[Excaliber]

Infletcher, good points! I to prefer my vacuum signal directly from the intake for a smoother idle AND it helps to stop a "run on" condition when the motor is hot and shut down. If for no other reason, I would vac advance for those features alone. Any increase in mpg while cruising is just icing on the cake.

Secondarily, the fuel/air mixture cannot be over looked! A "lean" condition will accentuate a "ping" problem. When I prep my car for some serious track work, I purposely change the jets to a "rich" condition, which would not be acceptable on the street. For ping control, rich is better than lean, especially if your running a LOT of advance, like I am.

Perhaps Azfordman's vac unit is not returning smoothly and quickly enough? Therefore, the added timing (vac timing) remains in play in spite of the loss of vacuum signal. That additional timing results in a ping condition when WOT is suddenly achieved.

After market vacuum dist's have a pretty strong return spring that responds quicker than a stock OEM unit. Plus, after market vac dist's typically have to get a fairly high signal before they even start to move, whereas a stock vac dist may move at a lower signal and return more slowly.

You will find very few, if any, professional engine builder that even recommend a vacuum advance. There just to risky (if they hang up, don't return quickly enough or add to much timing). Plus, there a pain in the butt to field questions about "how it works", how to tune it, etc. Example: THIS THREAD!!! I certainly won't recommend one, but some of us understand them, like them and are willing to deal with the RISK for the benefits. Choose wisely your distributor and tune carefully it's "curve".

That is an excellent article, Tom gave us the link too. While we are dealing with a small block here, it should be noted that FE's like a LOT of timing advance compared to many other engines. I specifically added "quench" (a whole 'nother can of worms there) to my FE to enhance the stock combustion chamber design and promote a faster and better burn. One BIG result of "quench" is that it suppreses "ping" so you can either run more advance or lower octane fuel. Quench is a favored "trick" that the Engine Master's competition builders use to gain horse power with pump gas. It is also one of the reasons I don't have a problem running a vacuum advance on MY engine.