The issue for power brakes is making sure they function "ALL" the time. That means Panic Stops, mountain driving and long steep downhill grades. That said I would like to see a minimum of 12hg at idle on the gauge for safe operation under all conditions. 14hg would be better. That equates to around 60* of cam overlap in a sbc. This depends on the motor and how efficiently it runs but it is a good ball park figure in overlap to shoot for. Less overlap, more Hg. More overlap and the Hg falls off more quickly. My 383 has 58* of cam overlap and I have 13 - 14hg at 650 - 700 RPM idle. JackHow much vacuum does it take to run a boost on a brake system?
1/4 turn from dead seat? If so, something IS wrong. What cam are you using?Jackel, Very good explanation Thank you. I am getting 10psi at 1000rpms no matter what I do. I have both mixture screws out about a 1/4 turn for max vac. any more and vacuum drops off. I may have multiple issues. I am on my way out to the garage to tweak things. I will update.
For a power booster to function properly you will need at least 18" of vacuum. Anything lower will give you a hard pedal. You can get away with 16" if you run a reserve canister.
Power brake upgrades, be sure to check for a minimum of 18 in. of vacuum prior to performing the upgrade. Vehicles with tall heads or superchargers most likely will not have sufficient vacuum. If you do not have sufficient vacuum, check your intake manifold for clogging, vacuum fittings and for collapsed vacuum lines. Also ask us about our 12 volt vacuum pump.