AutoCalc
Horsepower, torque, displacement, compression ratio, gear ratios, and quarter-mile predictions. The math under the hood.
Horsepower & Torque Converter
Convert between horsepower, torque, and RPM. Enter any two to find the third.
Engine Displacement Calculator
Calculate engine displacement from bore, stroke, and number of cylinders.
Compression Ratio Calculator
Calculate static compression ratio from bore, stroke, and chamber volume.
Gear Ratio & Speed Calculator
Calculate vehicle speed from RPM, gear ratio, final drive, and tire size.
Quarter-Mile ET Predictor
Predict quarter-mile elapsed time and trap speed from weight and horsepower.
About AutoCalc
AutoCalc provides free automotive performance calculators for gearheads, mechanics, and engine builders. Convert between horsepower and torque, calculate displacement, check compression ratios, plan gear ratios, and predict quarter-mile times.
All tools run in your browser. Share results via URL.
Horsepower and torque: these are related by the formula HP = (Torque × RPM) / 5252, where 5252 is the constant derived from unit conversions (1 HP = 550 ft-lbs/sec; 1 RPM = 2π radians/min). At 5252 RPM, torque and horsepower are always equal numerically. Below 5252 RPM, torque exceeds HP; above 5252 RPM, HP exceeds torque — this crossover is visible on any dyno chart.
Engine displacement: displacement in cubic inches = (bore² × stroke × number of cylinders × π) / 4. A V8 with 4.00-inch bore and 3.62-inch stroke: (16 × 3.62 × 8 × 3.1416) / 4 = 362 cubic inches (5.9L). Converting to liters: cubic inches × 0.01639 = liters. Displacement is proportional to air-fuel charge per cycle and therefore sets the baseline power potential before boost, cam, or flow changes.
Compression ratio: CR = (swept volume + clearance volume) / clearance volume. Swept volume is the piston displacement for one cylinder; clearance volume is the combustion chamber volume plus deck clearance plus gasket volume. Higher CR increases thermal efficiency and power but requires higher octane fuel. Pump premium (91–93 octane) is typically safe up to approximately 11.5:1 CR on a naturally aspirated engine; higher CR requires race fuel or ethanol blends.
Quarter-mile ET estimation (Hale's equation): ET = 6.290 × (vehicle weight in lbs / flywheel HP)^0.33. A 3,200 lb car with 400 HP: 6.290 × (3200/400)^0.33 = 6.290 × 2^0.33 = 6.290 × 1.260 = 7.93 seconds. Elapsed time depends on traction, launch technique, and drivetrain losses — actual track times often run 10–15% slower than theoretical for street cars.