The internal Combustion engines runs on a mixture of petrol and air inducted from the atmosphere to generate power. The fuel injectors, as their name suggests, supply an exact amount of fuel to the engine. That fuel then combines with the air delivered through your intake system. A spark plug (or glow plug for diesel engines) ignites the volatile gas and the small explosion forces a piston downward, turning the crankshaft. That piston-moving force is what produces horsepower.
Maintaining a healthy and precise air-to-fuel ratio is critical to engine performance. In fact, your engine needs almost fifteen times more air than fuel for efficient combustion. This is why a cold air intake system can have such a powerful impact on the overall health, durability, and performance of your vehicle.
Now, cold air is denser than warm air, so in cold weather the number oxygen molecules entering the combustion chamber per unit volume of air is significantly greater than when the temperature is high.
More oxygen provides better combustion, and hence a noticeable increase in power.
Similarly, on a damp day air sucked into the carburetor contains a lot more water vapor than usual. Although this has the detrimental effect of reducing the proportion of “useful” constituents of the combustible mixture, it is compensated for by the fact that water vapor, when heated, has a high coefficient of expansion; the net effect is more power than if the air were very dry.
Other factors also play a part. High barometric pressure, for example, increases the density of the air, so your car should perform better, other things being equal, driving through an anticyclone than when a deep depression is passing overhead.
And it should also run better at the seaside than on a mountain pass, because at high altitude the air is rarefied, again providing less fuel-burning oxygen than at sea level.
The accepted rule, apparently, is that the power from an internal combustion engine drops off by some 3 per cent for every increase of 1,000 ft in altitude.