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[oletrucks] vacuum gage vs. economy

To: Old Trucks List <oletrucks@autox.team.net>
Subject: [oletrucks] vacuum gage vs. economy
From: J Forbes <jforbes@primenet.com>
Date: Sat, 17 Mar 2001 20:21:37 -0700

If you do some research on engines, you'll find that
they are more efficient when running at low rpm,
under load.  This is a condition that gives
relatively low manifold vacuum.  

In overdrive, especially on something like an 86
Caprice, the engine is running very slowly.  You
have to open the throttle a ways to keep the car
moving at a constant speed on flat ground.  This is
good for economy!  even though the vacuum gage shows
it is not as economical as being in direct drive
(not overdrive).

The reason...when an engine is running with high
vacuum, the throttle is almost closed, and most of
the work the engine is doing is being used to
overcome engine friction.  In ovedrive, RPM is
lower, the throttle is open further, and a higher
percentage of work the engine is doing is used to
power the car.

I did some experiments on this in college for my
senior design project, which was a high mileage car
that used a 2hp lawnmower engine.  Using a
dynomometer and a scale to weigh fuel used, I tested
the specific fuel consumption (how much fuel is used
per horsepower-hour of work done), at part throttle,
and full trottle.  The efficiency goes way down at
low throttle openings, for the reason stated above: 
most of the work the engine does is used to overcome
engine friction.

Hope this clarifies things...although it may just
open a bigger can of worms   :)

Jim F
59s in AZ
oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959

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