Chart of brake booster size and psi to give you an idea of what happens when you move away from the 11". Problem is the Chart doesn't have the 9" dual but from my memory, it's awfully close to the 11".
I have 1 1/8 master but that was due to having 4 wheel disc brakes with 4 piston calipers, it wasn't just about pressure. The volume you need to feed those 4 big boys matters too so gotta have both on certain setups.
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69 GTO Convertible, 4000lbs
462ci, 606tq/569hp - 93 oct at 34 deg (207psi)
11.7:1, KRE H Ports, Lunati HR 282/290 w Johnson Lifters & 1.65 Scorp, E30, EFI, Holley HP + Dual Sync, 12-1 Crank Trig, 120lb Inj & 1000cfm TB, Torker II EFI Int & Rails, PTC 10" 0 Stat, Ricks SS Gas Tank, Magna 4303, Aerom EFI Reg, Aero Front & Wilwood Rear Disc Brakes, Dougs 1 7/8" Headers & Borla Pro XS 3" Muffs, Alum Rad & Dual Fans, 12:1 Box, UMI Control Arms & Viking Berz Fr + Rear CO Shocks, Hella UP28 Vac Pump
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