To add to STNGR69 Comments. (Reply#4)
Here is some text Credit Link Here:
https://www.crankshaftcoalition.com/wiki/images/e/e4/Vacuum_Advance_Specs.pdfDoes your engine require additional timing advance at idle in order to idle properly? Radical cams will often require
over 16 degrees of timing advance at idle in order to produce acceptable idle characteristics. If all of this initial
advance is created by advancing the mechanical timing, the total mechanical advance may exceed the 36-degree
limit by a significant margin. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can
provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A
tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the
additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a
vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control
unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing
while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note
on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating
problem at idle, and you have verified proper operation of your cooling system components, you can try running
manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps,
but it will also increase hydrocarbon emissions on emission-controlled vehicles. Running straight manifold vacuum
to the vacuum advance control unit is recommended for most applications where emissions are not an immediate
concern
