Cranking Torque Calculator

Estimate the torque required to crank an engine for starter motor sizing based on displacement, compression, and friction.

Input Parameters

Results

--
Compression Torque
N-m
--
Friction Torque
N-m
--
Total Cranking Torque
N-m
--
Min Starter Power
kW
Note: Cold cranking at 0°F can require 2-3x the torque vs 70°F. Multi-weight oils (5W-30) greatly reduce cold cranking loads. SAE J1100 defines cranking speed requirements.

Reference

Compression P = P_atm × CR^1.3 (polytropic) Comp Force = Comp_P × Displacement_per_cyl Comp Torque = Force × Stroke/2 Friction Torque = Disp × 8 × Visc_factor × (SAE_W/10) Visc Factor = 1 + (10 − T°C) × 0.03 Starter Power = Total_torque × RPM × 2pi/60