Here are a few excerpts taken from an article written by David Reher of Reher-Morrison Racing Engines. His discussion centers around small bore, long stroke engines versus large bore, short stroke engines used in drag racing, so you'll really have to "read between the lines" to pick out the information that applies here. My understanding, after reading what he has to say, is that if you're not limited in engine size, then go with the largest bore engine you can afford, and use a long stroke crankshaft. He points out two possible cons of a long stroke engine: 1) parasitic ring drag (which, in the long term,
could accelerate engine wear in a street car), and 2) windage and
oil foaming. However, street engines aren't typically called upon to rev in excess of 9000 rpm like a Reher-Morrison engine.
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If you are building an engine for maximum output at a specific displacement, such as a Comp eliminator motor, then the bores should be as big as possible and the stroke as short as possible. If you’re building an engine that’s not restricted in size, such as a heads-up Super eliminator or Quick 16 motor, then big bores are an absolute performance bargain.
The mechanical leverage of an engine is greatest at the point when the connecting rod is perpendicular to its respective crank throw; depending on the geometry of the crank, piston and rod, this typically occurs when the piston is about 80 degrees after top dead center (ATDC). So if torque is what accelerates a race car, why don’t we use engines with 2-inch diameter cylinder bores and 6-inch long crankshaft strokes? Obviously, there are other factors involved.
The first consideration is that the cylinder pressure produced by the expanding gases reaches its peak shortly after combustion begins, when the volume above the piston is still relatively small and the lever arm created by the piston, rod and crank pin is an acute angle of less than 90 degrees. Peak cylinder pressure occurs at approximately 30 degrees ATDC, and drops dramatically by the time that the rod has its maximum leverage against the crank arm. Consequently the mechanical torque advantage of a long stroke is significantly diminished by the reduced force that’s pushing against the piston when the leverage of a long crankshaft stroke is greatest.
A short crankshaft stroke reduces parasitic losses. Ring drag is the major source of internal friction. With a shorter stroke, the pistons don’t travel as far with every revolution. The crankshaft assembly also rotates in a smaller arc so the windage is reduced. In a wet-sump engine, a shorter stroke also cuts down on
oil pressure problems caused by windage and
oil aeration.
A sportsman drag racer can enjoy the benefits of big cylinder bores at no extra cost: a set of pistons for 4.500-inch, 4.600-inch or 4.625-inch cylinders cost virtually the same. For my money, the bigger bore is a bargain. The customer not only gets more cubic inches for the same price, but also gets better performance because the larger bores improve airflow. A big-bore engine delivers more bang for the buck.
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