The ability to match occurances together with events when they need to happen.

The interaction between the top of a piston's travel in a cylinder (aka top dead center) and when the spark plug fires. It also determines where the piston is located in the cylinder when the valves open and close. Poorly adjusted timing will cause increased fuel consumption, crappy throttle response, much less power and worse emissions to boot. Really bad timing can destroy your engine, like if a valve and piston both try to occupy the same space. Bad.

The crankshaft (which controls the pistons) and camshaft(s) (which control(s) the valves) are linked together through a timing belt or timing chain around the crankshaft and camshaft pulleys. There are often markings on a housing and on the crankshaft pulley for a good reference as to when one is near TDC on a particular cylinder. The timing is expressed as degrees before or past TDC.

A common trick to get a vehicle to pass emissions inspection is to bump the timing back (aka retard the timing). With now retarded timing, the engine makes less power and usually much better emissions. Advancing the timing usually results in an increase of power and much worse emissions. This only works for small amount of change - too much and you'll have a piston/valve crash (the engine wouldn't even run at this point, but the possibility of catastrophic damage is very real).

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