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.
A typical compression. for a diesel engine is 20:1, compared with 9:1 for a petrol engine. Compressions as great as this heat up the air to a temperature high enough to ignite the fuel spontaneously, with no need of a spark and therefore of an ignition system.