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The T337B is unique. Excess fuel returned from the engine goes to a fuel manifold, which looks like an aluminum whiskey flask mounted on the side of the fuselage. The engine is taking more fuel from the manifold than it is returning. The shortfall is made up for by fuel from the selected tank. The vent line comes from the high point of the manifold, and is connected to both the main and aux tank.
Consider what happens in that manifold: Fuel is drawn out by the pump and a lesser amount of fuel is returned by the pump. There is nothing forcing the fuel up the vent line except head pressure from the selected tank. Since the vent line is plumbed into the top of the tank, under normal circumstances, no fuel returns to either tank.
With this system, there is no need to burn an hour out of the mains before switching to the aux tanks. On a maximum range trip, the aux tanks should be selected early as aux fuel from the affected engine is not available to the surviving engine if one quits.
The POH for the T337B does not mention burning fuel from the mains for any length of time before selecting aux tanks. The manual does state on 2-1: "Vapor and excess fuel from the engine-driven fuel pumps are returned to the fuel line manifolds, and the returned fuel is recirculated to the engine-driven pumps." On page 7-1, regarding the auxiliary fuel system, the POH states: "Fuel vapor and excess fuel from the engine-driven fuel pumps are returned to the fuel line manifolds, and the returned fuel is re-circulated to the engine-driven pumps." Excess fuel does not return to the tank.
This system is distinctly different than that found on most other Skymasters including non-turbo models of the same year.
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