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Dec28-07, 08:03 PM
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Many modern engines use a “tuned” exhaust to improve the volumetric efficiency of a high-performance engine. The operating principle is simple in concept: When exhaust gases exit the exhaust port at very high velocities, a mild vacuum is created in the cylinder. Engine builders therefore use intake and exhaust valve timing to take advantage of this vacuum effect to increase the intake charge of air and fuel entering the cylinder.
Since most of the torque generated by the combustion process takes place by 90 degrees after top dead center (TDC), the exhaust valve usually begins to open before bottom dead center on the power stroke. The residual combustion pressure in the cylinder is then released into the exhaust port, through the exhaust system, and into the atmosphere.
In most naturally aspirated applications, the intake valve is timed to open a few degrees before the piston reaches TDC on the exhaust stroke. When the piston reaches TDC, both exhaust and intake valves are open, which results in valve timing “overlap.” The mild vacuum created by the pulse of exhaust gas exiting the exhaust manifold and header pipe will accelerate the air/fuel mixture entering the cylinder. This acceleration effect is called “exhaust scavenging,” and is used to a great effect in naturally aspirated engine designs.