RPM: what it actually means

RPM stands for revolutions per minute: how many times the engine’s crankshaft turns all the way round in one minute. It’s the engine’s speed, and it’s what the rev counter on your dashboard shows.

Most rev counters count in thousands, so a needle pointing at 3 means 3,000 rpm. Some typical figures for a petrol car:

• Idle, sitting still with the engine running: about 700 to 900 rpm. Any slower and the engine would stumble and stall.
• Cruising on the motorway: about 2,000 to 3,000 rpm in top gear.
• The redline, the red band at the top of the dial: often 6,000 to 7,000 rpm. Diesels stop at about 4,500 to 5,000.

Engine speed and road speed are different things. The engine’s revs only become road speed through the gears, which is why the same 3,000 rpm can mean a crawl in first or 70 mph (113 km/h) in top.

Engines turn far too quickly to watch, so the demo shows one cylinder turning 100 times slower than real life. Move the engine speed slider and watch what the rpm figure means every second.

At 3,000 rpm, the crankshaft turns 50 times a second, so each turn takes just 20 thousandths of a second. Each cylinder fires once every two turns, so every cylinder fires 25 times a second, and a four-cylinder engine makes 100 power strokes every second. That’s why an engine sounds like a hum rather than a series of bangs.

Even at idle, the crankshaft turns about 13 times a second.

Each piston has to stop dead and change direction twice every turn. At 6,000 rpm, a typical piston averages 39 mph (62 km/h) up and down the cylinder, and at the top of each stroke it is being pulled back with more than 2,000 times the force of gravity.

More rpm means more power strokes every second, which is how an engine makes more power as it revs. But it only works up to a point: at very high revs the cylinders can’t fill with air in the tiny time they have, and friction keeps growing. That’s why power peaks near the redline, as the horsepower vs torque lesson shows.

The gearbox decides how many times the wheels turn for each turn of the engine. In first gear the engine turns many times for each turn of the wheels, so the revs climb quickly as you speed up. In top gear, the same road speed needs far fewer rpm.

That’s what changing gear is about: keeping the engine in a useful band of revs whatever your speed.

• For economy, change up early, at around 2,000 to 2,500 rpm in a petrol car or a little lower in a diesel. Many cars show a gear-change arrow on the dashboard to suggest when.
• For hard acceleration, let the revs climb towards the redline before changing up, where the engine makes its most power.
• Don’t labour it. Flooring the throttle at very low revs in a high gear makes the engine shudder and strains it. Change down instead.

Electric motors are different. They spin as fast as 15,000 to 20,000 rpm, and make strong torque from a standstill, so most electric cars manage with a single gear.

The forces on the moving parts grow with the square of the engine speed: double the revs and the force pulling on each piston and rod is four times as big. At some point the valves can no longer keep up with the camshaft, and parts risk bending or breaking. The redline marks where the maker has drawn the safe limit.

To stop you going past it, the engine computer has a rev limiter. When the engine reaches its limit, it briefly cuts the fuel or the spark, so the revs bounce back down. You can hear it as a stuttering when a car is revved hard in neutral.

The limiter can only stop the engine driving itself too fast. If you change down into too low a gear at speed, the wheels force the engine to spin faster than the redline, and nothing can stop it. That kind of over-rev is one of the few ways a driver can wreck an engine in an instant.

Small, light engines can rev higher. Motorbike engines often rev past 14,000 rpm, and Formula 1 engines are limited to 15,000.