My project this weekend is to build an airbox for my race car. I’m going to take pictures during the build and present it in the next post. So lets talk about why I’m doing this project.
The purpose of an airbox on a race car is to aid in providing denser, cooler, and higher pressure air to the engine. Internal combustion engines make power by burning a fuel and air mixture at a proper ratio to each other. This is a ratio of volumes of air and fuel, and maximum efficiency is obtained at 14 parts air and one part fuel (the Stoichiometric ratio), while maximum power is usually obtained at a little richer mixture of 12 to 13 parts air, one part fuel. If we can find a way to add more air, we can then add more fuel, and thus make more power. But for a given set of engine components, there is a maximum volume of fuel and air an engine can ingest. (A forced induction system such as turbocharging or supercharging can mechanically increase this volume by compression, but that is not part of this discussion). What we need to do is get the most out of this volume constraint that we can. The component of air that actually burns in the combustion chamber is oxygen, so we can look for ways to increase the amount of oxygen in a given volume of air. This is done by increasing the density of the air, actually having more molecules of O2 in a given volume of air. For a given volume, denser air actually weighs more, which is what a barometer is actually measuring. We can gain density in air three ways: colder air, drier air, and higher pressure air.
For a given volume, humid air has more water in it than drier air, thus leaving less room for oxygen in it. At the track, we are stuck with the current atmospheric conditions, so we aren’t going to be able to change the humidity of the air. But we can use an airbox to get cooler and higher pressure air. Underhood temperatures soar into the hundreds of degrees, much higher than the air outside of the engine compartment, so why feed our engine with air drawn from under the hood? We can produce measurably more horsepower by just drawing air from outside the engine compartment, as long as we keep the correct fuel/air mixture ratio. Since the air must travel through some sort of ducting to the engine, we should design this ductwork in a way so that it will not collect heat and pass it on to the air flowing through it. If possible, the ductwork should be routed away from the hottest part of the engine, the exhaust manifold. If this is not possible, heat shields should be employed to reflect the manifold heat away from the ducting. The engine cannot ingest all of the air we are bringing to it at once (only one piston is sucking in air a time), so we need a place to collect this cool air for a moment until the engine needs it; voila, the airbox. And just like with the ductwork feeding it, we should employ heat shields to reflect as much engine heat away from the airbox as possible.
Higher pressure air is denser just like colder air is denser, it has more air molecules per a given volume. So we should take advantage of the the aerodynamics of a moving car, and feed the airbox with air collected from a high pressure zone around the car. Two of these areas are the leading edge of the car, and at the base of the windshield. Books have been written on why this is true, so I will just present it as fact for now to save time. Production cars with forward facing hoodscoops as far forward as possible, and with rear facing cowl scoops at the base of the windshield have been built just to take advantage of these properties. If the racer is considering minimizing or eliminating an air filtration system so as to maximize airflow into the engine, further consideration must be given to a clean air source. While the air from the nose of the car is both cooler and higher pressure, it can be quite contaminated with dirt, sand, tire rubber, and water; all things you want to keep out of your engine. So if you are going to supply your airbox from the front of the car, be sure to filter it or be prepared for more frequent engine rebuilds.
Next – building our budget airbox
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