RYAN - I have My flame-proof suit on so here goes....Air cooled engines run hotter, especially when pulling hard, and the flat head design has MANY points and places that create hot-spots that can cause pre-ignition that all lead to possible detonation which can break pistons, collapse piston ring lands, break piston rings, hammer out rod bearings, blow head gaskets, even cause valve damage in extreme cases. The problem is most severe in continous use engines, like mowing & plowing tractors. And especially tractors that get run hard on hot summer days. Hardest work I do with My Cubbies is blow snow in winter....but the 20-30 degree ambient air cools the engine so well even though it's pulling it's GUTS Out it won't detonate. In summer after mowing 3-4 hours it may detonate on 75-80 degree days even at less than half load.
Kentucky Ken posted a pic years ago of a Kohler head that detonated bad enough it blew a hole out of the combustion chamber between the intake & exh. valve. He welded it up and resurfaced the head and it was fine.
From tests I've run on My Kohlers the head temps can exceed 325-350 Deg. F on warm days with moderate loads. Since the combution chambers in water-cooled engines have circulating coolant just a small fraction of an inch away from the combustion event in the comb. chamber they rarely have temps over 250 deg.F.
Don V. has tested His pulling tractors on the dyno and commercially avail. regular grade 87 octane no-lead pump gas provides Him with the most HP on His pulling engines.