Greetings . . .

I am with strokersix on this one. An engine is first and foremost an air pump. Where there is pressure differential there is work to be done. Part throttle conditions create vacuum because of atmospheric pressure difference between the intake plenum and the crankcase. This creates pumping loss which uses fuel when cruising.

The crankcase doesn't need to be sealed in order to be under pressure. The weight of the earth's atmosphere (which varies a little depending on where you are with respect to sea level) is always there. Moreover, a vacuum in an intake manifold is still a positive pressure when compared to the true vacuum of outer space! What matters is the difference.

An overdrive unit reduces RPM and therefore the amount of air passing through the pump over a give period of time. The objective is to cruise by applying just enough fuel to do the work of maintaining speed.

Jets and circuits in a carburetor 'weep' fuel into a vacuum and abate as pressure equalizes. With a carburetor the throttle plate position is controlled by the drivers foot. In a modern engine the plate is controlled by a computer with input from a mass airflow sensor.

Cruising at low throttle openings is to be in too low of a gear and therefore at to high RPM.

I vote for the OD.

regards,
stock49

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