A Flathead engine, with its valves ensconced in the block, was still a competitive design in the low-compression, low-speed engines of the prewar era. That changed and changed quickly with higher-octane fuels allowing for higher compression ratios, and improved metallurgy and shorter-stroke engine designs enabling considerably higher crankshaft speeds.
The large-volume combustion chambers and upside-down valve placement required in a flathead engine conspired to limit the compression ratio to around 8.4:1, after which power and fuel efficiency both decreased.
Power output at higher crankshaft speeds was restricted in a flathead compared to OHV designs because of limited valve lift and diameter, both of which were constrained by the desire to limit combustion chamber size to maintain a decent compression ratio. Even when these were optimized, performance was encumbered by the excessive time required to fully burn the widely-distributed fuel mixture, often not fully-consumed until the piston was nearing the bottom of its travel, effectively reducing the motive force generated from the spent fuel.
For these reasons, most European manufactures abandoned their flathead designs in the early fifties. In the U.S., Cadillac and Oldsmobile went OHV for ’49 and Ford replaced its famous flathead V8 on 1954 models. Pontiac joined the club for ’55. All postwar German and Italian automobile engines were overhead valve designs.
But there were a few staunch holdouts…
The most egregious was Chrysler Corporation. Many people think that the Back to the Future DeLorean was the first time-travelling automobile, but how about the ’57 Plymouth Belvedere?
While Chrysler was touting its wildly futuristic Autodynamic Flight-Swept and Swept-Wing styling and space age push-button TorqueFlite Drive, hiding behind their sleek Forward Look grills were often as not inline-sixes dating back to Prohibition.
The base engine in Plymouth Savoys, Belvederes and Dodge Coronets would remain Chrysler’s PowerFlow and Get-Away flathead sixes through 1959.
The same year was the last for Britain’s flathead four-powered Ford 100E.
Studebaker soldiered right into the Swinging Sixties with their flathead six toiling away under the hoods of Larks and Champs through 1960, finally replacing it for ’61 with their Skybolt OHV Six.
Simca continued using a 2.4-litre (143 cubic inch) version of the venerable Ford Flathead V8 in their Vedette range through 1961, after which the engine continued to live on in Australia, Brazil and a number of military transport applications worldwide.
The longest surviving flathead honors go to the Rambler American, which was available with a flathead Super Flying Scot six halfway through the 1960s.
In 1965, as visitors glimpsed the future at the New York World’s Fair, Bob Dylan picked up an electric guitar at the Newport Folk Festival and Professor Roy Hinkley built a battery charger out of coconut shells on Gilligan’s Island, one could have visited a Rambler showroom to catch a glimpse of their brand-new Marlin fastback, one of only three U.S. cars available with up-to-date disc brakes; and out a corner of their eye spot a Rambler American whose motive power was provided by a 196 cubic inch (3.2 litre) inline flathead six that began its lingering career powering the 1941 Nash Ambassador.