Can someone tell me how brake boosters are measured? I assume the stated measurement pertains to the housing diameter (for instance: an 8" dual diaphragm booster would be 8" diameter). Is that measurement: the diameter of the back or front housing, or is it the diameter of the where the two halves join (which technically would be the largest diameter)? Thanks.

I believe it is the measure of the diaphragm itself which would be slightly smaller than the largest diameter. Maximum force varies with diaphragm area. Area varies with the square of diameter.

for a single 9': 9x9=81
for a dual 8": 8x8=64 times 2=128

The dual 8" would have about 60% more force vs a single 9"