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My mallory unilite distributor has 24 degrees mechanical advance at 3200 rpms. But it also has a vacuum advance canister attached to it, so it somewhat confuses me.

My question is what is the difference in mechanical and vacuum advance? is it simply in making the mechanical advance come in on a curve? if so, am I right in thinking that I can adjust the vacuum advance to make the mechanical 24 degrees come in sooner, like around 2700 rpm rather than 3200?

And also, what is a good total timing to go with for a 351C?
 

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The purpose of the vacuum advance is to advance the timing when there is a lot of manifold vacuum, like when you are cruising down the road or under part-throttle acceleration. This makes the engine run more efficiently and burn cleaner because at part throttle only a small air/fuel charge is getting into the cylinder, so the charge doesn't get compressed under near as much pressure, and thus can be ignited earlier. At wide-open-throttle, where there is no manifold vacuum, the vacuum advance has no effect on the timing curve, and the distributor behaves just like a non-vacuum advance distributor.
 
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