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I have been looking at the power bands for my car (2000 Opel Agila 1.2) recently, as I want to work out when it is best to change gear and stuff, as I have no life!

This is the approximate graph I made up in Excel comparing power and torque:

enter image description here

Why does the power drop after about 5500 RPM?

Why does the torque drop after 4000 RPM?

I always thought higher speed means more power, so why don't they both keep increasing past these points?

Note: The engine is naturally aspirated...

  • 3
    Your graph is a little deceptive to most who have seen/studied tq/power graphs before. This is because on most graphs, both the tq/power levels are shown from the same scale and not differing as you have them (ie: 60Nm on the same line as 30kW ... I'd expect to see 60Nm online with 60kW). Not a big deal, but may throw some people off if they aren't looking for it. – Pᴀᴜʟsᴛᴇʀ2 Jul 22 '15 at 18:22
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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" best between those speeds. That's why you have the most torque in that region. Another thing is that as the RPM increases, it gets harder and harder to get the optimal amount of air and fuel into the cylinder AND burn it at the optimal rate. The faster the engine revs, the less time there is to suck in, compress, burn and blow out. This affects your ignition timing in turn, which means you start the burn process earlier than you want so that the flame dies just before the exhaust valve opens.

Now, power is essentially just Torque X RPM, so it's just a matter of multiplying your engine speed by your torque to get the power and understand why your power curve looks the way it does.

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    I thought HP=Troque*Rpm/5252 am i missing something here.. – Shobin P Jul 22 '15 at 12:50
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    The 5252 is simply a unit conversion factor. The conceptual idea is that power is torque times frequency. – Bill N Jul 22 '15 at 12:58
  • Just wanted to add that ALL of the hard parts of your engine are tuned to operate in this range. The intake and exhaust manifolds, throttle body, cam, valves, valve springs, cylinder head, pistons, crank, rods. Spark and fuel are easy to adjust on the fly. VTEC helps with the cam. There are also variable intake manifolds. Its all a balance, and most vehicles prefer low/mid range power as that is where the engine spends most of its time. Lower RPM also has the advantage of better MPG and reliability. Some sportier cars will tune their engines to make their maximum HP at higher RPM(8000+) – rpmerf Jul 22 '15 at 13:45
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    It should be added that the Agila has a surprisingly "sporty" engine in that it runs best at mid-high range. I had a Corsa that was much the same. It felt happiest when you waited a bit before shifting gears. They're an absolute pleasure to drive in town for this reason. – Captain Kenpachi Jul 22 '15 at 14:07
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    if you want to conceptualize how detailed these engineers can get-- the air intake runners can be tuned to be just long enough so that the air shock wave of the intake valves closing will bounce to the end of the runner and back just in time to force a little more air into the cylinder as the shock wave reaches the now-open intake valve again. I think stuff like that is fascinating – seizethecarp Mar 7 '18 at 19:01
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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 transfer heat and convert it into energy efficiently beyond a certain point where the ambient temperature and cylinder pressure start to make more impact.
  • Geometry of the cylinder: Every cylinder is not built in a similar parameters , some engines make the pistons move faster and they basically have less time to do the complete otto cycle(intake, compression, combustion, exhaust) that is why motorcycles engines rev hard but produce less torque comparatively.
  • Friction: After a certain point of time due to high RPM there is a lot of friction in the engine torque is basically the amount of force required to turn a wheel for instance, when friction increases the amount of torque reduces..

Cutting it short , imagine you are pushing a cart from stand still, and your leg movement is RPM , typically to get the cart off the line you will need maximum energy at this time your legs are not moving very fast, but when you are upto a certain speed your legs will move fast but you will not need much energy to push the cart and keep it coasting so if we plot what you were doing on a graph it will be similar to the torque curve on a engine.

enter image description here

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Your engine loses torque and power at those rpms because the way the engine is tuned/designed. There are many factors to change this. (Some) of The factors are are airflow,fuel intake, air to fuel mixture, camshaft,valve springs etc. So when your engine starts losing that power/torque the factors mentioned play a big role in the power and torque your engine is making. For example a ls3 may continue to make torque up to 4400 rpm while a camed and valve spring upgrade may help it make torque up to 5900 rpm for instance.

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Well every car has a maximum power output. After a certain point, the engine doesn't produce any more power. The RPM power range of your engine is dictated by how powerful it is, whether it is naturally aspirated or super/turbo charged, etc...

Also past the 5500 RPM point on your car is most likely hitting the "red" section of your rev counter. The extra revs after this point are probably just there to account for any potential spike in RPM.

The torque of an engine dictates how fast an engine can produce the power needed to accelerate your car. At 1.2 in capacity, your engine will only produce so much torque and after a certain point the rate at which your car can accelerate will reduce until you can go no faster.

Sorry for my shoddy explanation, its the best way I can describe it.

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The engine starts to consume more power in order to keep the piston and crank at high rpm due to higher friction associated with higher rpm.

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