A: The abbreviation RPM stands for Revolutions Per Minute. In a motoring sense, the revolutions in question relate to a car’s engine. So, RPM in car speak becomes a measure of how fast an engine is spinning.
A petrol or diesel engine produces torque by burning fuel and air and using the force of that explosion to make the internal parts of the engine (crankshaft and pistons) spin inside the engine’s crankcase. It’s this spinning motion that drives the car’s wheels and that we measure the speed of in RPM.
RPM is calculated simply by counting how many times those same internal parts rotate in 60 seconds. Actually, it’s not quite that simple, because engine speed is not averaged over that minute; it’s a real-time expression of how fast the engine is spinning right now.
But it’s expressed in terms of per-minute to make it a workable, understandable number. That’s how revving a car engine will cause it to go from, say, 600rpm to 6000rpm in a second or two.
In most cars, motor RPM is displayed on a tachometer. But what is a tachometer? Essentially, by looking at the car’s ignition system, the tachometer counts each engine revolution. From there, it can work out how fast that means the engine is turning.
Early tachometers were the analogue type with a needle pointing out the RPM on a dial, while later cars often use digital tachometers.
Tachometers are also often called an RPM meter or RPM gauge or rev-counter, but they’re all the same thing. And regardless of the display methods, they’re all telling you the same thing.
Basically, as the car RPM goes up and down, the tachometer tracks this change of rotational speed.
Don’t have a tachometer? When it comes to RPM car owners who are experienced enough will be able to tell from the sound the engine makes how hard it’s revving.
More modern cars feature digital tachometers.
Why do you need to know your engine’s RPM? For the most part, you don’t. Modern automatic transmissions mean you don’t need this info to know when to shift gears and modern engines have rev-limiters which prevent the engine spinning too fast and damaging itself.
For many years, we all got along without tachometers in our mass-market cars, and only sporting or luxury cars were fitted with tachometers. But if you are driving the car hard (such as on a race-track) and you know the RPM automotive carnage can often be avoided.
But drivers who value outright performance love to know their engine’s RPM because it enables them to change gears at the best possible moment to keep the power coming hot and strong, and in cars without a rev-limiter, knowing when you’re at the engine’s maximum safe RPM is crucial.
It can also be a reminder to shift up a gear to lower the engine’s RPM and save a bit of fuel.
Should you ignore (or forget about) the tachometer, you risk blowing the engine to pieces, and that’s why sporting and race-cars often have rev limiters.
Things can happen pretty fast in the heat of battle, so having a limiter than can shut off the spark and/or fuel to the engine if the rev limit is reached can save an engine from total destruction.
In fact, on any high-performance or competition car from the fastest race cars in the world to a humble club car RPM limiter technology can save the day.
Should you ignore (or forget about) the tachometer, you risk blowing the engine to pieces.
Most petrol-engined cars have an effective RPM range of between 500 or 600rpm (a typical idle speed) all the way to 7000rpm or even more for some really sporty high RPM engines.
A diesel engine, by comparison, might have a similar idle speed, but will be at the upper limit of its rev-range by about 4000rpm.
The reason engines need to rev is because they (generally) make more power the faster they spin. Since power is simply the torque the engine makes multiplied by RPM, the more RPM you have the more power the engine will make.
(As a sideline, what does torque mean in a car? It’s the physical force the engine produces at its crankshaft. Power is simply a mathematical extrapolation of that force; how fast it can make that torque. So, in reality, it’s torque that moves the car, but power is what makes a car fast.)
So why not rev engines even harder and make more power? Because physics comes into things at high revs, and an engine that revs too high will destroy itself as the internal forces start to smash things to bits.
Think of it like a Lazy Susan. Spin the Lazy Susan slowly and a glass set on it will take a journey around the table. Spin the same Lazy Susan too fast and the glass will fly off and hit the wall.
Also, engine designers ensure the engine will make its best power and torque at a safe speed, so revving it harder is counter-productive.
RPM in car speak becomes a measure of how fast an engine is spinning.
Each engine has an upper limit on how fast it will spin without blowing up. This is what’s called the engine’s redline or its safe RPM meaning passenger car engines can’t rev as high as a (circuit) race car or a (speedway) speedcar engine can.
The limit of what RPM is too high varies from engine to engine, but generally it’ll be nothing like the redline of specialist race engine.
Current IndyCars, for instance frequently rev beyond 11,000rpm and NASCARS spend hours on end revving beyond 9000rpm.
And how much RPM does a F1 car have to work with? Well, Formula 1 cars have been reigned in these days to about 15,000rpm, but there was a time when 20,000rpm was the norm for an F1 engine.
When you consider that an engine doing 6000rpm has its crankshaft rotate 100 times per second, these race engines are amazing machines. Little wonder they sometimes expire in a shower of sparks and shrapnel.
So far, we’ve talked about RPM in terms of the crankshaft, but there are other systems on a car engine that revolve, too, and not necessarily at the same speed as the crankshaft.
The camshaft, for instance, only spins at half the speed of the crankshaft, so when the crankshaft is spinning at 4000rpm, the camshaft is only spinning at 2000rpm.
Knowing when you’re at the engine’s maximum safe RPM is crucial.
The alternator which produces electricity to keep the battery charged is usually belt-driven and thanks to the gearing involved, you’ll usually find car alternator RPM to be even higher than the crankshaft RPM.
So, what RPM should a car drive at? This is an interesting question, because in recent years, carmakers have fitted ever taller gearing to reduce the RPM at a given road speed and, therefore, reduce fuel consumption.
But in decades past, manufacturers tended to gear their cars to rev a little harder at common road speeds in order to be able to make the car accelerate faster in top gear.
Again, this was a time when four forward gears were about the best you could hope for and multi-ratio transmissions these days mean you can still have a ratio for accelerating at highway speeds, as well as one that reduces RPM and saves fuel.
The other big question is what RPM should car idle at a standstill. It depends on many things, but the rule of thumb is anywhere from between 500 and 900rpm for most modern petrol or diesel engines.
Too low and you might find the car sputters when accelerating at low RPM, to high and an automatic car might want to creep when stationary.
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