Posted by J-Gilly on Tuesday, August 17, 2004 at 6:32PM :
In Reply to: Can anyone explain the concept posted by Russ Prior on Monday, August 16, 2004 at 3:40PM :
It is difficult to think about how the vacuum is created when you think about one or two compression/combustion cycles. However, looking at the vacuum guage on a running engine, the individual firing of cylinders is not evident. The vacuum is created as the steady intake of air as the engine is running on all cylinders. Think of the engine as an air pump. The vacuum is greatest when the RPMs are highest they can get for the amount that the throttle is open. The highest vacuum reading tells you where the engine runs most efficiently because it is producing the most power, spinning the highest RPMs it can, for a given amount of air and fuel. At idle, when the throttle is almost closed, the vacuum is steady, and the motor will stabilize at the most efficient RPM for a particular adjustment. Open the throttle a little, and the vacuum will drop, as more air rushes into the manifold. As the motor accelerates, it will run a little less efficiently, until it stabilizes at a new, higher RPM, at which it is most efficient for that throttle setting. Retard the timing from the ideal setting, and the air/fuel mixture will not ignite completely in the cylinder (continuing to burn as it passes out the exhaust valve and into the exhaust manifold)so the burn will not all translate into the mechanical work of pushing the cylinder, and is therefore less efficient. Therefore, RPMS, and vacuum will drop. Advance the timing from the ideal setting, and the mixture will burn too early in the compression stroke. When this happens, the expanding gas works against the upwards motion of the cylinder, and slows down the RPMs, and with it, vacuum. Severe detonation, or premature combustion, causes knocking, and can cause catastrophic results. Hope this helps.
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