Intake Systems

The Intake system is a vital area of vehicle tuning. Original OEM systems are often designed with a to give certain characteristics to the engine. Original systems involved just an Airfilter and a Carburetor, and the airflow was given a short path into the engine. Changes to these systems often involved larger bodied Carbs, and cooler airflows. As air expands with heat, hot air will be less dense. There will be less mass of air in a given volume (for example, an engine cylinder and this will have correspondingly less Oxygen. Less Oxygen will mean less power can be produced from the burning of the fuel.
Similar systems were used on the first Fuel Injected engines, where a simple, easy path was made from the Airfilter to the cylinders.
Modern technology has studied the Airflow, and found certain effects can be beneficial to the power output. A key factor in engine power outputs, is the Volumetric Efficiency (V.E) . This is the ratio of Air inside a cylinder compared to what it can hold at atmospheric pressure. A slow turning engine, with large intake ports will have a high V.E. 100% would mean the cylinder has taken in as much air as possible. Turbocharging can increase upon this figure by pushing more air in than it would normally hold if open to the atmosphere.
Volumetric efficiency will drop at high RPMs on Normally Aspirated (N.A) engines as they will struggle to pull in enough air in the short time that the intake valves are open. Power is lost accordingly.

There are effects to increase the V.E on N.A engines. As with most tuning, this Intake tuning can be a compromise and only truly effective over a certain RPM range, but picking a choosing the design to fit in around the engine, the "boost" given can be used to smooth out the output over the engine.
Tuning can be done via the length of the intake runners, and their size. Long thin intake runners will speed up the gas velocity and the momentum of the fast moving air will help pack the cylinder full even as the intake valve is closing. This will boost power at lower engine speeds, but the thin runner design will restrict airflow at high speeds. Tuning and resonance of the pipework will also help.
As the engine sucks in air, there are pulses of high and low pressure. The tuning is designed use these pulses of pressure to get more into a cylinder, by adjusting the size and length of the intake. This effect is known as Helmholtz Resonance. The resonance effect will only work in a given intake size as a specific RPM range, so this can either be used to remove a flat spot, add extra torque in lower RPMs where most day to day driving occurs, or a variable length Intake system can be used to provide two resonant effects at different RPM ranges. An example of this is the BMW DISA flap in some manifolds.

Modifying your Intake

Intake systems are frequently modified to help the engine produce more power. The first step is usually a free-flowing Air Filter, and and Induction system. The Induction systems are designed to pull in colder air from outside of the engine bay, which increases the air density, and therefore the amount of oxygen available.
Care must be taken when carrying out these modifications, so that calibration of the Air measuring system isn't affected. M.A.P (Manifold Air Pressure) controlled engine can be adversely affected by changing filters and flow paths as they do not directly measure the Air Mass or Volume. Calculations are carried out inside the ECU based on pre-programmed data to estimate the air available based on the pressure inside the Manifold.
Air Flow Meters and Air Mass Meters are less affected as they can estimate the volume or mass of the Air by measuring what is coming in. Fitting a Cone filter directly onto the end of an Air Flow or Air Mass meter is not advised. A certain length of smooth intake tubing is needed so that turbulence from the air passing through the filter is dissipated and the Air flow become Laminar, and controlled. Fitting a Cone filter too near to the measuring element will result in turbulence over the measuring apparatus, either a Heated Film or a Hot Wire, and the turbulent air can swirl around this, passing over it more than once, and giving an apparent increase in air intake.
Likewise, a disturbed, non-laminar flow can show increased air passing through the centre, without noticing a decrease around the edges of the MAF/AFM tube, and again give incorrect readings. Changing the diameter of the tube will also throw out the calibration of the sensor, as it will estimate for flow in the very centre what the total flow is over the entire tube.
Alpha-N Intakes remove all air measuring devices, and instead estimate the air flow based on "Alpha" which is the throttle plate angle and "N", which is the RPM. The benefit of Alpha N is the removal of air measuring devices in the flow path, and no need to a restrictive tube where all air has to pass through. As it can only estimate air quantities, it only tend to be used in race applications where emissions and low speed engine operation are not as important. A similar and slightly more advances system is Speed-Density which uses a M.A.P sensor with the RPM to find the Volumetric Efficiency and] then add the required fuel amount. EndTuning can provide the popular Alpha-N system for the BMW S50B30 M3 engine when utilising larger airboxes, but low you may find low speed driving quality is affected. Details are in the Chips section.