Quite true for a lot of modern cars…those gauges are often a glorified “idiot light”.
The water temp gauge in my M3 is somewhere in between one of these and, say, an accurate aftermarket temp gauge that you might order from the Summit Racing catalog. My BMW gauge doesn’t just stick at one position, but it IS “buffered” so that small changes in the normal range are basically “smoothed out” – if the temp goes up ten degrees and back down ten degrees (while staying in the normal operating range) over a minute or so, you probably wouldn’t see it. This keeps the gauge from bouncing around and freaking people out, which is probably why they did it. Outside of the normal range (on either end), the buffering is not present – so it’s very accurate as the car warms up, and very accurate if you’re above normal operating temperature. It just doesn’t jump around the rest of the time.
A slight liability if the temp starts to creep up slowly, as you might not catch it as early, but not a terrible system.
EDIT: Wait, a guy named “Mr. Slant” talking about coolant temperature? Do you own a radiator cap business, by any chance?
Was there something in my post in this thread about passenger cars that gave you the impression I was talking about professional racers? I gave you a nod by mentioning someone who tunes their own engine. Most of the people who drive passenger cars for commuting, shopping, and other common activities do not need a tachometer. That doesn’t mean they are useless in specialized cases. The reason most cars have one is because they look cool, and help sell the car based on that factor.
Let me try to explain a bit further.
Inside your engine is a rotating shaft called the crankshaft. Along the length of the crankshaft (called a crank for short) are offset journals. Connected to these offset journals are rods called connecting rods. the upper end of the connecting rod (con rod or rod for short) is the piston.
As the crank journal rotates and approaches the cylinder, the piston is pushed up the cylinder. When the crank is at it very closest approach, the piston is at the very top of its travel (called TDC or Top Dead Center)
As the crank continues to rotate, the piston travels down the cylinder, when the crank journal is as far away as it can get from the cylinder, the piston is at the bottom of its travel. That is called BDC or bottom dead center.
(the following is somewhat simplified, so you gearheads don’t come in and talk about valve overlap etc, this is for illustration, not a full technical description)
so starting with the piston all the way up, the intake valve is open and the piston moves away from TDC. This sucks air and fuel into the cylinder
Near the time the piston gets to BDC (180 degrees of crankshaft rotation) the intake valve closes and as the piston rises the air fuel mixture is compressed. This takes it from flammable to explosive. near the time the piston gets back to TDC the spark plug fires (another 180 degrees of crank rotation, we are now up to 360 degrees of total rotation)
The explosion pushes the piston down HARD! Near the time the piston gets back to BDC (another 180 of crank rotation, we are now up to 540 degrees of total rotaion) the exhaust valve opens and as the crank pushes the piston up, the exhaust gasses are pushed out the exhaust valve.
the piston is now back to TDC for a total of 720 degrees of crank shaft rotation. The spark plug has only fired once in those 4 strokes.
so for any standard automobile engine* at any given RPM each cylinder will fire at 1/2 the rate of the number displayed on the tach.
Tach shows 4,000? Each cylinder is firing 2,000/ minute
What good is a tach? I find it helpful even in an automatic. Do I watch it like a hawk? No. If my car acts up, do I then watch it like a hawk? You bet.
Picture two people taking their cars in for a repair. Exact same problem on both cars
Cust #1 My car shifts funny sometimes
Service advisor: funny how?
C#1 I don’t know, just funny
Cust #2 After driving for about 15 minutes, I noticed that when accelerating away from a stop light, when the trans went to shift instead of the RPM dropping by about 500 they climbed by 1,000 or so, then dropped into the next gear.
Who do you think has a better chance at a proper diagnosis the first time though?
*yeah I know all about SAAB 93s. Nothing SAAB ever built was standard or normal.
You said if I have to look at the tach to shift, I’m not any good at it. I didn’t agree.
It was an example to illustrate that sometimes, more information than just sound and the speedo is needed for shifting, regardless of the skill set of the driver. I have a passenger car that gets driven every day to work…and also on the dragstrip. Of course, I also use my tachometer on the street (and I don’t street race). I use it on my commute, I use it on pleasure drives, I use it every time that I rev the car up to anything higher than about 5k RPM. On a high performance manual transmission car that gets driven like it was meant to be driven, a tachometer is an absolute must.
If you want to get in my car and see how close you can get to shifting at exactly 7,000 RPM without a tachometer, be my guest.
Do I use a tachometer to tune a car? Yeah, it can be used to set the idle on a carbureted engine, etc…but even those who never turn a wrench can make good use of the tachometer. I’d use one on a car straight off the showroom floor if I was driving it in a spirited manner.
