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RESONANCE INTAKE MANIFOLD PURPOSE Basic design of an internal combustion engine dictates the need for proper intake manifold length.  The length of the intake manifold has always determined the power curve of a particular engine type.  It has been established that very short intake tubes would create good low-end torque but can cause a rapid decrease of performance for higher rpm's.  For this reason, manufacturers have developed many ways of adapting to this need.  By strategically placing a resonance flap in the intake manifold, the Engine Control Module can control the length of the manifold tubes by opening and closing a flap. (See diagram below)  DESIGN (from SAE literature) The pistons on the intake stroke, which, with the intake valves open, moves the air quantity in the intake manifold, and create strong vacuum.  The intake airflow or the air/fuel mixture thereby develops considerable kinetic energy (momentum).  Back pressure, created when the intake valves close, immediately begins to expand in the opposite direction.  The level of pressure is dependent on the airflow velocity of the gas quantity mass that is present in the intake manifold. If the mixture column is quickly accelerated or braked, oscillation occurs, and the frequency of which these occur are dependent upon the intake manifold length.  If the engine timing and mixture column oscillations match, then a certain charge effect is attained by means of resonance oscillation.  Since the mixture column oscillations are also engine speed dependent, the resonance oscillations at a set intake manifold length can only be attained in particular engine speed range.  By varying the manifold lengths, better control of those oscillations are reached.  RESONANCE FLAP CLOSED (see pictures below) The intake air flows into the resonance intake manifold (19a) through intake ports, which branch off after the throttle valve, into the air collection housing of the oscillation pipe (19b).  The collected air volume is split in half by the resonance flap (22).  This leads to a distinctive increase in low-end torque. RESONANCE FLAP OPEN (see picture below) With the resonance flap open, the collected air volume in the oscillation pipe (19b) is not split.  The respective intake cylinder is served by both intake lines of the resonance intake manifold (19a).