Rod Ratio Calculator (Rod-to-Stroke Ratio, Rod Angle & Deck Height)

How it works

rod ratio = rod length ÷ stroke · maximum rod angle = arcsin((stroke ÷ 2) ÷ rod length) · deck height = rod length + stroke ÷ 2 + compression height + deck clearance · piston travel from TDC at crank angle θ = r + l − (r·cos θ + √(l² − r²·sin² θ)), with r = stroke ÷ 2 and l = rod length

The rod ratio describes how steeply the connecting rod swings as the crank turns. A short rod (low ratio) makes a larger rod angle, pushing the piston harder against the cylinder wall and accelerating it faster away from top dead centre; a long rod does the opposite and keeps the piston near TDC slightly longer. Most production engines fall between 1.5 and 1.8. Engine builders also use the same numbers to find the block deck height a rod, stroke and piston combination needs.

Worked example

An engine with 144 mm rods and an 86 mm stroke has a rod ratio of 144 ÷ 86 = 1.674, in the typical range. The maximum rod angle is arcsin(43 ÷ 144) = 17.37°, and at 90° after TDC the piston has travelled 49.57 mm (57.6% of the stroke). With a 32 mm compression height and 0.2 mm deck clearance the block deck must be 144 + 43 + 32 + 0.2 = 219.20 mm.

Sources: Connecting rod — rod ratio and side load · Piston motion equations

Frequently asked questions