Adjustable vacuum advance: What's it really mean?

[DocMel]

I'm installing a new distributor/  Ill be posting the make/install in another post.   

My questions deals with the adjustable vacuum advance (allen wrench inside of the canister nipple). 

Factory setting is 5-7degrees of crankcase advance at 15degrees vacuum,  so.............

1. If I turn clockwise to increase the amount of vacuum advance:  Whats the advantage of this?

2. If I turn CC,  Whats the advantage here?

Plz keep it simple for stupid

[brads70]

It allows the vacuum advance (timing)to come on earlier or later. Great feature for aftermarket cam and for fuel economy. A lot of people don't use it though.

[Chryco Psycho]

Having more advance by adding vacuum could in theory help fuel economy  but by shortening the advance curve you can have more initial advance which always helps power & milage especially with bigger cams .
 Generally I leave the vacuum disconnected & run the timing at 36* total & around 16* at idle

[blown motor]

Neil, I assume that is for both big blocks and small blocks.

[Chryco Psycho]

yes , if you dial the allen screw all the way CCW you will have zero vacuum advance

[dodj]

Plz keep it simple for stupid

Vac advance = better fuel economy at cruise.

[Chryco Psycho]

or not in my case

[73restomod]

This is nice to adjust even if you don't have a radical cam,  I run mine so that at any thing below 13" of vacuum it cuts out and resorts to mechanical timing. That's for my 79 w100 which makes 19" at idle, challenger 440 makes about 15" total, so I may let that hang out till 10-11". There will be much test and tune with that one, because the six speed really cuts the gearing. You can also run a vacuum gauge with a long hose on it and drive around observing your cruising vacuum and use that as your basis for tuning as well.