# Engineering formula to relate engine displacement and power

I am currently rating two motorcycles to decide which one to buy. The criteria which is most important to me is the power-to-weight ratio. Now, I want to calculate the power of the motorcycle by looking at its displacement. The engine displacement is the amount of air-fuel mix that an engine can burn at a time. Looking at google, I cannot find a concrete formula to convert the engine displacement (cubic centimeter) to power (horsepower).

Currently, I am looking at two models:

1. Suzuki Raider J 115 Fi (fuel injection)
2. Honda XRS 125 (Carburetor)

I know that the Honda model will provide more power but I want to look at the other model's fuel efficiency. I want to convert to see whether the power difference is not that significant. For the sake of comparison/formula, assume that you feed both with the same amount of fuel.

• displacement is only one of many factors affecting power. compression ratio is another significant factor. Really there is no simple formula for power or efficiency, you need to measure it or trust the manufacturers spec. Sep 25, 2017 at 11:56
• Oh, i see. Most specs does not show compression ratio. See here: carbay.ph/compare-motorcycles/… Sep 25, 2017 at 11:58
• This question is vague.. In first part you are asking for power to weight ratio. In the next paragraph you are talking about fuel efficiency.. Decide any one of the question and then edit the question. If you have 2 questions then you can ask them separately as new post Sep 25, 2017 at 13:21
• I'm voting to close this question as off-topic because this an opinion-based question about purchasing a cycle. Sep 25, 2017 at 14:29
• Power is irrelevant unless you include drag vs. speed. Further, torque is often far more important than power. And BTW there is no formula for turning displacement into power. There are a couple dozen other factors which affect power out per unit gasoline in. Sep 25, 2017 at 14:31

There's no simple relationship. When manufacturers quote a figure for power it's usually a peak value, which doesn't mean much without knowing the engine speed (rpm) at which this is developed. Power is time based, so an engine capable of spinning twice as fast should be able to burn twice as much air/fuel in the same time for a given displacement.

If you can find power and torque curves, these will tell you more than a single value. You'll get different power and torque curves depending on how the displacement is achieved (broader piston or longer stroke).

Power to weight ratio is a factor, but if you're looking at low speed acceleration you'll probably find that torque is more relevant.

• Thank you for the explanation. Most specifications does not show much details regarding torque, pistons and strokes, etc. Sep 26, 2017 at 0:00

Well, first I would research the manufacturer's specs for each engine and then you can divide the power by the mass which will give you a decent comparison.

However, another approach, based on more technical theory (used when they designed the engine)is shown here : https://www.quora.com/In-a-car-engine-is-compression-ratio-related-to-horsepower

An even more detailed explanation is available here, this is the approach taken in courses such as Power Plant Analysis : http://web.mit.edu/16.unified/www/SPRING/propulsion/notes/node25.html

• Does this mean that the displacement does not have direct effect in power? I am greatly concerned because 125 have the ability to suck in more fuel (noting that it is a carb) and if the 115 has the same capability, then I'll go with the fuel injection. Sep 25, 2017 at 11:14
• displacement of course directly affects power. The point is its not the only determinant. I highly doubt you could do a calculation to distinguish a 115 from a 125 even if you had all the specs at hand (compression ratio, bore stroke, cam profile, valve sizes, intake and exhaust design, etc etc) Sep 25, 2017 at 14:27