Compression should be taken cold BUT--a motor that hasn't run in a while will have unlubed cylinders and the rings may be a bit off there best. So run it up to temp and cool it down, then the test is done at wide open throttle, remove the plugs from all the cylinders durring testing. A 9 to 1 motor runs on regular. Carbon build up and/or engine work can make your motor more than 9:1. Running low test gas when you need high test gas mean self-distruction of the motor, as soon as you hit the peak tourque RPM's plus open throttle the gas will explode not burn (detonation) "ping ping rattel rattel snap crackel pop" keep it up then-- burnt valves, or pistons, or head. Even if you get it taken care of before distruction the detonations pit the piston crowns and the head. Too early timming and or too lean a fuel mix leads to detonation. The three things compression, timming, and mix are collective, a little of each works like too much alltogether. Worse case is an air cooled motorcycle motor, detonation wastes these motors fastest of all. Don't run low test gas if you need high test it's not an option.