vacuum advance

[swedefish]

Trying to figure out the ignition vacuum advance.
When I look at the shop manual for a -67 383 engine with 4 bbl carb, the vacuum advance should be set at 7.5 degrees at 12 " of vacuum. At what numbers do you guys set your vacuum timing advance or do you just tinker with it until best performance and gas mileage?
When I set the initial timing at approx. 22 degrees I get the total of 38 degrees with mechanical advance.
My reading of the vacuum at idle is 12" with the MP purple 284/484 cam.
With other words, how many degrees should the vacuum advance be set to.

Another perhaps stupid question, when reving to 2500 rpm and holding the engine at that rpm, is the vacuum advance activated at that rpm?
Guess I can connect a vacuum gauge to see how much vacuum I´ve got at 2500 rpm, right?
/Thanks, Lenny

 

[green1]

Vacuum advance will put you about 50 degree, at cruise, that is want your engine wants. Foot on gas, it drops out(lower vacuum). At idle, disconnect it to time engine, (high vacuum). Factory engineers developed this.
And here come the rebuttals, from non-engineers.

 

[dgc333]

The amount of vacuum advance is mechanically established in the module. The spring inside that the vacuum works against has an adjustable preloaded that is accessed through the vacuum nipple. This adjustment allows you to adjust the level of vacuum required to start the advance and the level required for full advance.

As stated previously mechanical plus vacuum will be around 50 degrees. But max vacuum occurs at small throttle openings which typically is also a relatively low rpm where you may not have full vacuum advance.

Mechanical advance is strickly rpm dependant and vacuum advance is load dependant.

 

[swedefish]

Thanks guys.
With the dist I´ve got I´m able to adjust the vacuum advance.