my understanding of mechanical advance & vacuum advance, tell me i'm wrong

[Argess]

Mechanical advance....what the engine needs when under maximum load.

Vacuum advance (really total advance as it includes mech adavnce).... what the engne needs with no load

Moderate load.... mechanical advance is there, but vacuum advance is less that no-load, but more than at full load.

As vacuum advance is really feedback to the ignition related to load, you car can work pretty well with in-correct timing. But that's not good enough.

As above, you have no idea wether the 20 Deg marked on the dist is cranksahft degrees or dist degrees (1/2 of crankshaft degrees). You'll have to ask them.

As a Cobra is light, and you don't push it hard down low, 20 deg intial and 38 total are good for mech advance only.

My engine (with mech only dist) idles best at 38 Deg (just a fluke the number matches the max advance number), but obviously if i set it there, I will get detonation.

For a decent cam (290 advertised duration), Barry Grant recommends 16-18 degrees. If you set it at 18, and you have 20 mech advance measured at the crank, you get a total of 38 max. I suspect that's what they set it for.

But in such a light car, you can get a bit better idle and low rpm response at 20 or a bit over. Just maintain the 38 max so you don't get engine damage.