[alfa] Re: Fuel economy vs. RPM, vacuum gauge?

[Sonny <maxid@xxxxxxxxxxxxxx>]

Joe Elliott cited a book saying:
"If minimum consumption is to be achieved you must remember that, in a
gasoline engine, the specific consumption decreases as the load on the engine
is increased.  This means that under given conditions and
for a given speed of the car, the engine uses less gasoline when it runs
comparatively slowly--say in top gear--with a wide throttle opening than when
it runs at a smaller opening and at higher revolutions in a lower gear."

My question is: how useful is a manifold vacuum gauge to determine the optimal
operating range for a vehicle, either in terms of economy or performance? When
I grew up in Europe many decades ago, I believe some people had vacuummeters
in their cars to help them save gas. A '73 Dodge Dart a buddy and I later
drove across the States had an arrangement with a light that would come on in
response to a high manifold vacuum. It would typically come on during
acceleration. Sometimes the light would come on when going up a hill.
Downshifting would make the light turn off, but I had a feeling that since the
instrument did not compensate for the smaller amount of ground covered in the
lower gear, it may have been a useful indicator only as long as you stayed
within the same gear but not useful to compare the consumption in two
different gears. Lastly, I believe that some 4x4 "rockcrawler" buffs use
vacuum gauges. Whether it is to get the most torque out of the engine or for
general surveillance of mechanical integrity, I don't know, but I don't
believe it is to save gas.

Can anybody add anything about the merits of vacuum gauges on motor vehicles?
Cheers,
Sonny
'91 164 S
Baltimore
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