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Car companies advertise power of an engine at different rpms.

Car 1: 80 PS at 6000 rpm

Car 2: 85 PS at 6500 rpm

What does the power at different rpm mean? Does it mean Car 1 has better power than Car 2 because it is at lower rpm?

Or does the rpm not affect the power comparison?

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very good question.. –  Anarach May 28 at 12:59

3 Answers 3

up vote 12 down vote accepted

What does the power at different rpm mean? Does it mean Car 1 has better power than Car 2 because it is at lower rpm?

It depends.

Or does the rpm not affect the power comparison?

tl;dr: The rpm of the power peak affects the engine's usability for different applications.

The "peak power" number is just one point on the power band of the engine. Ideally, you'd like to know the entire curve (and how its affected by ambient air temperature, altitude, humidity, phases of the moon, etc):

An example of a power curve showing both torque and horsepower.

In this figure, we see the horsepower and torque curves for two engines (where torque is solid and horsepower is dotted). Remember, torque is the engine working on the axles and, thereby, the wheels and tires. This is what accelerates the car forward.

If you know the torque of the engine at a particular rpm, you can calculate the horsepower at that rpm:

horsepower = (torque * rpm) / 5252

Admittedly, the above equation is specific to crazy old-time Imperial units. Insert appropriate conversion factors if you'd like to get metric units out instead.

Very casually, you can imagine horsepower (or "power" if you're being unit agnostic) as being "that which maintains speed in spite of drag." We generally expect a high horsepower vehicle to have a higher top speed (provided that it has the gearing to reach that speed).

Looking at the above equation, you can also see that, in these units, the horsepower and torque curves always cross at 5252 rpms (i.e., the scalar values are equal even though they're totally different units).

So what?

All of the above helps you understand what the manufacturer is telling you a bit better. What's missing is "what do you want from a car?"

Going back to the chart, you can see that the solid line has a torque peak at about 2500 rpm and that it doesn't drop off until about 4000 rpm. This means that, from a stop, the car will feel like it pulls away strongly, right away. However, as the rpms get much higher, the engine will seem to run out of breath, accelerating much slower at 6000 rpm than it was at 1000 rpm. Qualitatively, this is what we would expect from a large-ish displacement normally aspirated engine.

The dotted line has a torque peak at about 5500. It will feel sluggish from a stop and seem to wake up as the rpms increased. From 5500 to 7500, the dotted engine will out-accelerate the solid engine by a significant margin. This is roughly what we would expect from a smaller engine (and forced injection would only increase this late-rpm peak).

The question for the customer is: which do you like better?

Qualitative summary:

  1. The early torque peak of the solid line is fun from a standing start but you will need to shift early (trading mechanical advantage to get back to the torque peak). Hopefully, you have enough gears to get you to top speed. This profile is often preferred in a street car.

  2. The later peaks of the dotted line is sluggish from a standing start but becomes progressively more exciting as revs increase. You won't need to shift as early to stay at peak power, keeping the mechanical advantage for more revs. This profile is often preferred in a race car.

Full disclosure: I've owned low-end, high-end and (high-end + turbo) vehicles and I prefer the last combination. I haven't been interested in standing starts from a red light for many years.

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Nailed it!! As a person with little knowledge about cars trying to analyze what car makers mean by these figures, your answer was perfect . Esp the two in the summary –  user3041058 Dec 9 '13 at 14:11
Great explanation. Do you prefer the peaky curve even just for street driving, or is it mainly because you track your car? –  andrewb Dec 10 '13 at 2:13
@andrewb, the (high-end + turbo) in my car is still pretty modest. What that combination really does for me is provide a broad swath of torque from about 3000 RPM to the redline. Below 2500, it's super duper sluggish in anything except first gear. Back in the day, my low end car felt like it was running out of breath by 4000 RPM. –  Bob Cross Apr 2 '14 at 18:55

You don't need to worry about the RPM, just what the engine is capable of putting out. Engine 2 has more power, but together with differences in gearing and everything else, the fact that it does more RPM doesn't mean much. It could just be that the engine puts out about the same force as car 1 (Torque) but turns slightly faster therefore it has slightly more power. Power isn't actually how "strong" your engine is, that's torque. Power is essentially torque multiplied by how fast the engine turns. But aside from technicals, the RPM at which is produced its max horsepower doesn't matter much. There are more important things like the shape of the torque curve, etc. that they don't tell you.

The RPM does not affect the power comparison.

Based purely on what you said, Car 2 would have the engine better no matter what the RPM ratings were. It could be 85 PS at 3000 rpm and it would still be better (and probably a lot more torque-y!)

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The deciding factor of an engines output is BMEP. Brake mean effective pressure. The dimensions of the engine components such as crankshaft throw, cylinder bore diameter and stroke will decide the BMEP. Further consideration would be volumetric effciency, ignition timing and the number of cylinders. An engine may be designed for economy, or power, or simply to have dimensions to fit under a bonnet or a transmission assembly. So the output will vary for what the engine was designed to do, small economy vehicle or muscle car. The most widely accepted measure of an engines output is the kilo Watt. A greater number of kilo Watts does not always mean a better engine, it is design dependant. One horsepower is equal to 0.746 kilo Watts. 1 PS is slightly less then a 1 horsepower. PS is a German measurement which together with the horse power that has fallen out of general use but keeps on appearing. Motor manufactures will always use descriptions of their product which will be the most seductive to their target audience to maximise sales.

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