This is how they check for blown head gaskets, burnt/stuck valves, to make sure that it is blown or damaged and not something else. Its called a compression check. Take out the number 1 plug. Put the gauge into the number 1 plug. Crank the motor until the gauge reads at the highest point. Write it down. Do the same thing for cylinders 2 through 8. Add them up. Divide the number by the number of cylinders. This will give you a round about compression ratio. Example: number 1 cylinder - 9.5, 2 cyl - 9.6, 3 cyl 9.6, 4 cyl 9.5, 5 cyl 9.5, 6 cyl 9.4, 7 cyl 9.6, 8 cyl 9.5. This equals 76.2 total. Now divide 76.2 by the number of cylinders. 76.2 divided by 8 cylinders = 9.525. Your compression ratio is 9.5:1.