What is the minimum vacuum needed for the Brake Booster and Headlights to operate correctly?
I'm guessing, but I think 20Hg would be the norm on a stock 350. What if a longer duration cam is installed? I know that a cam change can effect vacuum, depending on the cam selection,
but by how much?

Any info is appreciated.

Gary

17-21 hg is generally considered normal for a non-performance engine. Bigger cams, etc. will reduce that. In my '78 L82 a previous owner installed a bigger cam. I installed an Edelbrock Performer intake, had the Q-Jet rebuilt (it also needed re-jetting), and removed the EGR stuff, and checked for possible vacuum leaks with none found. With the bigger cam the motor pulls somewhere about 11-12 hg vacuum and idles and runs fine with no problems with the power brakes or headlights.