[Roadsters] Maximum Advance and Octane -- how far can I go?

[Nathaniel Leeds]

Still tuning,

And now I'm starting to have evil urges. There's a little devil sitting on
my shoulder which wants me to buy a crate of octane boost and crank out my
advance (20+ degrees on my SU carb, Solex cam, U20). What is people's
experiance in this regard. How much maximum advance (mechanical + vacume)
can the U20 take on 91 octane, and then how much more can it take with a
little street-legal octane booster. I know there are standard formulas
depending on the engine, I just don't know what they are.

Also, does someone know where I can find the advance charts for the U20
distributors (plural because I know there are a couple of available curves,
and I don't yet know which mine is). I'd like to get a feeling for what my
maximum theoretical advance is without pissing off the neighbors by running
my engine at 6000 rpms.

Nathaniel Leeds
'69 2000 U20