Vacuum question

[Melchiah]

i am building a very mild 289 for my DD mustang (all stock apperance) and i am trying to fig out how much vacuum i would need at cruising 75mph max and passing say 85mph with a 2bbl carb. i am asking this to give me a ballpark figure for my carb size i am using the engine math *CFM = CID * RPM * VE * 1/3456* then *CARB = CFM / squareroot( desired vacuum / 3 )* i will give you some background info on the engine it has closed chamber heads small dish pistons and with larger 1.6 exh valves and the whole exhaust side is ported and polished. it has a very mild rv cam in it but i dunno the specs on it, it was installed when i got the car other than that its stock motor and i want to keep the 2bbl any recomendations?

[JJ98]

just a thought ive ran a 2barell on my mustang and found that when i put the 2barall to 4 adapter plate on i had better mpgs and go power i think it was a 600 with manel secondary but if you still want the 2barel i think its about 500 cfm all to gether sorry for the spelling just pulled a 16 hope this help but tomorrow ill look in my mustang manual ive got a old one

[BaronVonAutomatc]

IIRC, the OE 2bbl was ~350cfm. So if your engine is near stock that should work fine. Maybe a little choked off at higher RPM, but better low end and throttle response. Or Holley and Demon make electric choke 500 cfm vacuum secondary 2bbl carbs. With some hop-up, headers, duals, etc. that would be the way to go. Vacuum secondary with an auto tranny or mechanical for manual.