Manifold vacuum question

[60wag]

I've been running an in cab vacuum gauge for while. My 2F runs at 16" of water at idle but what has me puzzled is how any of the assortment of other vacuum operated devices can effectively function with the manifold vac jumping all over the place as the throttle position changes. At full throttle the vac is practically 0. So any spark advance at full throttle must be due only to the mechanical distributor advance? It seems like the combo of the regular vac timing advance and the HAC advance going on and off with manifold vac would make for a screwed up engine. What am I missing here? Is the vac advance additive to the mech advance, or are they in parallel?

 

[FJ40Jim]

Vehicles that have vac operated accessories (ie. climate control, cruise, etc) will have a vacuum reservoir and check valve. The vac reservoir is "charged" when manifold vac is high, and then maintained by the check valve if manifold vac drops low.

The vac and mechanical advances are additive. To further complicate things, the vac advance comes off a ported vac fitting on the carb which has different vac readings than the relatively straightforward manifold vac.

At WOT, there is no vac advance. That's a good thing.

The HAC advance uses an inline check valve to apply vacuum to the HAC advance diaphragm, and maintain that extra advance under all vacuum conditions.

 

[60wag]

Thanks, it makes more sense now. I just found the check valve in the HAC system. So after reading the the emissions book some more in addition to your info, I'm guessing that the vacuum spark advance is really there to just improve emissions while the mechanical advance system improves the driveability of the truck by adjusting the timing with rpm.