Consider the combustion chamber of a side valve (flathead) engine. The combustion chamber includes the valve pockets and area between the valves and the cylinder. After ignition, the flame must travel the distance between the spark plug and the furthest extent of the combustion chamber, i.e., the valve pockets, inches away. This limits CR and RPM.
That said, I like flatheads. Once in the late 1960s I saw a drag race between a SHP 409 powered altered with Hilborn injection and an old school rail dragster with a blown and injected flathead Ford mill. Of course the rail was lighter and the 409 altered was spotted more than a second but it wasn't even close. Standing by the rail I watched the old flathead overhaul the altered and win going away. I'm not sure if the rail was burning gasoline, alcohol or nitromethane but it was a sight to see. As they once said in the 1950s: "Flatheads forever."
Relatedly, flathead aficionados might want to again listen to the Beach Boys recording of Little Deuce Coupe. Unlike most of the bands that recorded car songs in the mid 1960s, the Beach Boys knew what they were talking about. Pay attention to the line at about 0:35: "She's ported and RELIEVED and she's stroked and bored...." Who knows what "relieved" means?
Unlike an OHV engine, both intake and exhaust gasses must travel between the cylinder and the side valves in a flathead engine. The cylinder head provides an area for this. This area, of course, limits CR and compromises combustion chamber design. It also limits high RPM performance as breathing is limited.
Relieving the block, i.e., machining the block to provide increased area for intake and exhaust gasses to travel between the cylinder and side valves, helped breathing and improved high RPM performance, but it was not enough.
By the mid 1950s, the handwriting was on the wall, and even the "flathead forever" folks accepted the reality that there was no way to make flathead Ford engines competitive with the OHV Oldsmobiles and Cadillacs in the automotive market.
I have digressed. There is nothing wrong with inexpensive side valve engines in relatively low speed industrial applications, especially in the throw away market, e.g., walk behind lawn mowers, etc. Your small, 1800 RPM Kohler generator engine is one such example.
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