In our AIR SCOOP SIZE article from “Freebies” on our web site, a vehicle MPH speed (called critical air scoop speed in our methanol book) is introduced. It is the vehicle or vessel speed where the air going into an air scoop matches both the speed of the racer and the air consumption of the engine. The article provides two drag race examples: a drag race mountain motor Pro Stocker with a critical air scoop speed of 54 MPH for a 55 square inch air scoop; and a drag race supercharge Pro Modified with a critical air scoop speed of 86 MPH for a 65 square inch air scoop. Below the critical air scoop speed, the engine is pulling air in from the scoop. Jetting for a mechanical fuel injection setup would match a dynamometer test. Above that speed, the air scoop is collecting air faster than the engine is using it. Pressure builds in the air scoop, effectively supercharging the inlet. Jetting in an MFI setup may need to be richened with an increase in speed. The equation to determine vehicle speed for the critical air scoop speed is:
vehicle MPH = engine CFM x 1.64 / scoop area (square inches)
SInce that article was written, a new level of performance was reached in many classes from air scoop inlet supercharging. That is, several racers such as Pro Mod drag race engines produce racer power levels in the vehicle that are well beyond attainable power levels on the stationary dynamometer. What is the amount of pressure increase in an air scoop of a racer as a function of speed, engine air demand, and scoop area? A theoretical analysis is made to begin this discussion. It will continue if future News Letters.
Assume the following:
engine CFM = 3400
scoop inlet area = 61 square inches
Vehicle MPH for critical air scoop speed = 3400 x 1.64 / 62 = 90 MPH for the speed at which the engine air demand from the scoop equals the air going into the scoop for a forward moving vehicle. Above 90 MPH, the air going into the air scoop exceeds the natural engine air demand. Pressure builds in the scoop. The air backs up. Some of it is spoiled around a pressure head at the front of the scoop. What is the potential pressure for the amount of air going in the scoop? For that answer, the volume of air going into the scoop is determined for a vehicle speed of 200 MPH.
air volume at the inlet = area of the inlet x length of a column of air at the vehicle speed.
area of the inlet = 61 square inches = 61 / (12 x12) = 0.42 square feet
length of a column at 200 MPH facing this inlet area = 200 x 5,280 / 60 = 17,600 feet per minute
volume of air at the inlet = area x length = 0.42 x 17,600 = 7,400 CFM
7,400 CFM provided / 3,400 demand = 2.18 atmospheres or 32 psi potential above the blower
That is a lot but unfortunately never attained. Last year, my sources reported blower hat pressures of up to 4 psi. So the remainder of air is spoiling around the hat injector inlet. At 4 psi above the blower, that is turned into an added boost of 10 psi in the manifold for a typical blown alcohol engine at speed. More on this subject in the future. What can be concluded is that large hat injectors and hood scopes are excessive in area inlet and beyond a certain size, may cause more loss from wind resistance than they gain in power.