Because vapor is constantly produced from the fuel in automobile fuel tanks, clean-air authorities mandated use of vapor-absorbing carbon canisters and have since then sought to generally reduce the vapor pressure of motor fuels, given as Reid Vapor Pressure or RVP. Now onward to another necessary quality of gasoline, its octane number, which is a measure of its resistance to the form of abnormal combustion called knock or detonation. Knock occurs when the last parts of the fuel-air mixture to burn (long after the spark) have been chemically altered by high temperature. The altered mixture then behaves like a sensitive explosive, auto-igniting and burning at sonic speed. This produces a shock wave whose collision with combustion-chamber surfaces is not only audible (knock or ping) but destructive. The resulting extreme-pressure spikes can hammer out bearings. Shock waves scour away stagnant gas from metal surfaces (the insulating “boundary layer”), accelerating heat flow into them. The combination of overheated metal and shock-wave impact erodes the outer edges of the piston crown, eventually destroying the sealing of the top piston ring. Bad, bad stuff and well worth avoiding! Today, few people have experienced detonation because so many vehicles carry systems that listen for knock and, when it is detected, retard ignition timing just enough to stop it.