What does RPM gage measure?

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Originally Posted By: HerrStig
An oil pressure gauge or amp meter might just tell the owner something, a tach with an automatic transmission is just a decoration.


Don't tow with a truck in the mountains, do you? :)

A tach is useful, if you know how to use it. So are all the others you mentioned- I vastly perfer ammeters over voltmeters because they instantly tell you whether the alternator is keeping up with the electrical loads, falling behind the load causing current to be pulled from the battery and discharging it, or actively pushing charge into a battery. When I used to drive my '66 Polara (ammeter in dash) every day and occasionally had to jump-start someone else, I could hook up the jumper cables and watch my car's ammeter to tell them when their battery was getting full enough to go ahead and crank their car. I could also tell them pretty reliably if their battery was bad, because the charging current would drop MUCH quicker than if it was good but discharged.

But in practice it is harder to make an ammeter reliable. Old Mopars all have ammeters, but it means passing the entire battery current (except for the starter) through the ammeter or through a high-power / low ohm shunt resistor and through a parallel voltmeter to make an ammeter. Either method inserts losses or high-current contacts in the system which are prone to fail over time. At a car show, look how many old Mopars have the alternator and battery wires through the dashboard bulkhead bypassed because the original spade connectors eventuallly got a little corrosion, which causes half an ohm of resistance, which at a mere 10A charge current creates 50 watts of heat in the connector and melts it. Don't ask me how I know this....... :p

Ammeters also move around a LOT just during normal operation, so like real oil pressure gauges they tend to confuse and scare uneducated operators.
frown.gif
 
You really thought the guy asking what does RPM gauge measure would know what a crankshaft is? Won't you agree with me that if he knew what a crankshaft is, he would have already known that it rotates!

"Flywheel" as answer would have been little better.

I would have said "when you open the hood and see something spinning? Locate something about a foot in diameter and a belt going over it and rotating? The RPM gauge is measuring how many complete circle that thing is going every single minute.

But I knew his question was not serious, so I did not answer. Did you notice even Jason stayed quiet even though the RPM labeling is his pet peeve? He knew ours chains were being yanked.
 
I just thought of a way I regularly use my tach (manual transmission). Sometimes, coming to a stop, I'll keep an eye on the tach and when the RPMs drop to about idle, I'll slip it into neutral without using the clutch.
 
Originally Posted By: Vikas
You really thought the guy asking what does RPM gauge measure would know what a crankshaft is? Won't you agree with me that if he knew what a crankshaft is, he would have already known that it rotates!

"Flywheel" as answer would have been little better.

I would have said "when you open the hood and see something spinning? Locate something about a foot in diameter and a belt going over it and rotating? The RPM gauge is measuring how many complete circle that thing is going every single minute.

But I knew his question was not serious, so I did not answer. Did you notice even Jason stayed quiet even though the RPM labeling is his pet peeve? He knew ours chains were being yanked.


I know what a crankshaft is and it's function. What I didin't know was what EXACTLY the gauge was measuring, the crankshaft, the wheels (now I know indirectly) or the pulleys (which is what I see spinning under the hood, according to what you "would have said"). There are so many things spinning in a vehicle, I wasn't sure exactly what the tachometer was measuring and why it's included in every vehicle.

With the assist of Clevy, Pete and other posts on here (exluding yours), I now have a BETTER understanding of how it works and why it's in every vehicle and how it can be used in an automatic.

If you think I was pulling your chains, why did you post, follow or even read the thread? You need to remember not everyone on this forum knows everything about a vehicle or how it works. Everything I've learned about maintaining on my vehicles, I've learned on here from experienced people who know how to work under the hood AND are wiling to assist those of us who don't.
 
