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22 votes
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What mechanical revolution is measured in "RPM"?

Ok, let's start from the same picture so we're on the same page: These engines are four stroke engines, which means the piston goes up and down a total of 4 times, twice up, and twice down for each ...
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17 votes
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How does the tachometer on my dashboard actually work?

While i'm not sure if there are mechanical tachometers like speedometers electronic tachometers are quite simple. An electronic tachometer works like an old analog volt meter. The speed of the engine ...
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16 votes
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Why does diesel engine have redline much lower than petrol engine redline?

In Short its based on low burn rate of diesel plus the longer stroke of the diesel engine. First you must understand the difference between these engines, the diesel works on purely compression of ...
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13 votes
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Why doesn't my rev limiter bounce?

Traditional rev limiters work by controlling the ignition and/or fuel. Ignition Control Retarding the spark (smoother) Completely cutting the spark (more aggressive) Fuel Control - Amount ...
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10 votes

Why does my RPM drop when sitting idle at traffic lights?

I am presuming this is a gasoline engine. If so, the symptoms sound very much like the throttle internals are dirty. If gunge builds up on the butterfly valve, it can restrict the air required for a ...
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9 votes
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Is it bad to drive slow at low rpms in 4th or 5th gear? Do you get better gas mileage?

To answer your question, no it isn't bad for your car to keep it in as high a gear as you can while still maintaining speed. As long as you are above idle in RPM and your engine isn't lugging, you ...
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9 votes
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2015 Honda Fit iVTEC - Shifted into 2nd Gear at Highway Speeds

Background I would be primarily concerned with the i-VTEC cam related mechanisms. I do not see that this engine is an interference engine. If it were an interference engine and you floated your ...
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9 votes
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Checking the motor's RPM without an in-dash tach

You have a few options out there. There are meters that clip onto the ignition system that show the engine RPMs that are quite simple and cheap: Tach meter at Amazon for example... You can also ...
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8 votes

Minimum RPM at idle speed

Idle speed is limited by the weight of the flywheel. A heavier flywheel allows for lower RPM idling and vice versa. If the flywheel is too light, it doesn't have enough inertia to keep the engine ...
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8 votes
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Optimal RPM range for manual transmission

It is in fact true that driving at slightly (key word) higher rpms is much better (for both fuel efficiency and engine life. The rpm range for the two can be different). Part 1. Engine life: Every ...
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8 votes

Optimal RPM range for manual transmission

It depends what you're trying to optimize. If you're looking for optimal acceleration, more RPMs are better. Most engines today produce peak power at redline. As for optimal efficiency, there's two ...
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7 votes
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Why does the power output peak and then drop as engine speed increases?

Your engine was designed in such a way that it is most efficient between 3500RPM and 5000RPM. That means that the valve timing and camshaft profiles were made in such a way that your engine "breathes" ...
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7 votes
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Can diesel engine be harmed as easily as gasoline engine by over-revving?

The rev limit of a petrol (gasoline) engine is generally set to protect the internals of the engine. Less sporty engines are designed to a cost and their components may not have the strength or ...
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7 votes
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How fast is one RPM on average?

This is a math-only question, and no one has explained the formulas. Also I like the idea of this simple question having multiple answers so... RPM is Revolutions Per Minute, but we want a time in ...
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6 votes

Why does diesel engine have redline much lower than petrol engine redline?

To expand on Anarach's excellent answer; the burn rate of diesel is slower than petrol and at higher RPM you would risk the exhaust valve opening whist the mixture in the cylinder was still burning. ...
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6 votes

Is it bad to drive slow at low rpms in 4th or 5th gear? Do you get better gas mileage?

tl dr - Yes to mileage; no to wear. Would you get better gas mileage? The general rule of thumb would be to run your vehicle in the highest gear possible without lugging the engine. This means if ...
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6 votes

2015 Honda Fit iVTEC - Shifted into 2nd Gear at Highway Speeds

It's very hard to say what damage, if any, will have occurred. All I can tell you is from my own personal experience. I once went from the red line in 3rd gear with a wide open throttle to 4th at ...
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6 votes

Driving car with half engaged clutch

To get us on the same footing, I'm going to assume by saying "clutch pressed halfway" you are suggesting the clutch be halfway engaged (meaning, you are still getting power from the engine, ...
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6 votes

Checking the motor's RPM without an in-dash tach

In addition to what JPhi1618 said... You can set any standard tach up with clips. I did this with a cheap tach when tuning my truck. They have 4 wires - positive, ground, signal, light. ...
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6 votes
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Why can a motorcycle rev higher than a car, in spite of both reaching the same speed?

One important factor is the mass of the components. Smaller, lighter components and shorter strokes allow higher RPMs. Given equal RPMs, an engine with a bigger stroke will put higher stresses on the ...
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5 votes

How do I reduce engine idling rpm without stalling it?

The issue with low idle is more significant when viewed from the perspective of oil pressure. Reducing the idle, albeit a noble cause of saving the planet, reduces overall emissions insignificantly ...
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5 votes
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Does using the entire rev range of a car damage it?

Engines are designed to withstand a certain amount of stress. When you exceed that threshold, longevity will start to deteriorate. I stated something to that effect in this answer. The engine has a ...
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5 votes

Why does diesel engine have redline much lower than petrol engine redline?

Paulster2 is right, varying the injection timing with respect to the engine crank angle timing is the main way to control the combustion process in a diesel engine. In a conventional gasoline engine (...
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  • 51
5 votes

Can 100% calculated load be reached in neutral?

Doesn't it suck up only as much as engine speed and throttle position allows? No, throttle position and engine load determine the quantity of air consumed. It can be quite hard to understand at ...
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5 votes

2001 Ford Explorer High Idle

The P1506 code is an idle air control overspeed error. From autocodes.com: The P1506 code means that the engine idle is out of factory specification. Sometimes the Idle Air Control (IAC) may have ...
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5 votes
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Volvo XC90 8V AWD car speed to piston revolution speed ratio changed after repair

Yes, I'd take it back to the shop to see if there is something they missed. While 300 rpm at 60mph is not a huge difference, to me this is about the difference of when a lock up torque converter isn't ...
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5 votes
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Does the flywheel really complete 33 revolutions per second when the engine is set to 2,000 RPM?

yes, that's correct. Now, rotating a flywheel at 2000 rpm is not a big deal: it's well within the capabilities of the material, as long as the flywheel is well-balanced. Other elements in the engine ...
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5 votes

Jeep Patriot RPM Gauge bounces / not right

I don't know for sure, but would bet the gauge itself is bad. One way you could check this is by putting an OBDII reader onto the port and check live data from the ECU to see if it matches what's ...
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4 votes

Why does the power output peak and then drop as engine speed increases?

There are various reasons as to why an engine is not efficient beyond its tuned range. Laws of thermodynamics, I do not want to get into scientific details but it simply means that you cannot ...
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4 votes
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Engine revs to 4000 RPM without pressing gas pedal

I'm betting your Jag is Drive-By-Wire (DBW), meaning, there isn't a direct connection between you and the throttle. If so, the gas pedal rheostat is probably telling the computer you are pressing it, ...
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