How would one go about setting an adjustable vac can properly? I understand about the Allen screw location just not sure what I would be looking to achieve in adjustment .
Yote
The basics are that you want as much timing in mid throttle cruise as you can get without pinging.
Boy, that was a lot easier and faster to say than it is to do. :D
For example: My own engine runs as high as 52 degrees in the mid light throttle range.
You can do this because the less load there is on the engine means the timing can be higher during that light load, but as load goes up so does cylinder pressure and therefor preignition can happen sooner as it climbs.
That is why cars with say 12:1 compression have to use higher octane fuel.
(Octane raises the detonation point of fuel) so the higher the octane rating the more compression you can run because the pressure and heat in a cylinder can cause fuel to ignite without any spark at all.
This is how diesel engines work, by compressing the fuel to the point it explodes without a spark at all.