Perform Mensuration and Calculation
What this domain tests
This domain covers measurement, unit conversion, and automotive calculations such as clearance, engine displacement, compression ratio, and torque. The exam expects correct units, sensible rounding, and formulas applied to real shop values.
Core rules and concepts
- Know the common conversions: 1 inch = 25.4 mm; 1 N·m = 0.7376 lb·ft; 1 L = 1000 cm³.
- Torque is force times lever length: T = F × L. Keep force in newtons and length in meters to get N·m.
- Clearance is the difference between mating dimensions, such as bore diameter minus piston diameter.
- Engine displacement for one cylinder is π/4 × bore² × stroke; multiply by the number of cylinders. Use consistent units.
- Compression ratio = (swept volume + clearance volume) ÷ clearance volume.
- Do not round until the final answer; record the instrument's precision and use the correct number of decimal places.
Worked example
A 4-cylinder engine has an 86 mm bore and 86 mm stroke. Convert to centimeters: 8.6 cm. One cylinder's swept volume is 0.7854 × 8.6² × 8.6 = 499.6 cm³. For four cylinders, 499.6 × 4 = 1998 cm³, or about 2.0 L. If the clearance volume per cylinder is 55.5 cm³, the compression ratio is (499.6 + 55.5) ÷ 55.5 = 10.0:1. The reasoning is to calculate one cylinder first, then scale.
Traps that cost marks
Mixing millimeters and centimeters; using diameter as radius; rounding 499.6 to 500 too early; confusing N·m with lb·ft; and reporting more decimal places than the instrument can support.
Key points
- Convert units before calculating.
- Displacement uses bore², not radius².
- Compression ratio uses swept plus clearance volume.
- Keep full precision until the final step.
- State the unit with every answer.