Car Parts Conversion Chart: Multiply-By Factors

Parts catalogues mix units without warning: a bore in millimetres, a displacement in cubic inches, a boost figure in bar and an injector rated in pounds per hour. The tables below give the multiplier for each conversion, taken from the exact unit definitions, plus the fuel-flow conversions that are not unit conversions at all and the one that cannot be a multiplier.

How to use a multiply-by table

Find the row that starts with the unit you have, and multiply. Each conversion appears in both directions, so there is no dividing and no reciprocals to work out. Factors are given to enough decimal places that rounding does not accumulate across a chain of conversions.

Length, volume, pressure, torque, power and speed

Fuel flow, and the assumption inside it

Injector ratings in pounds per hour and in cc per minute are not the same kind of measurement. One is a mass flow, the other a volume flow, and converting between them requires knowing what the fuel weighs. The convention is petrol at 6.0 pounds per US gallon, about 0.72 kilograms per litre, and every factor in the table below is derived from that one figure.

That assumption is the whole reason these conversions can mislead. Ethanol blends and methanol are denser than petrol and carry far less energy per unit mass, so an injector sized in pounds per hour on petrol is flowing a different mass and supporting a different power figure on anything else. Where the fuel is not straight petrol, work from its actual density rather than from this table.

Temperature: the one that is not a multiplier

Celsius and Fahrenheit do not share a zero, so no single factor converts between them. Multiply by nine fifths and add thirty-two to go from Celsius to Fahrenheit; subtract thirty-two and multiply by five ninths to come back. Minus forty is the same in both, which is a useful sanity check on any conversion you have just done.

Method and precision

Every factor in the first table comes from an exact unit definition — the inch is 25.4 mm, the foot is 0.3048 m, the US gallon is 3.785411784 litres — so the values are exact and are rounded only for display. The generator checks that each pair multiplies back to one, so no direction of any conversion can drift from its opposite. The fuel-flow factors are NOT unit conversions: they carry a density of 6.0 pounds per US gallon for petrol, and all six are derived from that single constant so they cannot disagree with each other. Temperature is given as a formula and a short table because an offset cannot be expressed as a multiplier. Sources: Exact unit definitions — no measurement involved.

Where conversions go wrong on a car

Torque is the expensive one. Pound-inches and pound-feet are a factor of twelve apart, and a figure read as the wrong one applies twelve times the intended load. Small fasteners, alloy castings and plastic housings are specified in pound-inches or newton metres precisely because pound-feet is too coarse a unit for them.

Pressure is the confusing one. Gauge pressure and absolute pressure differ by one atmosphere, so a boost figure of 1 bar means roughly 14.5 psi above atmospheric on a gauge and roughly 29 psi absolute in a calculation. The conversion factor is the same either way; the reference point is not, and that is what has to be stated.

Displacement is the harmless one that still catches people. A 5.7 litre engine is 350 cubic inches to the nearest whole number, which is where the familiar designations come from — but they were rounded to marketing figures, so converting back gives a number close to the badge rather than equal to it.

Measuring gear is in measuring equipment, and torque tools are in torque equipment and automotive tools.