So this talk got me thinking (may be dangerous ) that in my old days I never messed with a distributor that didn't have vacuum advance, and now everything I mess with is total electronic/computer controlled. As I'm thinking about it, the mechanical advance can be adjusted, via springs & weights, to yield the slope you want and the rpm where curve starts climbing but you've got one curve of advance vs rpm which means the advance at say 2500rpm is what it is, whether WOT or 1/4 Throttle. Adding the vacuum advance lets you overlay a second advance curve for part throttle operation, which lets you pull in a few degrees at cruise for improved economy, right? The ECU controlled spark goes a step further, allowing the spark to be mapped over rpm and vacuum independently to create a "topo map" of timing. So if you run a distributor with mechanical advance only you set your advance curve for WOT operation, and then tune the carbs to get the best economy you can? Are there any "tricks" for weights and springs to change the shape of the curve to get closer to optimum? Guess this is why the old shops had the Sun machines with O-scopes, right? Just curious, and not trying to get too far off the path here....