Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1 6/24/83; site cbneb.UUCP
Path: utzoo!watmath!clyde!burl!mgnetp!ihnp4!cbosgd!cbscc!cbneb!adm
From: adm@cbneb.UUCP
Newsgroups: net.auto
Subject: Re: engine revvs and fuel economy - (nf)
Message-ID: <920@cbneb.UUCP>
Date: Mon, 18-Jun-84 09:19:40 EDT
Article-I.D.: cbneb.920
Posted: Mon Jun 18 09:19:40 1984
Date-Received: Fri, 22-Jun-84 09:15:18 EDT
Sender: adm@cbneb.UUCP
Organization: AT&T Bell Laboratories, Columbus, OHIO
Lines: 33

#R:pertec:-14800:cbneb:8400004:000:1959
cbneb!wdg    Jun 18 09:09:00 1984

	Engine efficiency at a particular rpm is a factor of dozens of
parameters. Several major ones come to mind: engine temperature, compression
ratio, fuel mixture, cam timing, engine head design (including valve size and
shape), ignition timing, fuel quality, etc. For several reasons small engines
today are usually more efficient at high rpms than large "American V8's". This
is due to the smaller engines' usually having an overhead camshaft and hence
better breathing capability at higher rpms and larger valves per cc of 
combustion chamber. 
	Since each engine is different, and the load and environment is
unique to each there is no set answer. However, I have looked at several
dyno figures on brake fuel comsumption (the amount of fuel per horsepower
comsumed) and would venture a guess that smaller engines are probably most
efficient in the 3500-4500rpm range whereas larger engines are most efficient
in the 2000-3500rpm range. Of course these are guesses. If you discuss
say a BMW or a souped up American car then these figures could be quite higher.
	However, this is really not what you need to know. The engine is only
one factor in our MPG figure. The transmission and rear end gearing play just
as important a role. Engine efficiency does not nesessarily mean a high MPG.
In fact usually the higher rpm a engine is running the less the MPG. This
is due to the fact that an engine may be putting out say 75 horsepower at
4000 rpm, though at say 55 MPH it may only take 45 horsepower to overcome
the friction caused by drag. If in the above case the car were in say 3rd
gear then the driver should shift into forth, thus lowering the rpm and 
horsepower and fuel consumption; but still delivering plenty of power to
drive at 55.
	One point, don't run a engine too slow causing it to "lug". This
hurts fuel economy and is hard on the engine. Also running the engine at
too high an rpm will cause abnormally fast wear on the internal components.