A Question About Automobile Engines

One of the gadgets in my car to which I pay absolutely no attention is the gauge that shows rpms, but the other day I happened to notice that it was up to 4000 rpms. And it occurred to me that I had no idea at all to what this referred. Is there really a part of my car engine turning at 4000 revolutions per minute? If so, what is its purpose? If not, what does this gauge measure?

Every time your piston goes up and down that is one revolution. It’s not a circular movement, it’s up and down. The part that goes in a circle is the crankshaft.

The gauge to which your refer is a tachometer. It shows how fast your crankshaft is rotating. If you have a manual transmission, you can use your tachometer (as opposed to the sound of the engine) to time your shifts.

You engine generates the most power at a specific RPM (or range of RPMs) and it is likely to come apart above another specific RPM. That information is important to racers.

Nope. The tachometer measures crankshaft revolutions per minute. While the crank does convert this revolution to linear piston movement via the connecting rods, RPM refers to revolutions of the crank.

“Revolutions per minute” refers to crankshaft revolutions or 360 degree turns. Tachometers (the gauge you mention) can be useful when driving manual transmission cars because they can let you know when to shift gears for best performance or mileage. 4000 rpm’s a pretty high road speed in top gear- what gear were you in?

Apart from speed, that really depends on the vehicle (engine, transmission gearing, final drive/rear end gear ratio etc.). 4000 RPM might be redline on a diesel engine, but it might be a standard top gear highway RPM for a car like the Honda S2000, which redlines at an astronomical 9,000RPM.

I used to have a Honda CRX that would happily rev along on the interstate at 4 grand all day long. My current M3 redlines around 7,000RPM, so shifting well above 4000RPM is quite commonplace.

Sorry for the double post, missed the 5-minute window.

Since the OP asked about the purpose of the spinning part, the crankshaft is the heart of your engine. How fast it is spinning is basically how hard your engine is working (and how fast your entire rotating assembly is spinning), which is directly related to how fast you are going, although which gear your transmission is in changes the ratio between how fast your engine is spinning and how fast your wheels are spinning. This is why your RPMs increase with acceleration, then drop when you (or your automatic transmission) shift from, say, first to second, and then continue to climb again as you continue to accelerate.

The process by which this all happens is fairly complex, so you’d probably be best to read something like this How Stuff Works page to understand how it all goes down. In my opinion, anyone that drives a car should have at least an elementary understanding of this process, although this is coming from someone who doesn’t let a mechanic lay a hand on his cars. :slight_smile:


The crankshaft is spinning at 4000 revolutions per minute, or roughly 67 revolutions per second. The piston in each cylinder is moving up and down just as fast, so 67 times a second each piston goes all the way up and all the way back down again. Most cars use a 4 stroke engine, which means they explode gas only half of the time (2000 times a minute if your engine is revving at 4000 rpm). On the other down and up strokes the engine is spitting out exhaust and sucking in gas. Car engines go suck squeeze bang whoosh. The suck stage is when the piston goes down and gas and air get sucked into the cylinder. The squeeze stage is when the piston goes back up and compresses the gas. The bang stage is when the spark plug ignites the gas and forces the piston down (this is the only stage that actually makes power - every other stage requires the inertia of the engine to make the piston move). The whoosh stage is when the exhaust valves open up and the piston shoves the burnt gases out of the engine as it moves up.

If the engine drops below a certain rpm (a few hundred rpm on most engines) it won’t have enough inertia to get through the whoosh such and squeeze stages and the engine stalls. If you rev the engine too high (maybe above 6 or 7 thousand rpm on a typical engine) the mechanical parts have trouble moving that fast and you can damage the engine.

If your car is an automatic, you generally don’t need to ever look at the tachometer (the rpm gauge). It is more important if you are driving a stick shift, although you can generally feel and/or hear the engine speed and don’t necessarily need to ever look at the tachometer.

I’ve always said “suck/squeeze/bang/BLOW”. After all, if you’re going for sexual innuendo, you might as well “go all the way”. :slight_smile:

It is an automatic transmission. I was in drive accelerating up a long, gradually-rising hill.

Your engine’s computer sensed that it was operating at an RPM level that was too low in relation to the load on it. Ergo, it instructed the transmission to move to a lower gear, (i.e. 4th to 3rd). Doing so brought the RPM up to a level that produced more power. Prior to computers, a drop in manifold vacuum pressure triggered the downshift.