Originally Posted By: Eosyn

Personally, in automatics, I'd rather have other gauges that will give information I'm more interested in, such as outside temp, avg MPGs, more tripometers (use these to keep track of my fluid changes). I just don't see the need for a tachometer in automatics (then again, I'm not a mechanic. perhaps they are used to troubleshoot problems)?
21.gif



No offense to you or the other Corolla owners, but that sounds like something a Corolla owner would say!
crackmeup2.gif
"Forget about that engine thing, just tell me weather and MPG!"

Maybe it comes from first having a manual trans vehicle, but even in automatics I use the tach a lot, especially if I am familiar with the vehicle. It tells me how the engine is running, where it's at in its power band, what gear it's in, when the trans shifts/duration of the shift, and can indicate many engine and trans issues. The tach even tells me when it's time to clean the IAC valve.
 
Originally Posted By: 01rangerxl


No offense to you or the other Corolla owners, but that sounds like something a Corolla owner would say!
crackmeup2.gif
"Forget about that engine thing, just tell me weather and MPG!"

Maybe it comes from first having a manual trans vehicle, but even in automatics I use the tach a lot, especially if I am familiar with the vehicle. It tells me how the engine is running, where it's at in its power band, what gear it's in, when the trans shifts/duration of the shift, and can indicate many engine and trans issues. The tach even tells me when it's time to clean the IAC valve.



And the Corolla owner says, "huh?"
 
Originally Posted By: TFB1
Originally Posted By: 01rangerxl


No offense to you or the other Corolla owners, but that sounds like something a Corolla owner would say!
crackmeup2.gif
"Forget about that engine thing, just tell me weather and MPG!"

Maybe it comes from first having a manual trans vehicle, but even in automatics I use the tach a lot, especially if I am familiar with the vehicle. It tells me how the engine is running, where it's at in its power band, what gear it's in, when the trans shifts/duration of the shift, and can indicate many engine and trans issues. The tach even tells me when it's time to clean the IAC valve.



And the Corolla owner says, "huh?"


They probably ignore the CEL they eventually get too.

Pretending your car can't have issues isn't as good as knowing everything is fine.
 
Originally Posted By: 01rangerxl
Originally Posted By: Eosyn

Personally, in automatics, I'd rather have other gauges that will give information I'm more interested in, such as outside temp, avg MPGs, more tripometers (use these to keep track of my fluid changes). I just don't see the need for a tachometer in automatics (then again, I'm not a mechanic. perhaps they are used to troubleshoot problems)?
21.gif



No offense to you or the other Corolla owners, but that sounds like something a Corolla owner would say!
crackmeup2.gif
"Forget about that engine thing, just tell me weather and MPG!"

Maybe it comes from first having a manual trans vehicle, but even in automatics I use the tach a lot, especially if I am familiar with the vehicle. It tells me how the engine is running, where it's at in its power band, what gear it's in, when the trans shifts/duration of the shift, and can indicate many engine and trans issues. The tach even tells me when it's time to clean the IAC valve.



Your implication that corolla owners are not auto mechanics, you are correct, especially in this case. It's good that you can read and understand all that information a tach can give you, but to most drivers, it means nothing.
 
Originally Posted By: 01rangerxl
No offense to you or the other Corolla owners, but that sounds like something a Corolla owner would say!
crackmeup2.gif
"Forget about that engine thing, just tell me weather and MPG!"

Thanks for the chuckle.
smile.gif
 
Originally Posted By: Eosyn
Originally Posted By: Clevy
Rpm measures Crank rotations per minute.

They are there just as another point of reference. The driver can see at what point to shift(manual)and whether there is any slip,which could indicate potential problems.
In the 70's and 80's they were in sportier type cars. Now they are there to make the driver think they are in a sporty type car.
Real boys and girls use a tach with an audible indicator for redline so they DON'T have to take their eyes off the ROAD. That isn't OEM.

Thanks for the info Clevy
thumbsup2.gif


So it measures the rotations/revolutions of the Crankshaft?

