I know there's a third more surface area with the 11". Will I see a major difference if I go down to this 9" booster? It's for my 68 with stockish (69-71 disc brakes). Has anyone done this?

 

You're right on target with considering the difference in diaphragm area.

The braking force is proportional to the diaphragm area. With the 9" you'd be sadly lacking in boost. You will notice it when you need the brakes most.

If you need the clearance of a smaller booster, buy a dual diaphragm unit. A dual diaphragm 9" will have about a third more boost than an 11".

An 8" dual diaphragm will have the same boost as an 11".

 

I have a OEM 9" unit on my 66 with front discs and it stops on a dime and throws out 9 cents change. In other words,it stops great.

 

Mike,

My concern is concern is the difference I'll feel. I assume you've always had a 9" on the 66.

A guy I know locally bought this 9" and then bought a full disc brake kit from Right Stuff so now he has two boosters. If the difference between the two is significant I'd rather get a dual diaphragm booster or a standard 11".

My current OEM 11" works fine (though it's probably 50 years old), it's just painted black and kind of cruddy looking. I don't have any valve train clearance issues, I just need to loosen the booster to get the valve cover off.

 

1964-66 Cutlii came with 9" boosters. 1967-up Cutlii came with 11" boosters, presumably due to the greater pedal force required by the optional disk brakes. This is why 1967 big block cars use one notched valve cover on the driver's side. As noted, the difference is in the amount of boost due to diaphragm area. As noted, the 9" booster has about 2/3 the area of the 11" booster, thus only 2/3 the "boost" for a given manifold vacuum.

Like the butt dyno, the butt G-meter is not a reliable test instrument.

 

Can someone recommend a dual diaphragm power brake booster that is less than 11" and is completely compatible with a 68 type disc brake master cylinder?

There are quite a few booster/master combinations. I just bought a new master cylinder two or three years ago and it works fine though. I also have the stock hold off valve at the booster and the distribution block on the frame. It appears all the combination booster/master units will require re-plumbing brake lines.

There's no reason I can't use a stock type 11", I'd just like to be able to take my valve covers off without loosening the booster.