Disadvantages
The main disadvantages of a sidevalve engine are poor gas flow, poor combustion chamber shape, and low compression ratio, all of which result in a low power output.
The maximum compression ratio is low at about 7:1, reducing efficiency but permitting the vehicle to run on low-octane fuel.
The sidevalve configuration makes intake and exhaust gases follow a circuitous route, with low volumetric efficiency, or "poor breathing". High volumetric efficiency was less important for early cars because their engines rarely sustained extended high speeds; but designers seeking higher outputs had to abandon the sidevalve. A compromise used by Willys (Jeep), Rover and Rolls-Royce in the 1950s was the F-head (aka intake-over-exhaust configuration), in which there is one side valve and one overhead valve per cylinder.
An advance in flathead technology resulted from experimentation in the 1920s by Sir Harry Ricardo, who improved their efficiency after studying the gas-flow characteristics of sidevalve engines. The Ricardo head moved the exhaust valve farther from the center of the cylinder than the intake valve (whereas they had previously been equidistant). Ricardo observed how the form of the inlet and exhaust tracts affected gas flow and turbulence both in the inlet stream and within the combustion chamber.
Because the exhaust follows a complicated path to leave the engine, there is a tendency for the engine to overheat. In a T-block (aka T-head), a sidevalve engine has a crossflow configuration, so exhaust gases leave on the opposite side of the cylinder from the intake valve. American LaFrance powered their fire engines with T-head engines from the 1920s to the 1950s. The Cleveland Motorcycle Manufacturing Company produced a T-head four-cylinder in-line motorcycle engine in the 1920s; and early Stutz engines were T heads.
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