Power versus efficiency

We all want our cars to zoom, but we do not want to pay too much for fuel. How much is too much? We can figure this out. The average car gets 27.5 miles per gallon and is driven 10,000 miles per year. If the average gasoline price is $1.30 per gallon, the car owner spends $473 per year. A popular pony car, the Chevrolet Camaro, gets 19 miles per gallon. It costs $684 per year in fuel. Is it worth an extra $18 per month to zoom? You are probably saying yes. Then why do people care so much about fuel efficiency?

In the 1970's we had two major gasoline crises. As a result, the price of gasoline soared to about $1.30 per gallon in 1980. This may seem to be the same burden as in 1997, but adjusting for inflation, it is like paying $2.70 per gallon. Using the example of the previous paragraph, it cost the equivalent of $1421 per year to zoom in 1980. Assuming that gasoline prices remain stable in the future, it appears cost effective to zoom.

Assuming you want your car to zoom, how do you make it happen? Popular ways are to use a turbocharger or supercharger, use a small engine with a lot of horsepower, or use a large engine.

Turbochargers and superchargers are effective, but not economical. First, the manufacturer adds about $3000 to the price of the car. Second, when the insurance company finds out, it increases the cost of insurance by $300 per year. Third, these cars require high octane gasoline, which costs about 15% more than regular. Finally, turbochargers are rumored to break down unless the oil is changed very often. On the positive side, the word "turbo" will be on the side of the car, even if the turbocharger stops working!

Nowadays, many small cars have a lot of horsepower. For example [Reference], the 1993 BMW 325i gets 189 horsepower from a 2.5 liter engine. In contrast, the 1993 Ford Mustang gets 205 horsepower, about the same as the BMW, from a 5.0 liter engine. The difference is that the BMW needs to get to 5900 rpms (engine revolutions per minute) to get there, while the Ford needs only 4200 rpms. To make up for this, BMW adjusts the transmission so that it needs 2395 revolutions to travel one mile. the Ford needs only 1550 revolutions per mile. This means that the Ford gets less wear per mile. Reliability [Reference] and fatigue [Reference] analyses are based on cycles, so the number of engine revolutions is critical. Further evidence is on your bottle of engine oil. Castrol says

"In today's harder working engines, wear is a greater problem that ever before. These higher-revving engines shear oil and operate at extremely high component temperatures causing viscosity and thermal breakdown."

To summarize, a small engine with a lot of horsepower may be a false economy. Look at the number of revolutions per mile and make sure it is not too high.

Large engines, like on the Ford Mustang mentioned above, provide good acceleration with low engine wear. In many cases, they represent old and proven technology. As such, they cost very little ... when they are available. Large engines are becoming increasingly more difficult to find. Among inexpensive cars, only the Camaro and Mustang have V8 engines. These engines are rarely found among mid-priced cars. Luxury cars often have them, but they are expensive. This is like buying a $40,000 car to make sure you get a cupholder.

If you want plenty of power, but you want to be reasonable, get a car that you like and ask for the largest engine that comes with that model car.


Copyright 1996 Alacrity Research