Auto Repair Tips & Advice > Engine  I hear a sharp, metallic
pinging sound from my engine when I go up a hill or try to pass
on the highway. What gives?  It's called "spark knock" or
"detonation," and is a form of abnormal combustion caused by
excessive heat and pressure in your engine's combustion chambers
and/or using fuel with too low an octane rating.  The metallic
knocking sound is actually produced by shock waves inside your
engine's combustion chambers. When a spark plug ignites the
air\fuel mixture in a cylinder, it produces an explosion that
spreads outward from the point of ignition like an expanding
balloon. The outer edge of this balloon of fire is called the
flame front. During normal combustion, there's a single flame
front that expands and consumes all the air and fuel in the
cylinder until combustion is complete. Pressure rises gradually
then falls off producing a smooth power stroke. But when there's
too much heat and pressure for the octane rating of the fuel,
little pockets of air and fuel