If you want to say that they’re useless on a slushbox car, that’s fine. But they have a use beyond just tune-ups…and they aren’t merely a crutch for people that can’t shift.
I admit I overly generalized in that statement. Using the tach doesn’t make you suck at shifting. I should have pointed out that if you have to look at the tach to shift, then you suck at it. Are you telling me that you can’t shift your car if you can’t see the tach?
The only good use of a tach made by someone who never turned a wrench would be to train themselves to learn how to feel where to shift. I look at mine occasionally just to check myself out. That probably accounts for some tiny percentage of people. So again I’ve over generalized if you presume I meant nobody could ever find any use in a tachometer. But you are being ridiculously nit-picky in your interpretation of my statements, and admit it yourself by pointing out the uselessness of a tach in a ‘slushbox car’. I don’t know what you mean by that, but the vast majority of cars are not performance vehicles and shifting at a specific RPM is pointless anyway in road driving. Unless you calculate all the varying factors that affect engine performance and the optimal RPM for shifting, then you are just using the tach for entertainment. Entertainment is useful, but does not relate to your ability to drive.
Why on earth on you arguing over this? No is arguing that tachometers are needed in racing and possible usefulness in a tuneup (I say possible because a tach would be more useful in a tuneup if it was on the other side of the dashboard).
A “slushbox” is what us crazy car people call an automatic transmission. I like how you use that as an example of how nit-picky I was being without knowing what it meant.
Can I shift without looking at my tach? Of course I can.
Can I time a shift just perfectly to shift at redline without looking at my tach? No. Doesn’t matter if I work on my own cars or not. If I want to really wring the car out, and not bounce it off the rev limiter (or worse, in an older car), I need it. I don’t know how clear I can be in saying that I use it, on the street, very regularly. And trust me when I say there are plenty of people like me. Your opinions on the usefulness of the tachometer, though perhaps accurate for a decent percentage of the population, would get you laughed off of any car forum. Not everyone drives beige econoboxes back and forth to the grocery store all day. Useful only to train yourself how to shift? Seriously?
It’s certainly “entertaining” to drive in a spirited manner on a twisty country road, but that’s a lot different than saying the tachometer is only good for entertainment. It’s a purely functional instrument. I use it for its intended purpose. Period. That’s why it’s in my M3. It might be in a little Kia turd for a different reason, but so what?
I guess you’re either a “car guy” or you’re not…but if you don’t get it, just stop.
But you could if they would mark where the redline (or the optimum shift point, anyways) is for each gear on the speedo. The only reason why car makers moved away from this very elegant (IMHO) system is that tachs look cool and project a sporty image.
I didn’t use the ‘slushbox’ term as an example of your nit-picking, but your own self-contradiction.
I did not say it was useful only to train yourself how to shift. I said it was useful to check your shifting ability. The rest of your post is nonsense. You are using the tachometer for entertainment. It is not otherwise useful for legal, safe, economical, street driving. And I wouldn’t enter a car forum that contends otherwise. You have not presented any argument about the usefulness of a tachometer when driving. And if you are looking at it that often, you are not driving as safely as you can. And I suspect you have ruined car engines by over-revving them. My car engines have carried me over a light second without a rebuild because I don’t entertain myself with the sound of a high revving engine. Unless you are driving a lawn tractor, you don’t need to maximize the RPM for street driving.
The functional purpose of a tach is to see what speed the engine is running at. That is it’s intended purpose, and irrelevant while driving the speed limit. If you have some non-entertainment usage for your tach, say what it is or admit you are wrong.
I guess you either a ‘car guy’ like me who appreciates a good car and takes care of it, or an adolescent minded guy using his car as a substitute for something else.
Actually, a tachometer is quite useful for legal, safe, and economic driving. Just because I’ve used the example of high-RPM use doesn’t mean that’s the only thing it’s good for. You can use it to keep an engine in its most efficient range, if you so choose. Hypermilers, which are people hell-bent for some reason on getting the best gas mileage possible, rely on them heavily, since it is a tool that measures how fast the engine is turning. Some hypermilers are strict about ensuring that their vehicles never go above 2000RPM in order to get the best efficiency possible. Do a google search for hypermilers and tachometer and browse the 12,400 results.
In that case, I’d advise you to stay away from all of them.
And you can’t seem to grasp why knowing how fast your engine is turning over is useful to some people. Just because you don’t care, doesn’t mean the rest of us don’t. You also seem to have some sort of idea that I’m staring at my tachometer all the time, which is certainly not the case. I guarantee that I glance at my speedometer far, far more often. Is that OK with you? You’re kind of reaching here by making some sort of distracted driving argument.
