Calculate rod-to-stroke ratio from connecting rod length and crankshaft stroke, or solve for the rod length or stroke needed to hit a target ratio.
Rod-to-Stroke Ratio Formula
R = L / S
L = R * S
S = L / R
a = asin( (S/2) / L )
- R is the rod-to-stroke ratio (unitless)
- L is the connecting rod length measured center to center (in or mm)
- S is the crankshaft stroke (in or mm)
- a is the maximum rod angle from the bore centerline (degrees)
The calculator’s default mode divides rod length by full stroke. The other modes solve for rod length or stroke at a target ratio. The model assumes a centered, rigid slider-crank with rod length greater than half the stroke. It reports maximum rod angle and the maximum geometric side-to-axial force multiplier, tan(a). This multiplier is not the actual peak engine side load. Optional engine speed adds mean piston speed: 2 times stroke in meters times RPM divided by 60 gives meters per second.
How to Interpret Your Rod Ratio
At a fixed stroke, a longer rod increases the ratio and reduces maximum rod angle in this centered geometry. Actual cylinder-wall side loads also depend on pressure, inertia, friction and crank angle. The table compares geometry; it does not grade engine reliability, wear or safe RPM.
| Rod-to-stroke ratio | Max rod angle | Maximum ideal side/axial multiplier |
|---|---|---|
| 1.40:1 | 20.9° | 0.3824 times axial force |
| 1.50:1 | 19.5° | 0.3536 times axial force |
| 1.60:1 | 18.2° | 0.3290 times axial force |
| 1.70:1 | 17.1° | 0.3077 times axial force |
| 1.80:1 | 16.1° | 0.2892 times axial force |
| 1.90:1 and up | 15.3° or less | 0.2728 times axial force or less |
These listed dimensional combinations illustrate the ratio calculation. Their ratios agree with the supplied lengths, but the table is not a verified factory specification for every engine generation. Confirm the exact engine and parts dimensions with the manufacturer before planning a build.
| Engine | Rod length | Stroke | Rod ratio |
|---|---|---|---|
| Chevy 350 small block | 5.700 in | 3.480 in | 1.64 |
| Chevy 383 stroker | 5.700 in | 3.750 in | 1.52 |
| Chevy LS3 6.2L | 6.098 in | 3.622 in | 1.68 |
| Ford 302 Windsor | 5.090 in | 3.000 in | 1.70 |
| Ford Coyote 5.0L | 5.933 in | 3.650 in | 1.63 |
| Honda B18C | 138.0 mm | 87.2 mm | 1.58 |
| Honda K20 | 139.0 mm | 86.0 mm | 1.62 |
| Toyota 2JZ-GTE | 142.0 mm | 86.0 mm | 1.65 |
| Nissan SR20DET | 136.3 mm | 86.0 mm | 1.59 |
| Subaru EJ257 | 130.5 mm | 79.0 mm | 1.65 |
Rod-to-Stroke Ratio Example Problems
Example 1: An engine uses a 6.000 in connecting rod with a 3.750 in stroke. Dividing gives a ratio of 1.600:1. Half the stroke is 1.875 in, so maximum rod angle is asin(1.875 / 6.000), approximately 18.21 degrees. The maximum geometric side-to-axial force multiplier is tan(18.21 degrees), approximately 0.329, or 32.9 percent of net axial piston force in the ideal model. Actual peak operating side thrust is not calculated.
Example 2: You want a 1.70:1 ratio with a 3.480 in stroke crank. Multiply 1.70 by 3.480 to find the required rod length: 5.916 in. You would look for an aftermarket rod close to that length, such as a 5.900 in or 6.000 in rod, and adjust piston compression height to keep the same deck clearance.
FAQ
What is a good rod-to-stroke ratio?
There is no universal ideal ratio. Increasing the ratio reduces maximum rod obliquity in the centered model, but ratio alone cannot establish reliability, wear, cylinder filling or safe engine speed. Those depend on the complete engine, loads, materials, lubrication and clearances. Choose compatible parts and verify the complete design rather than treating a ratio threshold as approval.
Does changing the rod length change displacement?
No. Displacement depends only on bore, stroke, and cylinder count. A longer rod raises the rod ratio without adding a single cubic inch; it changes how the piston moves, not how far it sweeps. That is why rod swaps also require a piston with a different compression height, so the piston still arrives at the same deck height at top dead center.
Why does a stroker kit lower the rod ratio?
Increasing stroke S while retaining rod length L lowers R = L / S. For the listed 5.700 in rod, changing stroke from 3.480 in to 3.750 in lowers the ratio from about 1.64 to 1.52 and raises maximum geometric rod angle from about 17.77 to 19.20 degrees, an increase of about 1.4 degrees. The geometric side-to-axial force multiplier increases; actual peak side load requires dynamic-load analysis.
