Vacuum timing

[James903]

I’ve got a quick question about vacuum advance. I’m setting up a tunnel ram and my base timing is 20 deg and I have the 18 deg timing bushing for the mechanical. So basically I can get 38 total and it runs good I just have to set the carb set up a lil better. It’s running a lil lean. I’ve got a vacuum can on my msd I still haven’t bothered to run the lines to yet. My question is obviously the vacuum advances runs off manifold vacuum and it’s a 10 deg canister. So now will the vacuum canister give a additional 10 deg of timing on top of my total timing? Or does it just give 10 deg when there is higher vacuum pressure at lower rpms? I feel like I know the answer from stuff that I’ve read but not sure. You can only get the advance you have set mechanical right? The vacuum canister just helps advance when there is vacuum, and pulls the weights open in the distributor right? Thanks