🏎️ Engine Calculators

Engine Displacement Explained

Engine displacement is the total volume swept by all pistons as they move from bottom dead centre (BDC) to top dead centre (TDC) in one complete cycle. The formula for a single cylinder is: displacement = π/4 × bore² × stroke. For a multi-cylinder engine, multiply by the number of cylinders. A 2.0-litre four-cylinder engine has four cylinders each displacing 500 cc. Larger displacement generally means more air and fuel can enter each cylinder per cycle, producing more power at a given RPM. Displacement is the primary size descriptor for internal combustion engines and directly affects fuel consumption, torque output, and power band characteristics.

Compression Ratio and Octane Requirements

Compression ratio (CR) is the ratio of cylinder volume at BDC to cylinder volume at TDC: CR = (V_swept + V_clearance) ÷ V_clearance. A 10:1 compression ratio means the air-fuel mixture is compressed to one-tenth of its original volume before ignition. Higher compression ratios produce more thermodynamic efficiency and power, but require higher-octane fuel to prevent premature detonation (knock). Naturally aspirated performance engines typically run 10:1 to 12:1. Turbocharged engines often use lower static compression (8:1 to 9:1) because boost pressure adds effective compression. Diesel engines use very high compression (16:1 to 23:1) to ignite fuel by compression heat alone without spark plugs.

Horsepower and Torque Relationship

Horsepower and torque are related by the formula: HP = Torque (lb-ft) × RPM ÷ 5,252. The constant 5,252 comes from converting the rotational power formula. Torque is the twisting force at the crankshaft — it determines how strongly the engine can accelerate the vehicle at a given speed. Horsepower is the rate of doing work — it determines top speed and how quickly power can be sustained. Diesel engines typically produce high torque at low RPM (good for towing), while naturally aspirated petrol engines produce peak horsepower at higher RPM. The horsepower and torque curves always cross at 5,252 RPM regardless of engine type.

Bore, Stroke, and Engine Character

An oversquare engine has a bore diameter larger than its stroke length, allowing higher RPM operation and better high-speed breathing. Most modern performance engines are oversquare. An undersquare (long-stroke) engine has a stroke longer than the bore, producing high torque at lower RPM. A square engine has equal bore and stroke. The bore-to-stroke ratio affects the shape of the power curve, piston speed (which limits maximum safe RPM), and combustion chamber surface-to-volume ratio (affecting efficiency and emissions). Mean piston speed = 2 × stroke × RPM ÷ 60; most engines are limited to around 20–25 m/s mean piston speed by material and lubrication constraints.