Four-stroke power valve system

At low engine speeds, the wave pressure within the pipe network is low. A full oscillation of the Helmholtz resonance occurs before the exhaust valve is closed, and to increase low-speed torque, large-amplitude exhaust pressure waves are artificially induced. This is done by partial closing of an internal butterfly valve within the exhaust where the primary pipes from the cylinders join. The alteration of the pressure at this point controls the behavior of reflected waves at this sudden increase in area discontinuity. Closing the valve increases the local pressure, inducing the formation of larger-amplitude negative reflected expansion waves. A servo motor controlled by the ECU opens and shuts the valve. The valve goes from being almost fully closed at idle speed, through to fully open at higher engine speeds. This ensures low to mid-range performance, more linear power output and reduced exhaust noise levels while the valve is in its reduced opening position.