Posted by Dave in CA [172.68.47.165] on Thursday, November 01, 2018 at 10:23:46 :
In Reply to: Ignition QUESTION: Vacuum advance VS mechanical weights posted by DDD [172.68.133.225] on Thursday, November 01, 2018 at 09:01:03 :
I have an LA small block V8, 1970 vintage, rather than a flathead, but the same principles apply. It came with a distributor with both vacuum and mechanical advance. I set the initial timing to get the total mechanical advance to be in the range the engine performs best. For my engine, 34 to 36 degrees at WOT (wide open throttle) works best and setting the initial timing at 10 degrees gets me 35 degrees at WOT. At WOT, vacuum is low, and no vacuum advance is present. But, when cruising at part throttle, with high vacuum, I get another 15 degrees advance from the vacuum advance, for a total of 50 degrees advance. The vacuum advance does not contribute to performance but does contribute to fuel economy. The vacuum advance is connected to a ported vacuum connection (above the throttle plates on the carburetor) rather than to manifold vacuum. This is the way the factory connected things and I have not changed it.