A back-fire or backfire is an explosion produced either by a running internal combustion engine that occurs in the air intake or exhaust system rather than inside the combustion chamber or unburned fuel or hydrocarbons ignited somewhere in the exhaust system. A visible flame may momentarily shoot out of the exhaust pipe. Either condition causes an objectionable popping noise, together with possible loss of power and forward motion. A back-fire is a separate phenomenon from the fire produced by Top Fuel dragsters.
The term derives from parallel experiences with early unreliable firearms or ammunition, in which the explosive force was directed out at the breech instead of the muzzle. From this came the use of the word "backfire" as a verb to indicate something that produces an unintended, unexpected, and undesired result.