Which mixture theoretically produces the maximum RPM?

The mixture that theoretically produces the maximum RPM in reciprocating engines is the one with a richer air-to-fuel ratio, specifically 15:1 (air: fuel). In this context, the air-fuel mixture is crucial in determining the combustion efficiency and power output of the engine.

When the ratio is richer in air, there is more oxygen available for the fuel, enabling more complete combustion. This not only increases the engine's power output but also leads to higher RPM because the engine can burn fuel more efficiently. As a result, the engine can achieve a higher rate of energy conversion, which translates into increased rotational speed.

In contrast, a fuel-to-air ratio does not favor maximum RPM because it implies a higher concentration of fuel, which can lead to incomplete combustion and subsequently lower power output. Thus, while varying the ratios between air and fuel can impact engine performance, maximizing the amount of air relative to fuel is key to achieving peak RPM.

The choice of 15:1 (air: fuel) embodies this principle, supporting the notion that an excess of air is more beneficial for maximizing engine performance and RPM.