What pressure ratio causes knocking in a petrol engine?

What pressure ratio causes knocking in a petrol engine?
The pressure ratio does not cause pinging in an internal combustion engine. Pinging is the result of the air/fuel mixture igniying before the piston is in the propper position. Chep gas will cause pinging because it is igniting too quickly. Too high a compression ratio will do the same thing. Ingnition timing that is too far advanced will do the same thing. Back off your ignition timing two degrees at a time until the pinging stops, or buy a higher octane fuel.