When does vacuum advance kick in?
This is a repost from
Timing advance guru input needed
If you go there you can see the table the person posted.
All the same principles apply as with carburated systems.
from MTSC 1959
As rpms climb, more lead time in degrees is needed because the engine is turning faster and there's less time (seconds).
As efficiencies build, the combustion itself speeds up, reducing the rate of additional lead needed in the upper rpms.
On a distributor, this rpm related change in timing is handled by the weight and spring system.
Lean mixtures take more time for the flame to travel. So under these conditions more lead time is needed.
from MTSC 259. Ignition system analysis. 1969
This lead time for lean mixtures (vacuum advance) needs to go away when mixtures go rich again approaching wide open throttle.
If enrichment begins at 11"Hg (convert to MAP) then there should be no vac advance below 10 or 11"Hg.
If enrichment begins at 8"Hg, then there should be no vac advance below 7 or 8 Hg.
That's the basics of it.
More about mixtures relationship with load and manifold vacuum in these posts:
How lean is too lean?
Power Valve Selection: The Definitive Answer
This is a repost from
Timing advance guru input needed
If you go there you can see the table the person posted.
All the same principles apply as with carburated systems.
from MTSC 1959
As rpms climb, more lead time in degrees is needed because the engine is turning faster and there's less time (seconds).
As efficiencies build, the combustion itself speeds up, reducing the rate of additional lead needed in the upper rpms.
On a distributor, this rpm related change in timing is handled by the weight and spring system.
Lean mixtures take more time for the flame to travel. So under these conditions more lead time is needed.
from MTSC 259. Ignition system analysis. 1969
This lead time for lean mixtures (vacuum advance) needs to go away when mixtures go rich again approaching wide open throttle.
If enrichment begins at 11"Hg (convert to MAP) then there should be no vac advance below 10 or 11"Hg.
If enrichment begins at 8"Hg, then there should be no vac advance below 7 or 8 Hg.
That's the basics of it.
More about mixtures relationship with load and manifold vacuum in these posts:
How lean is too lean?
Power Valve Selection: The Definitive Answer
