A while back, I rebuilt my head and upped the compression to 9.5:1. Since then, I've had to run 2 degrees initial timing to avoid detonation. The vacuum advance unit froze, rendering it unadjustable and it brought timing in way too fast. I finally built a new distributor. With this one, I welded up the advance slots on the cam and filed them down to give 14 degrees total mechanical advance. I'm also using a vac unit with 16 total as opposed to the previous one with 22. So now I'm running 14 initial with moderate advance dialed into the vac unit. What a difference! The car runs much better at low speeds. And no detonation on the highway. I've got a little tinkering yet, but this is the best thing I've done in a while.

Mitch

On this same subject ,how much advanceis safe to run on a internally stock slant with headers and dual carbs.Im talkin mechanical and vacumm both together.this engine is in a longbed truck and it does get used as atruck(hauling engines transmissions and general gophering).
thank you

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Ive got a slanted attitude and a heavy right foot.

Mitch has the right idea, more initial with less total is the "hot ticket".
Another good idea is to set the soft spring so the mechanical advance starts to happen just off idle. (1000 to 1500 rpm area) I like to use the two advance springs, the second heavy one with a large end loop so it slow the advance rate way down for the final 4-6 degrees.

A stock SL6 will take up to 14-18 degrees of initial advance before the idle goes rough. (test this with a vacuum gauge)
(Turning the initial timing up will not pass a smog test but the engine sure will run well)
All the mechanical advance should be in by around 3000 RPM. 28 to 32 degrees total, many Slants have too much total advance.

Disclaimer All engine combos and vehicles are different. Fuel quality and the way you drive also has a big influence. Distributor recurving is NOT a "one size fits all" thing and you need to experiment via trial and error to find (and maximize) the best distributor curve for your combo.
DD