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The roof over each cylinder in an engine forms a combustion chamber. It is the space between the top of the piston and the cylinder head, which the combustion of the compressed air-fuel mixture takes place. The chamber's shape determines how well the engine will run, what kind of fuel it needs, how much fuel it burns per horsepower produced, and the volume of pollutants that come out of the tailpipe. The shape of the combustion chamber also has a considerable effect on its knock tendencies. An efficient combustion chamber must meet several requirements. The chamber must be compact to minimize the surface area that absorbs heat when the fuel burns. There should be no nooks or crannies, which can cause spontaneous combustions or knock. The chamber must move large volumes of the air-fuel mixture into the combustion chamber conveniently. Lastly, it must provide available room for the spark plug since the ignition flame spreads across the chamber at a finite speed.