Tech Tips/Calculators

Stroke or No Stroke?

Many believe that cubes equal horsepower, and that’s true, though long strokes are not as effective at pure horsepower production as large bores are.

Why?

Horsepower is a function of torque [hp=(lb-ft x rpm)/5,252], and stroke alone is rpm limiting. Stroking increases piston speed (a 3.480 stoke engine at 6500 rpm moves the piston at 3,770 feet per second; a 3.750 stroke does it at 4,063 fps) and the fast piston speed reduces the time available for cylinder filling at high rpm. Therefore, the torque production drops off at a greater rate than rpm increases, and horsepower production is cut short. Strokers can be made into high-rpm race engines, but there’s a greater cost in heads (to flow with less pressure drop at high rpm), valvetrain (to survive the more aggressive profiles required), and lightweight internal components. On a street engine, lengthening the stroke (beyond a certain point) and changing nothing else tends to favor low-end torque over high-end horsepower. That’s less true in the case of very small engines such as 289 fords, 273 mopars and 283 chevys; in witch case the sheer increase in displacement is worth power everywhere.