Torque

Torque is the measure of an engine's rotational, twisting effort — the force that actually turns the crankshaft and, through the gearbox and final drive, the wheels. Where power describes the rate at which work is done, torque describes the magnitude of the force available at any given instant. In everyday terms it is what a driver senses as pulling strength: the ease with which a car accelerates from a standstill, hauls a load up a gradient, or surges forward when overtaking without needing a downshift. An engine that produces strong torque low in the rev range feels muscular and relaxed, because abundant effort is on tap before the engine has to be worked hard.

Mechanically, torque is generated by combustion pressure pushing each piston down its bore, with that linear force acting on the crankshaft through the offset of the crank throw — the perpendicular distance between the connecting-rod journal and the crankshaft axis. A longer stroke or higher cylinder pressure therefore tends to yield more torque, which is why long-stroke and forced-induction engines are prized for their pulling power. The standard metric unit is the newton-metre (Nm); the imperial equivalent is the pound-foot (lb-ft), with roughly 1.356 Nm to each lb-ft. A modern 2.0-litre turbodiesel might develop around 320 to 400 Nm, while a large performance V8 can exceed 700 Nm.

The relationship between torque and power is precise rather than vague: power equals torque multiplied by rotational speed. In practical units, power in kilowatts is approximately torque in newton-metres times engine speed in rpm, divided by 9,550. This is why a small engine spinning very fast can match the power of a large lazy engine that produces far more torque at low revs. Manufacturers quote both a peak torque figure and the rpm, or rev range, at which it occurs; a flat, broad torque curve that holds from low revs is generally more useful on the road than a high but narrow peak.

Torque delivery shapes a car's character more than any single headline number. Naturally aspirated petrol engines typically build torque progressively and peak fairly high in the rev range, rewarding revving. Turbocharged engines, especially diesels, develop a strong slug of torque from just above idle, giving effortless mid-range flexibility but sometimes a more abrupt delivery. Electric motors are different again, producing maximum torque from zero rpm, which accounts for the instant, seamless shove of electric vehicles.

Torque at the engine is only part of the story, because the transmission multiplies it. A low first gear and a numerically high final drive trade outright speed for greatly increased torque at the wheels, which is how a modestly powered vehicle can still tow heavily or climb steeply. Understanding torque alongside horsepower, brake horsepower and power-to-weight ratio gives a far truer picture of how a car will actually feel and perform than any one figure alone.