"so there is no vacc. advance and i want to keep it that way." So, yet another Top Fuel engine on the street.
Did I read you are adding a B28 vacuum advance? How can that be if the distributor has no place for it?
Then, what Lars says is true, but too many people interpret it as needing to bring the vacuum advance timing in along with the mechanical curve, creating TWO acceleration advance curves, DEAD WRONG TO DO. The way it needs to be done is as a supplement to INITIAL timing, controlled by vacuum loading, as Lars says, and then, NOT added during acceleration, but when the engine can tolerate it, from full manifold vacuum changes as engine loading changes. Acceleration, NO vacuum advance, light to NO load, high vacuum, add the vacuum advance timing into the mix, add throttle, drop it back off until load reduces again, and on, and on. An added benefit to doing it the right way is that idle timing, which likes to be 22 to 24 degrees, can be done easily by adding the vacuum advance degrees directly to the initial timing number, to get there, without over advancing the timing so far the engine doesn't like to spin over and start.
An engine, even most stockers, can tolerate up to 12 or so initial degrees before they get into starter problems, bigger cams make it a bit higher. Setting 'initial to the moon high' timing isn't the way to do it. Using the vacuum advance to supplement the 12 initial, to get the 22 to 24 IDLE timing the engine wants....IS, and, that makes the vacuum advance work as both myself, and Lars describe it, NOT the way the 'Top Fuel for the Street' top tuners do.
Don't mistake ported vacuum sourcing vs full manifold vacuum sourcing, world of difference in it.
No vacuum advance, not set up right, LOSER, vacuum advance, set up correctly, applied correctly, great benefit. top tuner ignorant and arrogant about lack of knowledge on vacuum advance is a serous LOSER.
"The vacuum advance control unit on the distributor is intended
to advance the ignition timing above and beyond the limits of
the mechanical advance (mechanical advance consists of the
initial timing plus the centrifugal advance that the distributor
adds as rpm comes up) under light to medium throttle settings.
When the load on the engine is light or moderate, the timing can
be advanced to improve fuel economy and throttle response due,
in part, to the slower flame travel in the combustion chamber
under these lean conditions. Once the engine load increases, this
“over-advanced” condition must be eliminated to produce peak
power and to eliminate the possibility of detonation (“engine
knock”). A control unit that responds to engine vacuum
performs this job remarkably well."
Another top notch "must read" article is here on these boards, in the 'Performance' section as a "sticky", it is by another former GM engineer, John Z, " Timing & Vacuum Advance 101