Stovebolt cylinder heads have some interesting geometric features that make it difficult to improve the gas flow much. Enlarging and smoothing the ports helps somewhat, but making those twists and turns to get the gases in and out of the head is the primary drawback to getting good volumetric efficiency and the siamesed ports definitely aren't a plus. Making what is essentially three 90 degree turns with the intake airflow before it gets to the cylinder is not a good thing, and the exhaust flow is almost that convoluted.
When David Healey chose to use a 6 cylinder engine from an Austin delivery truck in the sports car he designed in the mid-1950's (the Austin-Healey 100-6 and 3000 models) he encountered similar problems. He chose to use side draft S-U carbs, two or three of them depending on the series of car. The split exhaust manifold on the Healey was very similar to the Fentons we use on stovebolts, but the header pipes were the exact length necessary for proper exhaust tuning when the muffler and tailpipes were removed for road racing. Sneaky!
Jerry
