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Ram pressure formula3/17/2023 The suspension of more methanol as vapor with fewer fuel droplets is beneficial to combustion, as is the additional cooling from the vaporization.Īt the Critical Air Scoop Speed, the fuel injector behaves normally. At low-speed, the small vacuum is adding to methanol vaporization. It may be more of a resistance to airflow than the power that is contributed by the positive pressure.”Ī portable weather meter can be used to indicate the speed of the air from a fan into a stationary engine air scoop on a dynamometer test, which can then be used to determine a fuel system adjustment trend.Ĭonsider a forward-facing mechanical fuel-injection hat on a methanol-fueled racing engine. According to the Kinsler Handbook: “At a certain speed, extra air is deflected around the scoop. The pressure increases with vehicle speed, and the associated “boost” can require fuel enrichment, or as mentioned earlier, a reduction in ignition timing.Ī fuel system may have differing mixture requirements for the various modes that can be numerically categorized by CASS. In this state, there is positive pressure in the air scoop. Above that speed, the volume of air the scoop accumulates is greater than the volume of air the engine is ingesting. That is the critical air scoop speed or CASS. Then, at a certain speed, there is a balance between the volume of air forced into the air scoop and the air consumption of the engine. There is a small vacuum within the scoop from the engine (or supercharger), drawing in more air than is provided from the forward-velocity of the vehicle. With a forward-facing air scoop at lower vehicle speeds, the engine is pulling air into the scoop. Alternatively, an increase in fuel at speed will usually be a better alternative and make more power. Many tuners compensate for the extra air at high speed with ignition retard while leaving the fuel system the same as the low-speed setup. So, a faster speed or a larger inlet area is needed for increased horsepower from the ram-air effect. More air consumption from a larger engine will empty the volume of the scoop more rapidly. Not much speed or inlet area is needed before horsepower is increased from ram-air, and there is a need for more fuel. Low air consumption from a smaller engine doesn’t empty the air scoop very much. A faster speed differential rams more air in, resulting in a greater need for fuel enrichment.įinally, the amount of ram-air pressure is dependent on the air consumption of the engine. A slower speed differential has less air entering the scoop with a reduced need for fuel enrichment. The amount of air being rammed into the scoop is also dependent on the speed at which the scoop is moving relative to the air. Conversely, a larger frontal-area brings in more air with a greater need for fuel enrichment. A smaller frontal-area brings in less air with a reduced need for fuel enrichment. The first factor is the size of the scoop inlet facing the air. It is dependent on the relationship between the amount of air being rammed into the scoop and the amount of air the engine is actually consuming. Tuning for ram-air is usually trial-and-error and quite an extensive process. ![]() ![]() Inlet Area, Airspeed, and Engine Air Demand This setup was considerably faster with the scoop than without it. Popular stack-type Hilborn mechanical fuel injection is shown on an early Ford Flathead V8 racing engine with an air scoop for added ram-air effect.
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