated that the dual 8" was better than the single 11".

An all wheel disc set-up requires more pedal pressure than a disc/drum set-up. But needing a dual vs. a single diaphragm booster is based on the required pedal force and the available vacuum. Based on that data then you back into what type booster is needed, and yes the dual is better from a space consideration.

But, as we previously discussed, a dual 8" isn't better than a single 11", in regards to the amount of force it can produce. The laws of physics aren't changed by salesmen.

[SCOTI]

Originally Posted by lakeroadster

....The laws of physics aren't changed by salesmen.

I'm liking this statement. I do believe it's going to wind up on my sig.... .

[nicke]

Good call. Thanks for clearing that up. I really do learn something new everyday on this board

[argonaut]

Hmmm. Here was my thinking, and the reason I asked:
Obviously the effective diameter of the diaphragms is smaller than the nominal case size, and the size of the second diaphragm on the 8" dual is even smaller than the primary. So I estimate:

8" Dual
primary diameter=6" => Primary Area=28.27"^2
Secondary diam=5" => Secondary Area=19.63"^2
=> Total Area=47.91"^2

11" Single
Primary Diameter=9" => Primary Area=63.62"^2
=> Total Area=63.62"^2

Therefore the 11" would actually have more effective force due to the larger effective diaphragm surface area.

But CPP did tell me that the dual 8" was the best, so I'm confused.