Engine R/L Ratio Calculator
Calculate the ratio between crank radius and connecting rod length, compare different stroke and rod combinations, and analyze how the geometry of the crank–rod mechanism changes the engine proportions.
PowerTunePro also calculates engine displacement, rod/stroke ratio, and bore × stroke geometry, with optional compression analysis.
How to Use the R/L Ratio Calculator
Under Original Setup, enter the cylinder bore, crankshaft stroke, and connecting rod length measured center to center.
Calculate the current setup, then enter the dimensions of the configuration you want to evaluate under New Setup. PowerTunePro compares both combinations and shows the changes in displacement, R/L ratio, and rod/stroke ratio.
If you want a deeper analysis, enable Compression to include the corresponding compression parameters.
Compare Before Changing Stroke or Rod Length
The R/L ratio depends directly on crankshaft stroke and connecting rod length. Cylinder bore does not directly change the R/L ratio, although it is used to calculate displacement.
With the other parameters unchanged:
| Change | Effect on R/L |
|---|---|
| Longer stroke | R/L increases |
| Shorter stroke | R/L decreases |
| Longer connecting rod | R/L decreases |
| Shorter connecting rod | R/L increases |
| Cylinder bore | Does not directly affect R/L |
This comparison is especially useful when evaluating changes in stroke, connecting rod length, or different crankshaft, rod, and cylinder combinations.
How to Interpret the R/L Ratio
The R/L ratio compares the crank radius (R) with the connecting rod length (L):
R/L = (stroke ÷ 2) ÷ connecting rod length
For example, for an engine with a 54 mm stroke and a 105 mm connecting rod:
R = 54 ÷ 2 = 27 mm
R/L = 27 ÷ 105 = 0.257
A higher R/L value means that the crank radius is larger relative to the connecting rod length. A lower R/L value represents a relatively longer connecting rod for the same stroke.
This proportion affects the geometry and kinematics of the crank–rod mechanism, including connecting rod angularity and piston motion throughout the cycle.
The R/L ratio, however, should not be interpreted by itself as “good” or “bad.” Actual engine behavior also depends on operating speed, reciprocating mass, piston geometry, cylinder design, combustion, mechanical loads, and intended application.
R/L Ratio, Rod/Stroke Ratio, and Displacement
Although they are geometrically related, R/L ratio and rod/stroke ratio are not the same quantity. PowerTunePro displays both to make comparisons between different engine configurations easier.
Engine displacement, on the other hand, is determined by cylinder bore and piston stroke. Changing only the connecting rod length does not change displacement, but it does change the geometric proportions of the crank–rod mechanism.
For this reason, when comparing two setups, do not look only at the final R/L value. Also consider displacement, bore × stroke geometry, and rod/stroke ratio.
Compression Analysis
The Compression option expands the analysis without being required for the basic R/L calculation.
In simple mode, you can work with the fundamental compression data. In advanced mode, the calculator allows additional engine geometry parameters to be included and, for two-stroke engines, specific information related to port geometry and crankcase volume.
This allows the same Original Setup vs New Setup comparison to go beyond basic crank–rod geometry and provide a broader view of the engine configuration.