The whole point of performance tuning and engine modifications, whether it involves supercharging, turbocharging, NOS, or gas flowing the cylinder head is, firstly, to improve the efficiency of the air flow into and out of the engine; and secondly, to improve the quality of the air/fuel mixture. The process of improving the air flow into and out of the engine is often described as improving engine breathing and the main aim here is to identify restrictions that may impede the air flow into and out of the engine or eliminate or minimize these restrictions. We could also use forced induction to force more air into the engine. Either of these strategies will increased air flow into the engine, which will improve the quality of the fuel/air mixture and will, consequently, increase horsepower and torque.
In our sections on Turbochargers, Superchargers and NOS dealt with using forced induction to get more air into the engine, while in our section on Exhaust Systems we dealt with getting air more efficiently out of the engine through correctly tuning the exhaust system or designing a more efficient performance exhaust system. In this section we'll deal with the air intake side of engine breathing. We will discuss ways in which we can remove restrictions and improve the efficiency of the air flow into the engine. We'll look specifically at improving the efficiency of the air filter and air filter box, sizing and positioning of the mass air-flow sensor and the throttle body, improving the inlet ducting, cold air induction, how to size and tune the intake manifold runners and how to design an efficient intake manifold. The information discussed here is meant for naturally aspirated engines but it may also be applied to engines that use turbochargers and superchargers.
Let's begin with a little discussion on air temperature, power and the intake system. In the basics of increasing engine power, we raised the point that air density is important to producing engine power. Denser air produces more power because it has more air molecules per volume and air temperature is one of the factors that affect air density. It is for this reason that intercoolers are used on turbocharged and supercharged cars – to reduce the air temperature in the intake charge. On a normally aspirated engine, you can also try to reduce the temperature of the intake charge by ensuring that the intake duct is not exposed to hot spots in the engine, such as the exhaust manifold. An aluminum heat shield fitted between the exhaust manifold and the intake system, for example, would help prevent the intake charge from absorbing too much heat that is radiated from the exhaust manifold. Also, most intake manifolds feature cooling ducts to aid the atomization of the fuel at low rpm. At higher rpm, where you want the power, these cooling ducts are counterproductive as they heat the air in the intake manifold. Blocking off the intake manifold's cooling ducts at the cylinder head would be advisable.
With that behind us, let's move on to improving the efficiency of the air filter and the air-filter box...