Personally, in automatics, I'd rather have other gauges that will give information I'm more interested in, such as outside temp, avg MPGs, more tripometers (use these to keep track of my fluid changes). I just don't see the need for a tachometer in automatics (then again, I'm not a mechanic. perhaps they are used to troubleshoot problems)?
21.gif



Originally Posted By: Clevy
Originally Posted By: mechjames
Revolutions per minute. Basically how many times your engine turns over per minute at whatever throttle you're at. It usually measures spark from the ignition coil(s) on a gas engine, and charging output from the alternator on older diesel engines. I'm not sure what they measure on the new ones, possibly using a crankshaft position sensor or the cam position sensor with multiplication.


What?

um, that really confused me too
confused2.gif
I really don't understand "engine turning over." I'm quite sure it doesn't mean the engine is physically turning itself round and round under the hood. I'm not being a smartbutt here, I just really don't understand some of the jargon used in mechanic world. Thanks for the post, nonetheless, Mechjames
cheers3.gif
 
Originally Posted By: Hokiefyd
I believe Rick is not correct. A 4-stroke engine does not take two revolutions to move the displacement of the engine. By definition, an engine's displacement is (essentially) bore X stroke X number of cylinders. You take the square area defined the bore's circle and multiply that by the stroke to get the volume of that one cylinder. It takes exactly one engine revolution for the piston to travel down and travel up again, thus moving one displacement's volume of air. Depends on your definition of "move" ONE of a 4 cycle's strokes doesn't MOBE it in or out, it COMPRESSES the air.

Now, it does take two engine revolutions of a 4-stroke engine to complete the 4-stroke cycle, as Stelth notes above. But indeed takes one engine revolution to move one displacement's volume of air.
 
Originally Posted By: Hokiefyd
Originally Posted By: SteveSRT8
Man, so far off it's not even funny!

What kind of engine? A regular 90 degree V8 needs one revolution to fire FOUR cylinders. One cylinder can indeed complete a full cycle in ONE revolution of the crank.


By 'full cycle', I think we mean the entire 4-stroke cycle. As in, intake, compression, power, exhaust. Every 4-stroke engine I'm aware of, regardless of the number of cylinders, needs two revolutions of the engine to complete the 4-stroke cycle for any given cylinder in that engine.
+1 and displacement is not stipulated by two or 4 cycle.
 
Originally Posted By: 440Magnum
Originally Posted By: HerrStig
An oil pressure gauge or amp meter might just tell the owner something, a tach with an automatic transmission is just a decoration.


Don't tow with a truck in the mountains, do you? :)

A tach is useful, if you know how to use it. So are all the others you mentioned- I vastly perfer ammeters over voltmeters because they instantly tell you whether the alternator is keeping up with the electrical loads, falling behind the load causing current to be pulled from the battery and discharging it, or actively pushing charge into a battery. When I used to drive my '66 Polara (ammeter in dash) every day and occasionally had to jump-start someone else, I could hook up the jumper cables and watch my car's ammeter to tell them when their battery was getting full enough to go ahead and crank their car. I could also tell them pretty reliably if their battery was bad, because the charging current would drop MUCH quicker than if it was good but discharged.

But in practice it is harder to make an ammeter reliable. Old Mopars all have ammeters, but it means passing the entire battery current (except for the starter) through the ammeter or through a high-power / low ohm shunt resistor and through a parallel voltmeter to make an ammeter. Either method inserts losses or high-current contacts in the system which are prone to fail over time. At a car show, look how many old Mopars have the alternator and battery wires through the dashboard bulkhead bypassed because the original spade connectors eventuallly got a little corrosion, which causes half an ohm of resistance, which at a mere 10A charge current creates 50 watts of heat in the connector and melts it. Don't ask me how I know this....... :p

Ammeters also move around a LOT just during normal operation, so like real oil pressure gauges they tend to confuse and scare uneducated operators.
frown.gif




How hard is it to attach new spade connectors, bypassing the shunt because the connectors fail is pretty wimpy. Personally, I crimp and solder them in high current applications. The shunt resistor is NOT in the starting circuit on old mopars. I have several pieces of test gear which can do RPM counting for trouble work.
 
