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The timing gear is connected by chain, gears, or a belt to the crankshaft at one end and the camshaft on the other. It is marked with tiny increments all around its perimeter, which correspond to degrees of timing from the straight-up timing position of the camshaft and crankshaft. These marks assist the individual who is tuning up the engine to set the timing to the determined optimal timing degrees of the camshaft and engine designers.
In order to set an engine's timing gear to the correct inclination, the mechanic must confer with the engine manufacturer as well as the camshaft manufacturer. The purpose of timing an engine with the gear is to ensure that the valves are opening and closing at the correct time to best fill the cylinder with an air/fuel mixture as well as to release all of the spent fumes from the exhaust cycle of the cylinder. A mere few degrees off can be the difference in an engine that performs perfectly and one that will not run correctly. A poor running engine will make less power and use more fuel than a properly-timed engine.
While the gear rotates a full 360 degrees, the timing marks are concerned with just a few degrees before and after top dead center of the piston's rotation. Top dead center is when the piston is at its absolute highest point of travel within the cylinder or at the top of the stroke at the dead center of when the crankshaft is neither traveling up nor down in the cylinder. The gear is used to measure the amount of rotation in degrees in relation to when the valves begin to open and close.
In the perfect relationship of timing, the timing gear can be set so that the intake valve will begin to open as the piston is still moving downward in the cylinder walls and sucking the fuel into the cylinder. This would aid in getting as much fuel as possible into the combustion chamber. Also, the exhaust valve would remain open as long as the piston was moving upward in the cylinder and pushing the exhaust fumes out of the combustion chamber so that there is no contamination to the incoming fuel mixture. The problem in this is that there must still be sufficient time with both valves being closed to produce the combustion pressure known as compression during the ignition and power stroke function.