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.