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I have looked at small engines. Once I saw Mitsubishi G350L (made around 1986), the displacement is 141cc, but it maximum power is only 3.5HP at 2200RPM. I saw another engine, Honda GX200, 196cc, 6.5HP at 3600RPM. And another amazing Bajaj (automotive maker from India) with its Qlue, the engine is 200cc but can produce 13.3HP at 5500RPM. Indeed they are different. But the difference looks to be too much.

My question is how come the engine power so different? What is the relation between the piston's displacement and its power? Those 3 engines are relatively close in the cylinder displacement.

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  • $\begingroup$ Design design design: look at the speed and they are slow... $\endgroup$ – Solar Mike Oct 20 '18 at 4:44
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Power is the product of torque times RPM. For a given amount of engine cc's displacement, you can get more power out of those same cc's by designing the engine to spin faster. This requires changing the valve cam profiles to get the engine to spin faster and also retuning the carburetor and exhaust systems, but it is done all the time when designing engines for motorcycles.

The disadvantages of pulling more power out of an engine by getting it to spin faster are 1) at low speeds, an engine "cammed" for high speed runs very rough and will not idle at all below about 1000RPM, 2) a faster-spinning engine runs hotter and is at increased risk of overheating and failing catastrophiclly, and 3) the faster the engine turns, the faster it wears out the piston rings and the crankshaft bearings.

This means that different engine manufacturers will design their engines to balance power output against reliability and longevity in different ways, as you have seen yourself.

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Power generation in a car has to do with many factors four of which are easily modified by manufacturer and sometimes even mechanics.

One - fuel, the more fuel you get efficiently into the cylinder and the smoother you explode it better performance you get, but of course unless you are a race care driver you want to have the engine that gives you the best MPG rather than competes with a formula one.

Two - compression ratio, if you can compress more air in a short cylinder head, and use a turbine to squeeze it more, you get more power. But at the same time more wear and tear.

Three - RPM, and that's why we used to rev-up the engine before take off when we were at high school. Same engine if you make it turn faster will give you more power, but at a cost of wear and tear.

Four - designing an efficient engine, such as using lighter alloys, smaller radiators and water pumps, steeper coolant temperature gradient, minimizing moving parts, using fuel injection, designing smooth aspiration and flew of air and managing heat around the engine.

Every manufacturer has their own design which is based on a balance between their cost of design and manufacturing, and their market.

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