Advantages
The main advantages of a sidevalve engine are simplicity, reliability, cheapness, compactness, responsive low-speed power, and ability to use low-octane fuel. The absence of a complicated valvetrain allows a compact engine that is cheap to manufacture. In particular, the cylinder head may be little more than a simple metal casting, and the simple valve gear engenders reliability. These advantages explain why overhead designs began their displacement of flathead designs in higher-cost, higher-performance applications (such as aircraft, luxury cars, and sports cars); only gradually worked their way into applications such as economy cars, trucking, and commercial agriculture; and still have not extensively displaced flatheads in applications such as lawn and garden equipment or subsistence agriculture.
An important benefit of the a sidevalve design is that, if a valve should drop, only limited damage would occur, and the engine might continue to operate with a broken valve. (By comparison, an OHV or OHC engine can suffer piston damage and catastrophic engine failure when a valve drops.) The design is intrinsically not an interference engine design, and provides safety from damage at high rpm.
Because intake and exhaust gases pass through the same corridor between the block and head, and because the light valve gear allows the valves to open and close quickly, flatheads are designed with minimal valve overlap (where both intake and exhaust valves are simultaneously open). This gives better low rpm performance than with OHV or OHC engines which use valve overlap to gain high rpm. Although a sidevalve engine can safely operate at high speed, its volumetric efficiency swiftly deteriorates, so that high power outputs are not feasible at speed.
Read more about this topic: Flathead Engine
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