When you have long manifold runs, the accelerator pump volume becomes

Perhaps I am dating myself, but the old Chrysler 300 had four-barrels
outboard of the valve covers and the manifold runs crossed the entire
engine. The accelerator pumps on those carbs were enormous. Blipping the
throttle was like flushing a toilet.

         The reason this is so is because you must instantly provide the
fuel for a whole manifold full of air. You go from a pretty good vacuum to
a full atmosphere in the time it takes for the butterfly to open. As this
initial slug of air is flying past the butterfly, you must quickly dump
fuel into it.

         On the quickly closing side of the problem, the fuel wetting the
walls of the manifold is, indeed, a serious issue. Fuel doesn't evaporate
that well at room temperature and pressure. Thus, the less volitile
fraction of the gasoline wets the manifold walls. When you suddenly shut
the throttle, you get a big dip in pressure. The surface area of the

walls is quite large and the fuel layer is thin, so when the pressure
drops, all the fuel flashes into vapor.

         If the manifold is very long, you can get enough fuel to cause a
few misfires which fill the exhaust with fuel/air mixture. Then as the
mixture becomes leaner, the engine fires, touching off the mixture in the
exhaust system. Ka-bang.

         On cars with carburetors and long manifolds (like air-cooled VWs)
they put a throttle position damper to prevent the throttle from closing
too quickly.

         Bill Dube'


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