Tom68 wrote: ↑Wed Feb 01, 2023 1:05 am
The sidevalve 2 barrels had vacuum brake distributors, high air speed through the venturi ported distributor port braked the advance in the dizzy.
If you are referring to the vacuum brake distributors on Flathead Ford, 32-48, that isn't how they work. When no vacuum is on on the brake piston it applies and drags the advance mechanism to full retard. Load and large throttle opening drops the vacuum and allows the spring to push the brake against the slip ring to retard the timing.
When the brake is applied it has the effect of slowing the mechanical advance so it may not reach total until 3000 or higher RPM.
When the vacuum is high the brake is off and the mechanical advance is allowed to move to full advance, which was only 17 degrees on early and "Denver Head" engines and 25 degrees on later engines.
Early engines with light load high vacuum (brake off) were full advance at 12-1400 RPM. Later engines full at about 2000. Standard initial advance was 4 degrees.
The mechanically inclined user was encouraged to listen for knock and adjust the initial and vacuum brake spring tension accordingly.
3-bolt distributors up to '41 got manifold vacuum through an external steel line from the manifold below the carb and '42-'48 2-bold distributors got manifold vacuum through passages in the manifold and block and timing cover.
In 1949 Ford introduced the "Load-O-Matic" distributor advance system which used a vacuum diaphragm instead of the vacuum brake piston of the '32-'48. The Load-O-Matic used venturi vacuum to advance the WOT timing at full load and manifold vacuum for additional timing at part-load.
The carburetors for this system had internal passages arranged to change from manifold vacuum to venturi vacuum using steel and nylon ball check valves and a vacuum operated valve that looks like a Holley power valve, but its open/closed functions are opposite of a PV. That is the deal that looks like an external power valve on Ford carbs of the 49-56 model years when they used that Lode-O-Matic system on all Ford cars.
1957 and up the V8 got mechanical advance with ported vacuum advance, however the Load-O-Matic was was still used on 6 cylinders until the late '60s.