[johnsjmc]

BTDC is advancing. Before top dead center means the spark occurs in advance of the piston reaching the top. The idea is to have flame propagation reach a max just after the piston changes direction to achieve the max pressure(BMEP) downwards on the crank.As it takes TIME for the flame to burn it must be lit earlier when the engine is going faster. The problem with setting by ear is that inaudable detonation may wreck havoc without hearing knock. Some dyno research points to the most efficient timing values to be short of audable detonation as well.