360 Tune Up
It's not that different than the timing for the late 60s 340s. Those started with an initial around 5* (for reduced CO) and started quickly advancing right off idle ( around 700 rpm) to get to the timing that would be more efficient for driving. With a stock engine that timing should work OK. In terms of efficiency, a little more initial would probably produce more power at and put less heat into cylinder walls and exhaust at idle. To keep the rest of the timing the same with higher initial, the inside of the advance slots are shortened .
The Direct Connection guidelines for setting vacuum advance require checking the engine's manifold vacuum at your typical highway cruising rpm. Then adjusting the vacuum advance as needed to get the mechanical timing plus vacuum timing so it will be around 50 (go check) at 2600 rpm or so.
So for your current setup, it looks like having the vac advance adding about 20* would be about right. Just need to find out the vacuum 20* should be added at.
This looks like a typical early smog curve. I don't have a mid 70s or early 80s book. Those might be similar, I just don't know.
It's not that different than the timing for the late 60s 340s. Those started with an initial around 5* (for reduced CO) and started quickly advancing right off idle ( around 700 rpm) to get to the timing that would be more efficient for driving. With a stock engine that timing should work OK. In terms of efficiency, a little more initial would probably produce more power at and put less heat into cylinder walls and exhaust at idle. To keep the rest of the timing the same with higher initial, the inside of the advance slots are shortened .
That seem like pretty good vacuum at idle. Maybe too much to get away with using manifold vacuum on the vacuum advance. Its going to depend on the cam and build etc.
The Direct Connection guidelines for setting vacuum advance require checking the engine's manifold vacuum at your typical highway cruising rpm. Then adjusting the vacuum advance as needed to get the mechanical timing plus vacuum timing so it will be around 50 (go check) at 2600 rpm or so.
So for your current setup, it looks like having the vac advance adding about 20* would be about right. Just need to find out the vacuum 20* should be added at.