Here's a good read: http://yarchive.net/car/air_induction.html
From the article:
"I realized over 10 years ago that a lean-of-stoich cruise mixture, and
the resultant higher manifold pressure, would improve mileage dramatically and
built a device to prove it. These devices were fitted to many of my cars but
the best result was on my 79 El Camino. The device consists of a fresh air
metering valve and some controls. The metering valve is a modified
Datsun A.I.R injection anti-backfire valve. The seat orfice is about 10 mm,
a relatively high flow device. This device feeds air from the air
cleaner to a plate fitted where the EGR valve once went. (This is one of
those garbage EGR implementations that could be felt) The controls
consists of ported vacuum switched by a thermal valve to defeat the
actuation when the engine is cold and an adjustable orfice and volume
arrangement whereby the slope of the applied vacuum to the metering valve
could be adjusted and time-delayed.
The anti-backfire valve was modified so that instead of pulsing open
it opened and remained open when vacuum was applied. The original had
a diaphram across a sealed chamber with a small orfice to bleed pressure
across the diaphram. When vacuum is applied, the diaphram opens the
valve until the pressure in the sealed side is equalized by bleed through
the orfice. My modification was simply to drill a hole through the housing
into the sealed chamber so it always had atmospheric pressure on it. The
orfice bleed is low enough not to matter.
The thermal valve was already fitted to the engine for some other
purpose. It is closed until the engine is almost fully warmed.
This mechanism, fitted to the 305 small block, improved the long
trip mileage from 18-20 to 25-27 mpg. Town mileage improved some but
not dramatically because the delay, used to improve throttle response,
kept it un-actuated most of the time. The down side of the device is
that throttle response at cruise was somewhat soggy, since additional
throttle had to be dialed in to respond to the opened air valve.
My homemade cruise control system handled that quite nicely :-) I'm sure
electronic controls would have greatly improved the performance of this