Are you sure that it’s engine vacuum opens those secondaries? I think there's a pretty good chance that you're confusing what pulls the secondaries open on those vac-sec Holley carbs.
The whole intent and purpose for the vac-sec on these carburetors is to improve driveability in street-cars and heavier vehicles. It accomplishes this by opening the secondary throttle plates in such a way that the fuel metering signal is not abruptly interrupted by the transition of going from airway velocity of two partially open throttles to four throttles quickly whacked wide open.
Here’s a scenario: You’re sitting at a stoplight in your beloved LTD station wagon with its vinyl bench seats and 4bbl 460 inch powerplant. The engine is happily loafing at 800 RPMs, and there’s about 17 inches Hg of intake depression. Suddenly, the light changes, and you drop the hammer! Depending on which 850 cfm (for sake of discussion) carburetor you happen to be sporting, a number of things are going to happen.
First off, that 17 inches of depression is going to go away REAL QUICK. A big gulp of air is going to rush through the open throttle bores into the intake runners faster than you can think. Now the intake runners are suddenly FULL, and there is no inlet depression. Now what? While all that’s happening, the carburetor is having to transition from running on the idle metering circuit to running on the main metering circuit. Despite what a lot of people will try to convince you, ‘engine vacuum’ does NOT pull fuel into the engine. The depression formed within the booster venturi as air passes through it (in most carburetors anyway, some don’t have a booster venturi) creates a pressure differential between itself and the fuel bowl. This DP causes fuel in the fuel bowl which is at a higher pressure to flow through metering orifices or a ‘jet’ to the area of lesser pressure, which is at the discharge nozzle(s) within the booster venturi. This metering signal is a function of the mass airflow through the venturi, not the depression that may or may not exist within the engine inlet.
Anyway, that big gulp of air will cause the A/F mixture to lean excessively causing the engine to misfire/stumble while the engine tries to accelerate, unless you take steps to prevent that occurrence. Carburetor makers do this by implementing an accelerator pump to provide an adequate amount of additional fuel to aid the carb/engine in transitioning between the idle metering circuit and the main metering circuit. In addition to the accelerator pump, most carbs found on automobiles will also make use of some sort of power enrichment circuit. This is typically done by using a diaphram operated orifice (power valve) or spring/piston operated metering needles to allow more fuel to flow into the main metering circuit in addition to what flows through the main metering orifice or jets. These sense the intake manifold depression and operate based on engine load (a loaded engine having higher intake pressure than an unloaded one at idle, part throttle cruise, or deceleration.) When your foot is no longer planted on the floorboard keeping the throttle plates pinned open, engine ‘vacuum’ returns, causing the power enrichment circuit to close. Thus, the main metering circuit takes over again until such time as the throttle plates close far enough to cause main metering circuit signal to decay, and creating a depression across the idle circuit nozzles and fly-holes.
Somewhere within all this mess are a couple alternative means to an end. Whereas a mechanical secondary carburetor will use accelerator pumps on both the primary and secondary sides, a ‘vacuum secondary’ carburetor feature an accelerator pump on the primary side ONLY. Why, you ask? Because it uses a spring opposed operated diaphram to open the secondary throttle plates. This diaphram begins to open the secondaries as mass airflow through the primaries allows, thus ensuring an adequate metering signal across all four main metering circuits and eliminating the lean-stumble/misfire/backfire that would otherwise require an accelerator pump circuit on the secondaries as well.
Because you don’t have another accelerator pump squirting fuel every time you blip the throttle, fuel economy tends to be a lot better in a vac-sec carb. Heavy cars with tall gears don’t allow an engine to rev quick enough for mass airflow to quickly reach a point where the main metering circuit can react to the metering signal. So, they need an extremely LONG accelerator pump stroke while the engine accelerates to the point of making a stronger metering signal. Or, you can use the above-described vacuum operator to slowly open the secondary throttles to match the ability of the engine to consume enough air to maintain a strong metering signal.
Does this make sense? I left out the air bleeds and trims to avoid unneeded confusion. Point out any confusing points, and I’ll do my best to elaborate.
Brian Kennedy
[ November 20, 2002: Message edited by: jetenginedoctor ]