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Rod/Stroke Ratio - What's Your Angle?

Dip into the intricate world of internal engine geometry and look closely at something you know very little about: rod/stroke ratios. (C'mon, admit it!) Editor Bob demystifies.

If there's one truth about Honda engines, it's that they like to scream. And Honda enthusiasts like to make them scream. The numbers on the tach reach so high, Honda practically offers the license: Go ahead. Make it sing. It's good at it. So long as you don't miss a shift, all is bliss.

Lightweight components, stronger materials and shorter strokes enable modern four-cylinder engines to spin very fast, yet last longer than ever. A tremendous amount of science goes into engineering and creating these high-spinning machines, most of it rooted in elementary principles of physics and geometry-fundamentals anyone planning to build an engine should know.

Understanding rod/stroke ratio, or the amount that a rod deviates from an imaginary straight line extending from the center of the crank journal to the center of the piston, is key to knowing how these machines deliver power at high rpm.

Determining the Rod/Stroke Ratio
To determine a motor's rod/stroke ratio, divide rod length (distance in millimeters from the center of the big and small ends) by stroke. A B18C1, for example, combines 138mm rods with an 87.2mm stroke for a 1.58:1 ratio.

Most engine builders shoot for a ratio between 1.5:1 and 1.8:1 on a street motor, with 1.75:1 considered ideal, regardless of application. (The most highly developed four-stroke engines in the world-F1 and motorcycle engines-have rod ratios of more than 2:1.)

The rod/stroke ratio affects several engine dynamics, including piston speed and acceleration, piston dwell at top dead center and bottom dead center, piston side loads, cylinder loading and bearing loads. Many of these elements play roles in engine aspiration, combustion and wear.

Generally, a lower ratio means a high rod angle, creating greater potential for accelerated wear to cylinder walls, pistons and rings. A low enough ratio, due to the severity of its rod angle, can drive a piston right into the cylinder wall.

Higher ratio engines, on the other hand, don't have the same friction concerns, but compromise in other areas. Air does not fill the intake ports with the same velocity, and there is less demand for the ports to flow as well since there is more time to fill and scavenge the cylinder (we discuss this phenomenon later). This typically means stagnant airflow at low revs and weaker torque. Hey, you can't have it all.

Lower Ratios-A Honda Characteristic
As the chart on this page indicates, many Honda ratios-designed for economy-fall on the low side. Honda produces compact, short four-cylinder engine blocks that don't require long rods. Most Honda blocks also feature a small bore. When coupled with a short stroke, the rod angle is still harsh, though not as bad as if the piston were larger in diameter.

Some tuners take the geometry into their own hands with longer rods. A longer rod makes more torque with the same piston force, and since it's less angular than a shorter rod, reduces sidewall loading and decreases friction. All of this adds up to more power.

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