Most of the 7" boosters are a single diaphragm, the 8" or usually a dual diaphragm which helps the braking power with a bit less pedal effort. I've used both, the 7" will stop a car just fine, but the 8" dual diaphragm takes a bit of the pedal effort out of it. Big thing is to make sure the master cylinder you get is designed for 4 wheel discs, the diaphragm IMO is just a personal preference deal. Hope that helps, somebody will probably chime in with a bit more thorough explanation....