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It replaces the OEM air intake with a short metal pipe and a conical air filter inside the engine bay. A significant increase in intake air volume will only occur in an engine where the factory intake piping was designed with baffles and other sound absorbing materials (rubber, etc.).
The term ram, albeit commonly used, is somewhat a misnomer. A true ram air intake device is something that enhances intake pressure or in the case of resonant systems such as Chrysler's Sonaramic, increases the torque with a peak at a certain RPM.
There is still some open debate on this issue, but some believe that a problem with short ram air intakes is that the air entering the intake is at a higher temperature due to the proximity of the engine, which may reduce some power. This may be partially offset by an increase in the volume of air entering the engine. To counter intake heat problems, many short ram intakes include some form of heat shield. Moving the intake inlet port further away from the engine block will also help to alleviate the problem and some users use a cold air intake where the inlet air is at or close to ambient temperature. However, others claim that Short Ram Intakes (SRIs) or similar Warm Air Intakes (WAIs) offer benefits over Cold Air Intakes (CAIs) which include better MPG due to a more complete burning of fuel. Users with forced induction engines often opt for short ram intakes because compressors adjacent to the engine, especially turbochargers, heat the incoming air and negate much of the benefits of a cold air intake. Additional problems can result from using a short ram intake in cars utilizing a mass airflow sensor though most of today's sensors automatically adjust without issue. Turbulence in the intake airflow produced by the filter or piping, or a change in intake diameter at the point where the airflow is measured can produce inaccurate airflow readings. The error in airflow then translates to an error in the amount of added fuel. In the worst case, the air/fuel ratio can run lean, causing detonation and possibly engine failure although the airflow sensor will generally detect and correct the throttle to compensate for the warmer air. This can also be solved by modifying/elongating the intake piping, replacing the airflow meter, or by replacing/remapping the engine control computer to provide the correct amount of fuel for the intake airflow at all engine speeds.
A harmless but noticeable result of using a short ram air intake is an increase in intake noise, which may manifest as a sucking sound, and the loss of any silencing properties that the factory intake had.