Otto also came to understand one other super-important fact. If the combustion of a correct fuel-air mixture inside a sealed volume raises its pressure seven times, doesn't it make sense to compress that mixture before igniting it? That way, the factor of seven would be applied to an already compressed gas, rather than to gas at atmospheric pressure. The result would be a tremendous increase in peak combustion pressure—and much more power. To compare: Igniting the charge without pre-compression means peak pressure is about atmospheric pressure, 14.7 psi, times seven, or 103 psi. But by first compressing it at, say, five-to-one, combustion would raise peak combustion pressure to more like 500 psi. Big difference.