RootesRacer
Donation Time
Thanks for the informtion. I also thought that the vacuum advance was a transitional advance and wasn't really added into the total.
It does get added to the total, but only when you are a light loads and very small throttle angles.
Picture being at 3500 RPM, and cruising at light throttle in a low gear.
The engine makes good vacuum being its not loaded, the vacuum ports get a decent vacuum signal due to the high vacuum and due to the passages being uncovered by the throttle plates.
At high manifold vacuum, the cylinder pressures are low and the engine needs more timing to make peak cyl pressure at the correct crank angle since the flame front is slow.
This means light throttle timing can be 40+ degrees, which is needed for best fuel economy and engine response.