Hitting 4000 RPM sounds reasonable for a situation like that, especially if the vehicle is neither exceptionally light nor exceptionally powerful.
If you were driving on flat ground at legal US highway speeds and turning 4000 RPM, that would be unusual.

The main purpose of a tachometer is ‘coolness’. You get to see the needle jump as you accelerate the engine. If you have to look at the tach to shift gears, you’re not very good at it. You can compare the tach and the speedometer to see if your getting slippage in the clutch or automatic transmission, but you should be able to tell that by feel also. It might be useful if you tune the engine yourself, but that’s about it.

engineer_comp_geek, I’m sure you must have meant gas gets exploded at rate of half the rpm times the number of cylinders. So there are 8000 bangs per minute in a four cylinder engine running at 4000 rpm.

Okay, here’s a question. Why in the hell are they putting tachometers in cars anyway? Back when I was young, there wasn’t a single car with a Tach. In fact, on the hood tachs were popular in the 1960s and early 1970s. I even believe that the Firebird actually gave you an option for an on the hood tachometer.

However, even back then, at least 30% of the cars were manual transmissions, and in most cars, manual transmissions were standard equipment.

But, in todays age, almost all cars are automatic, and you can’t get a true manual even in many so called sports cars. So, why suddenly has an instrument that does absolutely nothing in 95% of the cars on the market become a standard piece of equipment? When manual transmissions were more common, and a tach would have been useful, you didn’t see them. So, why now?

On a manual transmission car, the engine speed is a direct function of the vehicle speed and what gear you’re in. A tachometer gives you no information you couldn’t roughly deduce from the speedo. The little hashmarks on the speedo that tell an unfamiliar driver when to shift (like on old VW’s) work just as well as a tach.

So even on manual transmission cars, tachometers are just for decoration.

I’d say it starts the other way around, with the crankshaft converting the linear motion of the pistons’ power strokes into rotational motion. Of course the pistons’ other three strokes are driven by crankshaft momentum, so it goes both ways.

(Replying to the last few people who thinks tachs aren’t necessary here):

Not in my world. If you’re casually tooling around town, sure, you can tell when it’s a good idea to shift by the sound of the engine or by checking the speedometer. In an automatic transmission-equipped economy car, they’re probably not that crucial.

However, if you want to take the car all the way to the redline without bouncing off the rev limiter, if you want to know exactly when your car is in the powerband, and if you want to maximize your 1/4 mile ET at the dragstrip (including being able to launch and shift consistently from run to run), a tach is a must. If you know exactly where 7,000 RPM in first, second, third, and fourth gear is on your speedometer, and can recall that information in a split-second with the throttle floored at speed, more power to you. I’ll keep the tach. “Roughly” deducing something like engine RPM isn’t going to cut it for me – especially on my cars that don’t have a newfangled rev limiter.

TriPolar: are professional drag racers who have shift lights to tell them when to shift “not very good at it”? No, they just need to know, with a very high level of precision, when to shift in order to maximize acceleration. Knowing when to shift when you’re cruising to the grocery store isn’t hard. I don’t look at the tach then. When I’m shifting at redline on the dragstrip or during a track day, you better believe I’m looking at it…and I’m not lousy at it.

Heck, why have a temp gauge? When you see smoke, you’ve overheated it. That’s all you need. Why have an oil pressure gauge? When the engine seizes, it was too low. When your engine sputters and dies, you’re out of gas. They’re all decoration, if you don’t care what your engine is doing.

I wondered if someone would say that, haha…I thought about it as I wrote it, but I figured it would involve more explanation :slight_smile:

Then again, what if I’m engine braking?

I’m with Speedway on most of his perspective on tachs and other gauges. I do differ a bit when it comes to oil pressure and water temp. While I haven’t raced since the late 90’s, many years of drag racing saw me keeping a close eye on oil and water, at least until I was staged. Once at that point, however, never saw anything except the tach and the track.

Bear in mind, the oil and water gauges on modern cars are generally inaccurate.
There’s an ‘off’ position, an ‘okay’ position and then a ‘you’re about to die’ position.
My Hyundai’s temp. gauge reads the exact same setting from about 175 to 220 F.