Why did they recommend no advance?
The vacuum advance was not used.
This is over simplification, but gives you an idea of how this works.
If you do use vacuum advance you have two choices - manifold vacuum or a ported vacuum source on the carb which will do the same thing once the throttle is opened. The maximum vacuum advance will be the same in both cases, its just based on the vacuum source and how it is brought in. If you had your initial timing set at 12 degrees at the crank and had manifold source
, the advance might be 24 at idle
. If you had 12 degrees at the crank and used a ported source
, the vacuum would be 12 degrees at idle until you cracked open the carb (not wide open), it would then become 24 degrees, so you now would get the same vacuum advance as you would with manifold vacuum. (You car may run best using manifold vacuum versus ported. Ported vacuum and retarded timing was also one of those things used to keep engine emission gasses in check. The use of vacuum advance also improves gas mileage during part throttle/cruising type driving)
However, when you open the carb wide open, vacuum drops and in both these cases, the advance would drop to the crank setting of 12 degrees until manifold vacuum recovers and builds back up. (I am not covering the function of the mechanical advance and its role in all this)
The vacuum advance can be eliminated and the engine's initial timing can be set to what would be the 12 degrees at the crank plus the vacuum advance of 12 degrees for a total of 24 degrees initial at the crank - which is what was done on Wendt69's engine. With the vacuum advance eliminated and set at 24 degrees at the crank, the remaining advance is brought in and tailored by the distributor's mechanical weights. The optimum total advance for Wendt69's engine combination, as tuned on the dyno, showed the most power/torque to be at 36 degrees. So the timing curve and total advance was adjusted by the rate at which the weights pulled in the total advance (using different weight springs) and the maximum number of degrees which in this case was 12 degrees more (24 initial plus 12 mechanical) for a total of 36 degrees and as I recall it was all in around 3,000 or 3,500 RPM's.
Normally aspirated drag cars do not use vacuum advance as the engines are wide open all the way down the track providing no vacuum signal to be used and you will always see the old factory dual point type distributors without vacuum advance as they were purpose made for high-performance/racing cars. They also use high octane racing gas so the engine will not detonate/ping at the higher initial crank timing.
So very basic explanation. Actual timing using the initial crank timing, mechanical timing, and the vacuum advance to custom tailor the engine's timing curve has been cover more thoroughly in other posts. Each engine can be different in how all these settings are used to maximize the engine's HP/Torque and running condition and what each change can effect.