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TECH WIZARD F or a given power density (hp per liter), increas-ing the bore (cylinder diameter) will increase output roughly twice as much as increasing the stroke (piston travel) a similar amount. This is because displacement is a factor of the square of the bore, while displacement increases linearly with stroke. Think of it this way: power is a function of displace-ment. But more bore can increase the length of the engine, which can increase the weight. At the same time, a long stroke can mean a taller engine which also increases weight. More insidiously, long stroke means greater piston speed and more piston travel per mile, which can mean more wear. For a given displacement and output, a smaller bore with a larger stroke will have greater piston travel and will scrape past more feet of cylinder liner per mile, unless bore is proportion-ally increased. (Larger displacement engines have higher output and can be geared to run slower.) The optimum bore/stroke ratio for modern heavy truck diesel engines will be higher than 0.8:1. The specifi cations of several heavy truck engines are shown in the attached graphics. Study them care-fully; you will fi nd some real surprises in here. QUESTION: Which results in a greater increase in power and torque output, increasing the bore or the stroke? ANSWER: Each dimension has its advantages, but increasing the bore results in a greater increase in power. Tech Wizard Ask the Have a question for the Tech Wizard? Send your question to paula.willis@volvo.com. DRIVER'S DIGEST 12