General vacuum advance question

[Fern]

Timing at idel is 15* and with vacuum advance plugged up goes to 35* all in. sounds good.
When I plug up the vacuum advance goes to 54* all in. sounds bad.
Can someone explain why this?
Also how could you use the vacuum advance and still remain at 35* all in.

[DeathProofCuda]

Mostly it's just confusing terminology.  You'd like to think that 35 degrees "all in" means the maximum advance that you will get out of the distributor, but that's not the way that many folks use the term.

The 34 to 36 degrees of advance that you are shooting for with the vacuum advanced unplugged is your mechanical advance.  Sounds like you are in the right neighborhood there.  When you hook the vacuum advance back up your timing may sometimes advance beyond that point.  This will typically be at part throttle cruise situations when your manifold vacuum is high.  If your timing is in the 50+ degree range at this point, you are fine as long as the engine isn't pinging.  If the engine is detonating, then you may need to adjust your vacuum advance to provide less advance during part throttle conditions.

I won't even ask whether you have your vacuum advance connected to manifold or ported vacuum.  No need to replay that endless pissing match.

[Chryco Psycho]

 This is why I I leave it disconnected although at cruise you can run over 38* with light load