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In very early engine designs the one-way exhaust valves were 'upside down' in the block, parallel to the cylinders - the so called L-head engine because of the shape of the cylinder and combustion chamber, also called 'flathead engine' as the top of the cylinder head is flat. Although this design makes for simplified and cheap construction, it has two major drawbacks; the tortuous path followed by the intake charge limits air flow and effectively prevents speeds greater than 2,000-2,500 rpm, and the travels of the exhaust through the block can cause overheating under sustained heavy load. This design evolved into “intake over exhaust”.