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Originally Posted By: Eosyn

Personally, i It's good that you can read and understand all that information a tach can give you, but to most drivers, it means nothing.


Dear lord, I hope its not MOST drivers! I've been assuming that a slight majority of drivers are at least semi-aware mechanically, maybe its now become a minority.
 
Originally Posted By: HerrStig

Now, it does take two engine revolutions of a 4-stroke engine to complete the 4-stroke cycle, as Stelth notes above. But indeed takes one engine revolution to move one displacement's volume of air.


To move it where??? Just squeezing the air/fuel charge till the piston is at TDC isn't moving air in my book, that only happens on the second revolution when the exhaust is expelled from the engine...
 
Originally Posted By: HerrStig
Originally Posted By: 440Magnum
Originally Posted By: HerrStig
An oil pressure gauge or amp meter might just tell the owner something, a tach with an automatic transmission is just a decoration.


Don't tow with a truck in the mountains, do you? :)

A tach is useful, if you know how to use it. So are all the others you mentioned- I vastly perfer ammeters over voltmeters because they instantly tell you whether the alternator is keeping up with the electrical loads, falling behind the load causing current to be pulled from the battery and discharging it, or actively pushing charge into a battery. When I used to drive my '66 Polara (ammeter in dash) every day and occasionally had to jump-start someone else, I could hook up the jumper cables and watch my car's ammeter to tell them when their battery was getting full enough to go ahead and crank their car. I could also tell them pretty reliably if their battery was bad, because the charging current would drop MUCH quicker than if it was good but discharged.

But in practice it is harder to make an ammeter reliable. Old Mopars all have ammeters, but it means passing the entire battery current (except for the starter) through the ammeter or through a high-power / low ohm shunt resistor and through a parallel voltmeter to make an ammeter. Either method inserts losses or high-current contacts in the system which are prone to fail over time. At a car show, look how many old Mopars have the alternator and battery wires through the dashboard bulkhead bypassed because the original spade connectors eventuallly got a little corrosion, which causes half an ohm of resistance, which at a mere 10A charge current creates 50 watts of heat in the connector and melts it. Don't ask me how I know this....... :p

Ammeters also move around a LOT just during normal operation, so like real oil pressure gauges they tend to confuse and scare uneducated operators.
frown.gif




How hard is it to attach new spade connectors, bypassing the shunt because the connectors fail is pretty wimpy. Personally, I crimp and solder them in high current applications. The shunt resistor is NOT in the starting circuit on old mopars. I have several pieces of test gear which can do RPM counting for trouble work.


Oh its technically easy enough... but if the factory were to make it truly long-term reliable in modern vehicles the way 80+% of old Mopar owners do (the rest bypass the ammeter and cram in a voltmeter) it would cost 5 pennies more per car than a voltmeter, and that's what makes the voltmeter the universal instrument nowdays. :-(

Its interesting that in the late 70s or early 80s, Chrysler went from a full-current ammeter to a shunt resistor with a low-current meter movement in the dash. The problems in the field changed from burned connections with the old system to high-resistance forming in the low-current path to the meter over time so that while the charging system contined to work, the meter gradually stopped showing anything. Darned annoying.
 
Originally Posted By: 440Magnum

Its interesting that in the late 70s or early 80s, Chrysler went from a full-current ammeter to a shunt resistor with a low-current meter movement in the dash. The problems in the field changed from burned connections with the old system to high-resistance forming in the low-current path to the meter over time so that while the charging system contined to work, the meter gradually stopped showing anything. Darned annoying.




Ford did that with the Fox Stang, T-Bird etc, talk about a limp one Viagra couldn't fix...

The connections are generally fine, problem is due to the "C" magnet inside the movement going dead... I "recharged" one from a large speaker magnet and it worked great, for about three weeks...
 
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