Pounds per Square Inch (PSI) refers to a standard measurement of pressure.
There are several pressure measurements on a vehicle. These include pressure in the automatic transmission, cylinder compression pressure. fuel pump pressure, power steering pump pressure, and tire pressure. In the USC system, pressure is measured in pounds per square inch.
A cylinder compression tester measures the pressure in the engine cylinder at the end of the compression stroke. If the compression pressure is low, there is trouble in the engine.
In the metric system, pressure is measured in kilopascals (kPa). To convert compression pressure, multiply PSI by 7 ( 1 PSI= 6.895kPa)