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.
Compression Ratio is defined as the ratio of Maximum engine volume to minimum engine volume. In Mechanical Terminology, its ratio of Total Volume to Clearance Volume. It is fundamental specification of engine. For Spark Ignition engines, the ratio will be around 10:1 to 15:1.