Slow Driving Won’t Keep Engine Young

Question: Will the 55-m.p.h. speed limit save not only the life of a driver but also the life of a car? In other words, will a car run longer if it is run at 55 m.p.h. rather than 70 m.p.h.?--L.L.

Answer: Depending on the care that you provide and the type of driving you do, you can expect to see up to a 100% difference in an engine’s service life before it is time to have it rebuilt or shipped off to the junkyard. But I doubt seriously whether consistent driving at 55 m.p.h. rather than 70 m.p.h. would make a measurable difference in the engine’s life.

If two identical engines were mounted on test stands and run continuously under laboratory conditions, an engine running at a higher speed would probably accumulate just as many miles as the one running at lower speed. That is certainly true of almost all engines except for very small four-cylinder engines that might be underpowered for the size car they are put in.

An engine in a car going 70 m.p.h. is working harder and running faster than one in a slower-moving car. But if the higher speed is not taking the engine up to the limit of its power range, it is unlikely to damage it. If this were not so, a car going down the highway at 35 m.p.h. would last longer than a car going 45--which is hardly the case.

An engine’s output is measured in two basic ways, horsepower and torque. Horsepower, which is a measure of the total power of the engine, peaks at about 5,500 revolutions per minute on most of today’s four-cylinder engines. Torque, which is a measure of the turning force of the engine, peaks at about 1,700 rpm. At highway speed, the engine should be operated at above its torque peak and well below its horsepower peak. Almost all of today’s engines can attain 70 m.p.h. at well below 5,500 rpm.

There are two important areas that can affect engine life. An engine that receives regular maintenance, including scheduled oil changes, engine valve adjustments and tuneups, will come much closer to fulfilling its expected service life. Second, driving habits have a major affect on engine life. If you confine your driving to short city trips and seldom put the engine under a heavy load, you may think you are “babying” the engine to a long life. That’s a big misconception.

An engine needs to be driven hard occasionally, meaning at freeway speeds and accelerated with a wide open throttle. Under such circumstances, engine combustion temperatures reach a peak and keep the engine clean by burning off deposits. An auto engineer I know claims if he had two cars to chose from, one driven carefully and one driven by a hot-rodding teen-ager, he would always choose the car driven hard.

Q: I have a 1983 Ford XLT Camper. When I drive more than 20 miles and I have to make a stop and turn off the engine, I can hardly get the engine to turn over to start. It sounds like the battery is dead, but I’ve had the battery and starter checked. I need help.--C.G.

A: I’ll assume the engine is in good condition and you aren’t heating up the block so much that you have a near-seizure problem. One thing you did not mention having checked is your battery cables, which can cause the symptoms you mentioned. A hot engine compartment, which increases resistance in the cables, and dirty cable connections, which further increase resistance, can starve the starter for enough amperage.