[Sun Tuned]

Bill,
The boys here above, as usual have already nailed it down.

I would like to add youre correct, you have to advance the curve ahead and somewhat faster as the RPM goes up to “get ahead” somewhat of what’s happening. There is a fixed amount of time to complete one combustion cycle, as the rpm changes, theres less time to get that done as the speed goes higher.

However theres a law of diminishing returns with this. After a certain point advancing anymore just doesn’t produce the gains you get early on. In fact up top it’s advantageous to pull a couple degrees back out.

To illustrate a point. If you hook a gas analyzer up on say a 360 Hp 389 with an 068 cam…. And set the timing dead stock @ 6 btdc, and set the carb and get a hydrocarbon and CO reading,then you watch it and as you bring initial timing up towards 12 , youll actually see the HC go down and clean up a bit. Now obviously as you do that the rpm will come up and you’ll need to readjust carb just a bit and reset idle speed but it will clean that up from 6 degrees.

But as I said there’s a diminishing returns thing at play, if you go past 12, on towards 14,16 etc.. you’ll see the HC start climbing back up again…

So there’s a sweet spot for a particular setup, and you can get greedy.

Hope that helped.