Compression Pressure: The pressure created in the cylinder when the piston moves toward top dead center (TDC) after the intake valve closes, trapping the induced charge (normally a fuel/air mixture) within the cylinder. Compression pressure can be measured by installing a pressure gauge in the cylinder in place of the spark plug and "cranking" the engine with the starter motor. To improve measurement accuracy, the throttle is usually held wide open and the remaining spark plugs are removed to minimize cranking loads and optimize pressures in the cylinder under test.