The Ram-air Principle



The Ram air principle is the effect where by air is encouraged to enter the engine at a far higher rate than would be expected through the mechanical action of the machine alone- rather like the effect of forced induction, but without the need for a turbo- or supercharger.

For the air intake system on normally aspirated engines there are two forms of ram air effect:

Inertial Ram Air Effect

Air has mass. Therefore it has inertia. It is rather like push starting a car, it takes some effort for you to get the vehicle rolling, but once rolling it is very difficult to stop without the aid of the brakes! Similarly, once air is moving into the intake manifold, it too will take some stopping. This can be utilised by engine designers to encourage faster, and therefore greater amounts of air into the combustion cambers (giving a higher volumetric efficiency).

Cone filers, by eradicating the tortuous air intake path of the standard MGF 1.8i K series engine, will reduce frictional losses in air movement- and thus the air, as it enters the inlet manifold, will have a higher velocity (and inertia) to the benefit of high end power output.

Wave Ram Air Effect

Engines are reciprocal air pumps. Air tends to enter the engine in pulses- at both high and low pressures concomitant with the induction, compression, ignition and exhaust cycles of the engine. Thus, like a musical air instrument, the inlet path can be tuned. Instead of tuning for musical pitch, the inlet tract can be tuned to actively encourage air movement into the engine.

Some air filters have been fitted directly to the throttle body on the MGF. This is bad for the harmonics of the air intake- not only does it make the system louder, it also detrimental to power output. More developed systems have a spacer tube between the throttle body and the filter- which makes the installation more refined, but is beneficial to power outputs at particular engine speeds.The trick involves ensuring that the low pressure pulse in the inlet tract coincides with the intake cycle of the four stroke engine, thus encouraging more air to enter the engine by effectively sucking it in. The length of the tube determines which engine speeds benefit most- approximately 6 inches has been found to benefit the top end power production of the 1.8 K series most.

Some manufacturers have developed variable induction length manifolds- to benefit not only high output at high speeds (short inlet tract) but also low end torque (pulling power) at low engine speeds (long inlet tract). Sadly, MG/ Rover have yet to adopt this technology.

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