" it's casued by fuel igniting outside the combustion chamber "

I don't think so.

Fuel igniting outside the combustion chamber while the intake valve is still open, would cause backfire, not pre-ignition. An ignition in the intake would (especially on the compression stroke) would cause an explosion, or flame, that you would hear as a loud explosion; or you would see as flames licking the seams of your hood after the air filter ignited.

Pinging, or pre-ignition is caused when the cylinder is in the compression stroke and the valves are closed. As the fuel is compressed, it may autoignite, for various reasons. But because the valves are closed, you can sometimes bearly hear pinging, unless you are paying attention to the sounds of your engine.