I'm using manifold vacuum for two reasons. One, my AED Holley 750DP doesn't have a timed port. Also, my 340 wants a ton of initial advance. I'm at 24 initial, 36 total right now. Manifold vacuum will allow me to lower initial a bit, raise mechanical a bit and still get the initial timing she craves off the vacuum advance (provided it kicks in before 15"). That's the plan, at least. I've never used vacuum advance before with this motor and she runs fine. But I hooked up vacuum advance just as an experiment -- and because so many say it's a must for a 95-percent street-driven car -- and took her for a spin. No pinging, no detonation, and a noticable improvement with idle/drivability.
