According to Carroll Smith, author of Prepare To Win, Tune To Win, and Engineer To Win:
A properly designed air box (scoop) can do several things:
1. Increase engine power by increasing the flow of air through the engine and by providing the coolest available air to the engine.
2. Even out the distribution of air to the intake stacks.
Functional air boxes of the early Formula One Teams all had very large intakes, complete with generous radii. The intakes invariably lived well up in the breeze -- just behind and over the driver’s head. The inlets fed large diffusers which also served as plenums, and bases of the diffusers were sealed to the intake stacks. The outside shape was carefully sculpted to reduce drag.
So what is actually required to make an air box work -- and why do so many of them not work?
First, the intake must have enough area to get a sufficient volume of air to the engine at low speed when the ram doesn’t work. Remember that airflow into the box varies with speed while air required by the engine varies with RPM. We’re not trying to ram at low speeds -- we are merely trying to avoid choking the engine. A large enough inlet for low speeds probably means that we will over ram at high speed. The most common method of dealing with this situation is to depend upon leakage between the plenum base and intake stacks or on small bleed holes in the plenum. Along these lines, it is absolutely necessary, if you are using carbs, to make sure that the float chambers are seeing the plenum pressure. Otherwise your mixture strength is going to be off. If the air box is really working it will also be necessary to supply more fuel to the engine or you will run lean -- maybe even enough to burn a piston.
Next the edges of the intake opening must be well radiused to avoid partially stalling the inlet air. The intake must be so located that it cannot be blanked out by some other part of the vehicle (or driver). It must also be served by high energy air which means out in the free airstreem for a ram duct.
The purpose of the diffuser/plenum is to persuade the airflow to turn ninety degrees as smoothly as possible and to provide an expanding chamber in which the velocity component of the airstream energy will be converted to pressure which will be equal at each intake stack. We also have to avoid turbulence at any of the intakes (venturi).
Theoretically the plenum should clear all of the intake stacks (venturi) by at least 1/2 diameter and a full diameter would be better. This is sometimes a bit difficult to achieve. It is probably more important to get the shape right and to keep the last six to eight inches of the vertical walls vertical. There is a theory that the stacks should extend into the plenum chamber to minimize turbulence but it doesn’t seem to make any practical difference.
Speaking of inlet stacks, any basic air conditioning book informs us that for maximum undisturbed flow the lip of an inlet stack should have a full radius.
The outside shape of the air box is a question of minimizing drag. You should make certain that the whole thing is securely enough attached that there is no possibility at all of its coming off.
Assigning some basic numbers to the features under discussion:
Intake Ram (PSI) = (Air Density (lb/ft^3) * (Air Velocity in FPS) ^2) / 288g
At 80 MPH
Intake Ram = (.076 * (118)^2) / (288 * 32.2) = .11 lb/in^2
At 160 MPH
Intake Ram = (.076 * (235)^2) / (288 * 32.2) = .45 lb/in^2
In both cases we have assumed a 100% efficient duct, which is not possible -- 75% efficient would be a good one. So the figures become .083 and .34 respectively. They don’t sound like much -- and they aren’t -- in the quantitative sense. But a gain of .34PSI inlet pressure is a percentage gain of 2.3% over standard atmospheric pressure -- which is worth talking about.
The largest gain in engine performance, however, will come from the even distribution of inlet air to the individual intake stacks (venturi) that is provided by a well designed and efficient plenum.
My last work in airboxes is that they work. They work on any type of race car and they work with either carburetors or fuel injection. They only work if they are correctly designed. It is worth taking the time and trouble to make a good one.
