TDC-Top Dead Center - this occurs when No. 1 piston is at it's uppermost point generaly at the end of the compression stroke and start of the power stroke. This is used to set the timing of the cams and ignition. Since 1985 very few road cars have the inlet valve open before the exhaust is fully closed, this may allow unburnt fuel into the exhaust and out the tailpipe, this is considered polution.
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