Igneous Rock is formed when a magma cools underground and crystallizes or when it erupts unto the surface of the ground, cools and crystallizes. Magma that erupts onto the surface is called lava. When magma cools slowly underground the crystals are large enough to see.
Readings for compression tests usually vary between 175 to 250 PSI (Pounds per Square Inch). A higher reading generally indicates better engine performance; however you should check your vehicle manual to find out what the ideal reading should be for your vehicle.
The condition of the engine is determined by measuring the degree to which a cylinder with valves closed leaks air. In simplified terms, if air is pumped into the cylinder at 100 PSI and the gauge reads 97 PSI, then the leak down percentage is 3%. Doing a leak down test is a fairly simple matter. The tester is connected to an air source and is attached to the engine through the spark plug hole. There are two types of instruments, those with single and double gauges.