Your suspicion is unfounded. I haven’t ruined any car engines. I have had one engine fail in my life, at age 16, and it was because a quick-lube oil-change place made a grave error (leaving the old gasket stuck to the block, allowing the oil to leak out in minutes). Since then, I’ve always done all the work on all of my cars, and nothing like that has ever happened again. See, I don’t over-rev my engine, because I have a tachometer that shows me exactly where to shift to avoid doing so. Congratulations on your mileage. Cars to me are more than an appliance to get me from point A to point B. I don’t NEED to maximize RPM on the street. I can certainly get to where I’m going without doing so. Perhaps you have some image of me as timing myself on the way to work, racing past pedestrians and barely avoiding collisions with other vehicles, just because I sometimes perform the unheard-of act of driving a car near the high end of it’s normal operating RPM range.
The non-entertainment usage of my tach IS to see what speed my engine is running at. That’s exactly what I (and everyone else) use it for. Why would that be irrelevant while driving the speed limit? Even if you were to assume that a tach is only useful when redlining the car (which isn’t true), it’s certainly possible to do so without breaking the speed limit.
That said, there are certainly other reasons to know how fast your engine is turning than the high-RPM example. I used the hypermiler example above. Another would be when using a vehicle to tow something: knowing when your vehicle is in the RPM range where it makes maximum torque is pretty handy, especially if you’re in hilly terrain and shifting often. They are useful for keeping a cold engine under a certain RPM until it’s up to temperature, when to downshift to avoid bogging the engine, all sorts of things. Some of us care about what are engines are doing…how hard they are working, keeping them within their normal operating ranges, and keeping an eye out for anything out of the ordinary. A tachometer is a tool that allows us to do those things. I don’t use it to entertain myself any more than I use a temp gauge or a speedometer to entertain myself.
I take care of my cars. That’s why I have a garage full of tools. I do everything from replacing brakes and changing oil to swapping engines, because I don’t trust other people to work on my cars to my level of satisfaction. If I do it, I know it’s done right.
Ahh, do I catch a whiff of the old “compensating for a small penis” argument? Fantastic! I greatly enjoy cars and the art of driving. It’s a hobby and a lifestyle. Cars are far more than an appliance to me…and the millions of other car enthusiasts out there. If enjoying and having a passion for something in my life means that I’m “substituting for something,” then go right on thinking that.
I’ve wasted far too much time on trying to explain something to a stranger halfway across the country. I’m going to go “substitute for something else”.
Setting aside the pissing match that this thread turned into, here’s a nice animation of the working “guts” of an engine. I’m assuming your car has a 4-cylinder engine, so this should be pretty close to what you have. As has been said, the “4,000 rpm” refers to how fast the crankshaft is spinning.
It’s pretty amazing how fast machinery can be made to spin. Yes, as has been stated, when your tach reads 4000 RPM, that’s how fast the engine’s crankshaft is spinning.
Larger engines tend to spin more slowly; whereas your car probably can’t idle at less than 600 RPM, this massive marine diesel engine cruises at around 100 RPM.
Big V8’s (5-7+ liters displacement) will cruise down the highway at less than 2000 RPM. My own car (3.5-liter V6) cruises at about 2500 RPM; a typical four-cylinder automotive engine (1.5-2.5 liters) will cruise at around 3000 RPM.
Likewise, smaller engines tend to turn faster. Whereas your car hit 4000 RPM while accelerating uphill, my motorcycle (with a 1.2-liter engine) cruises at about that speed.
I think we see the difference between “car guys” and the rest of the population. Neither right or wrong, just a different perspective/insight on things mechanical.
My car is used for legal, safe, economical, street driving. It’s got a Diesel engine and a stick shift. And a tachometer. I like to keep half an eye on the tachometer to keep the engine running at low RPMs when it’s cold (because that’s better for the engine, and I can hardly hear the engine when I’m driving), to check that I’m in the most economical RPM range on the highway or to double-check whether I’m in the highest-torque region or if I should shift down before overtaking someone.
Do you still think that the tachometer isn’t useful for legal, safe, economical, street driving?
By listening to the sound of the engine or kinda just getting a feel for how much you have to blip the throttle. You don’t need a tach. I’ve had to do it when my clutch went out and I was busy looking at the road and not the tach.
Unless you are racing or are driving a car made in about 1940 (or your clutch died like mine did) rev matching isn’t exactly important. They invented synchromesh transmissions for a reason, you know.
A tachometer is also necessary when someone is learning how to drive a manual. Sure, once you get the hang of it you can ignore it and just do everything by “feel”. But lots of people who have spent years driving don’t really understand what their automatic transmission is doing.
So for these new manual drivers they absolutely need the tach for a little while until they learn to shift by feel.
