Getting the right mixtureMaximum horsepower is essential for winning races, and the correct fuel mixture is essential for making maximum horsepower. Proper jetting is one of the major tools used to accomplish this.
Fuel, oxygen, and a heat source are the three components necessary for combustion. The heat source (the ignition system) ignites the combustible charge, and the fuel and oxygen burn to create energy. The amount of energy that can be produced is based on the amount of fuel that is burned, which depends upon the oxygen available. The density or weight of the air affects the amount of oxygen carried in the air. The three factors that influence air density are temperature, barometric pressure, and humidity. Higher temperature, lower barometric pressure, and higher humidity all cause a decline in air density. Therefore, air density increases with lower temperature, higher barometric pressure, and lower humidity. As air density increases, more fuel can be burned and power output increases. As air density decreases, less fuel can be burned and power output drops.
This is why lap times often show substantial improvements when the heat of the day fades and the coolness of the evening sets in. Racers and engine tuners can monitor these changes in air density and use the information to calculate fuel-mixture adjustments. Chassis or driveline changes may also be necessary to compensate for higher or lower power levels.