The weight of gasoline in relation to the weight of water. The important thing is to always use the same density fuel. Changing fuels can richen or lean an engine. Good race fuels never change their density. Fuel density is kind of like oil viscosity.
Heavier weights flow less volume through a given opening.
This is an intense pressure wave within the cylinder. This is premature ignition before the spark plug fires, usually caused by something glowing inside the cylinder. Do not confuse this with detonation. It is not the same. The sum total of all the factors in your motor's equation.