windage

Windage is energy lost to the mechanical stirring and aeration of lubricating oil as rotating engine components move through it at high speed. The crankshaft, connecting rods, and pistons displace oil within the sump, creating turbulence and drag that opposes rotation. This loss increases sharply with engine speed and oil viscosity, becoming especially pronounced above 5000 rpm in high-revving gasoline engines.

The primary cause is the crankshaft itself. As it spins, the crank throws and journals physically push oil out of their path, creating vortices and splashing. Piston skirts also churn the oil near the cylinder walls. At low speeds this energy cost is trivial, but at 7000 rpm a poorly designed sump can waste 1 to 3 percent of brake horsepower to windage alone. Drag racing and performance engines often employ baffles, crankshaft scrapers, and modified sump geometry to reduce it.

Engine designers manage windage through multiple strategies. Deep-sump pans with internal baffles keep oil level away from rotating components. Crankshaft scrapers, thin plates that nearly touch the rotating crank, deflect oil downward rather than letting it slosh up into the crankcase. Racing engines may use dry-sump systems that scavenge oil continuously into a remote tank, eliminating the problem entirely. Thinner synthetic oils reduce viscous drag compared to heavier conventional oils, though adequate film strength must still be maintained.

Windage loss also contributes to heat generation within the sump. Churned and aerated oil loses cooling efficiency and may foam, degrading its protective properties. Proper ventilation of the crankcase through positive crankcase ventilation (PCV) helps manage pressure buildup that exacerbates churning. Modern low-viscosity oils like 0W-20 minimize windage losses in production engines while still meeting film strength requirements under load.

The term reflects its mechanical reality: the oil resistance to the passage of rotating parts through it, akin to air resistance or drag on aerodynamic surfaces. For street vehicles, windage is a minor efficiency concern masked by other losses. For motorsport applications, particularly those limited by fuel consumption or operating at sustained high rpm, reducing windage through sump design and oil selection becomes part of competitive tuning.