[mylesdw]

Rob: nice interesting reply.

In answer to your "dunno":
At 600 rpm and 20 degrees btdc you are firing 5.5 milliseconds btdc. When you lift off at 5000 rpm and get the all in advance of 45 btdc it only equates to firing 1.5 milliseconds btdc so in real terms you are still significantly retarded compared to tickover.

Set up correctly vacuum advance should make no difference to the power output at any point but should allow it to run leaner and cleaner at cruise. I agree with your point about not driving these things for fuel economy but if the set up is slightly rich under WOT, to be on the safe side, then it is nice to have some way of leaning out somewhat under light load.