Agreed, Ignition timing of 35 degrees BTC is a good place to start experimenting when you lack any model specific recommendation. The longer the flame front takes to travel from the spark plug location to lighting off the majority of the charge, the more ignition advance (head start!) is required. Douglas used shrouded plug locations on both side valve and ohv to resist plug oiling, and that led to some slow ignition.
We have a model A Ford with a period Miller ohv cylinder head 'performance upgrade'. Reputed to lift the power from the factory 40hp to 60hp. When we finally got around to installing it the performance was disappointing, not much more than the factory flat head it replaced and less than high compression aftermarket flat head available for the model A like the Winfield. It turned out it needed a lot more ignition advance than expected, a lot more. Then it started to make a noticeable improvement in power. Seem there was so little turbulence in filling the cylinder (the combustion chamber was basically an extension of the cylinder bore, vertical valves, and a near vertical spark plug off to one side) that the flame propagation was glacially slow. Perhaps at higher revolutions things got mixed up more and the ignition was more efficient; it was after all a cylinder head intended for racing, not putting around on the road.
Generally though, the lower the compression (molecules not pressed together so close) and less efficient the engine, the more advance will be needed. Turbulence helps, hence the discovery that 'squish' areas in side valve head design (Riccardo?) really improved the power.
-Doug
