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From: jackh@zehntel.UUCP (jack hagerty)
Newsgroups: net.auto
Subject: Re: engine revvs and fuel economy
Message-ID: <1540@zehntel.UUCP>
Date: Mon, 18-Jun-84 13:20:44 EDT
Article-I.D.: zehntel.1540
Posted: Mon Jun 18 13:20:44 1984
Date-Received: Fri, 22-Jun-84 07:43:02 EDT
References: <148@pertec.UUCP>
Organization: Zehntel Inc., Walnut Creek, CA
Lines: 33

Most smaller engines have their POWER peak at higher revs (and, until
recently, their torque peak too) meaning that if you need fast acceleration
to pass or to merge on the freeway, etc., then you *should* use higher
revs.

However, if you're after maximum economy, then you should use the lowest
engine speed possible. There are at least two reasons for this:
   First, despite the best efforts of Castrol, Mobil, Valvoline, et. al.,
engine friction between all of the sliding, rubbing, rotating parts of
your engine increases with speed. This is, in part, why your engine will
wear out faster if you keep the revs up all the time.
   Second, the so-called pumping losses are lower at lower engine speeds.
Your engine is just a large pump, sucking air and fuel in, burning it and
pushing it out the tailpipe. A certain level of power requires a certain
amout of fuel/air mixture. If you force the engine to turn slower (by 
using higher gears) then it must draw in a larger amount of mixture each
revolution. The amount of mixture is controlled by a movable restrictor
in the intake passage called a throttle plate. This is what your "gas"
pedal is really attached to. Therefore, at lower engine speeds the throttle
plate is opened more lessening the restriction and reducing the amount of
work your engine must do to pump the air through.
 
The conventional wisdom says that you should never "lug" your engine. This
is a vaguely defined term that has to do with operating your engine below
the "green line", i.e. the lowest reccomended engine speed, which could
result in all kinds of nasty things happening to your engine.  This has
been largely discredited and current thinking says that if your engine is
not pinging or knocking and you're still getting enough power to do the
job, then you're not running too slow.

                                    -- Jack Hagerty, Zehntel Inc.
                                        !ihnp4!zehntel!jackh