...known only to Chrys, they did not use man vac adv [ MVA ], instead used the useless PVA. GM used MVA. My 1966 showroom stock Pontiac idled @ 26*: 6* init + 20* MVA; with a very mild cam & 10.75:1. [ Chevs used about 24* ] With more cam & lower CR, vac is reduced & even more timing is needed...