Horsepower vs torque

Every engine has two headline numbers: power, in bhp, PS or kW, and torque, in Nm or lb-ft. People argue about which one matters. They are really two views of the same thing, and you need both to understand how a car will drive.

Torque is a twisting force. Pull on a spanner to undo a wheel nut and you are applying torque. Pull harder, or use a longer spanner, and you apply more. It is measured in newton metres (Nm): the force, times how far from the centre you push. Stand someone weighing 80 kg (176 lb) on the end of a spanner 500 mm (20 in) long and they apply about 400 Nm (295 lb-ft), the peak torque of a typical 2.0-litre diesel.

In an engine, burning fuel shoves the pistons down, and the connecting rods turn each shove into a twist on the crankshaft. That twist is the engine’s torque.

Power is how fast the twisting gets work done. Heaving on a wheel nut that won’t budge takes plenty of torque but does no work, because nothing moves. An engine does work by twisting while it turns, and the faster it turns while still twisting hard, the more power it makes:

Power = torque × engine speed

So torque is how hard the engine twists; power is how much twisting it gets done each second. Twice the torque at the same revs is twice the power, and so is the same torque at twice the revs.

The demo is what a rolling road measures. The engine runs from idle to its redline at full throttle, and the graph plots its torque (Mustard) and power (Coral) at every speed. Press play to watch a run, or drag the slider.

Watch three things happen:

1. Torque rises, flattens, then falls. At low revs the engine fills its cylinders poorly. At high revs the air has too little time to get in and friction grows, so each bang gets weaker.
2. Power keeps climbing past peak torque. Each twist is a little weaker, but the engine is turning so much faster that the work done each second still goes up.
3. Power peaks when torque falls faster than the revs rise. Past that point, revving harder makes less power. That is why the quickest time to change up is usually just before the redline.

Now switch between Petrol and Diesel. The diesel makes nearly twice the torque, 400 Nm (295 lb-ft) against 210 Nm (155 lb-ft), yet both peak at about 138 kW (185 bhp). The petrol engine makes up for its weaker twist by turning much faster: it revs to 7,500 rpm, against 4,500 for the diesel. Diesels run out of revs sooner because their fuel burns more slowly and their heavier parts can’t safely spin as fast.

The turbo petrol shows why modern turbo engines feel so strong. The turbo packs the cylinders with air from low revs, so peak torque arrives by about 1,800 rpm and stays flat for thousands of rpm.

If the diesel has so much more torque, why isn’t it faster? Because of the gearbox.

Gears trade speed for torque. In first gear, the gearbox and the final drive together can multiply the engine’s torque more than ten times, but the wheels then turn more than ten times slower than the engine. Torque goes up, speed comes down, and the power stays the same, apart from a few per cent lost to friction.

That makes power the number that decides how hard a car can accelerate. At any road speed, you can pick the gear that puts the engine near its peak power, and the gearing turns that power into the most push at the wheels. Geared to suit, the petrol engine near 7,000 rpm pushes just as hard as the diesel near 4,000 rpm.

Torque still shapes how a car feels. An engine with lots of torque low down pulls hard without being revved or changed down, which is why diesels feel relaxed when towing or overtaking on the motorway. An engine whose torque arrives high up needs revving to go quickly, which some drivers love and others find tiring. For everyday driving, a wide, flat torque curve matters more than one peak number.

Electric motors make their full torque from a standstill, which is why even ordinary electric cars feel quick away from the lights.

Power and torque are quoted in several units, and mixing them up makes cars look stronger or weaker than they are.

• kW (kilowatts) is the official unit.
• PS is metric horsepower, which most European car makers quote. One PS is a little smaller than one bhp, so a 150 PS car makes 148 bhp.
• bhp is brake horsepower, measured at the engine’s crankshaft. The “brake” is the dynamometer that loads the engine to measure it. American figures in hp are the same size.
• Wheel horsepower is what a rolling road measures at the tyres. It is often 10 to 20 per cent below the crankshaft figure, because the gearbox and driveshafts take their share.
• Nm and lb-ft are both torque. One lb-ft is about 1.36 Nm.

One oddity: on a graph in bhp and lb-ft, the two lines always cross at 5,252 rpm. That is a quirk of the units, not of engines. Plot the same engine in kW and Nm and they cross at 9,549 rpm instead, well past most redlines.

Most power mods change the torque curve, and the power follows.

• Remaps on turbo engines raise the boost, so each cylinder burns more air and fuel and pushes harder. Torque rises across the middle of the rev range, and power rises with it. That is why remap figures quote both, such as “+40 bhp, +80 Nm”.
• Parts feel torque, not power. The clutch, flywheel, gearbox and driveshafts are each rated for a maximum torque. A diesel remap can add a quarter more torque, and a clutch that coped with the standard figure may start to slip at full throttle in a high gear. Careful tuners limit torque at low revs and in low gears to protect them.
• Raising the rev limit only adds power if the engine still makes good torque up there. Most standard engines’ torque falls away quickly past peak power, so a higher limiter on its own adds very little.

Rolling road figures can differ by several per cent between machines, and even between a cold morning and a hot afternoon. To judge a mod, compare runs on the same rolling road on the same day.