I think the vacuum advance on my '68's distributor may be bad. When I disconnect it and reconnect it, I don't see any change at all in my timing. I think the timing, which is currently 4degBTDC, is supposed to drop closer to TDC when I disconnect the vacuum line from the vacuum advance unit, right?

So what's the purpose of the vacuum advance unit and what effect would it have on the engine (327/300) if it was bad?

--Steve

The distributor has 2 ways to advance. First thing to know is the reason that it is needed. The engine will have the best performance when it is about 36 degrees. That is where the power is maximized. However, at that setting the engine would be hard to start and idle rough. Then there is emmissions to think about. The lower the timming is is better for that. Side note: a motor in a boat doesn't have adjusting distributors because there isn't any load on it when it is in gear at idle.
That being said, the vacuum advance is a way for the distributor to advance as soon as you step on the gas (ported vac). This brings it closer to the 36 degrees (normally less for stock set-ups) right away. As the motor biulds rpms, the mechanical advance takes over an keeps the advance up in the perforance range even after the vacuum drops off when you let up on the gas pedal.
The answer to your question is a properly working/setup distributor will have more power then one with no vacuum advance.

Depending on the vaccum and the advance setting, your timing should change to 14 to 18 DBTC when you have the vaccum line attached to the distributor. This depends on your intial setting and is different pending engine combination. Check the tune-up portion of a manual for your year model and engine combination. Your distributor should also advance the total timing to around 23 to 25 DBTC at approximately 3500 rpm. If you change the springs and weights in your distributor, you can tune in more advance for increased performance. The timing advance lets you adjust when the spark plug ignites the fuel mixture.
You can get or borrow a vaccum pump and hook it to the vaccum advance. Take off the distributor cap and watch the advance linkage as you pump up vaccum. The linkage should move as vaccum increases. It should also hold vaccum and not leak down. Your linkage may need attention or the vaccum advance may need to be replaced. When in doubt, always consult a repair manual. Good luck!

if your advance is hooked to ported vacuum, it won't move at idle when you pull the hose. joe

After I changed to headers, large exhaust system and a larger carb, I went to the guy who did the heads for my 454. He told me that " I am now close enough to a race engine that I no longer need vacuum advance ". Notice that was in quotes and it is his words not mine. Anyhow, he completely removed the vacuum advance, replaced the springs in the distributor, set the timing to around 34 or 35 degrees, changed jets by two sizes and the car now runs and idles fine.
Can this can be done as a normal configuration, I don't know. Surely there are people out there who know if it would work ( no vacuum advance ) on any engine or not.

Yes you can run engines with out vacuum advance. I know some ford engines came from the factory with mechanical advance only. Really what you are trying to do by using an advance is allow the spark plug to fire at the exact moment the piston reaches maximum compression. Believe it or all your doing is timing the spark plug to fire at that exact moment the piston reaches TDC. As the engine speeds up you need more advance to deliver the spark on time. I read somewhere and I was trying to find the source that in most stock ignition systems it takes a 1/4 degree of advance for every 250 RPMS of engine speed above intial timing to keep up with the time it takes for the coil to generate the energy and discharge it to the plug. So for if your engine reved to 5000 RPM's divide that by 4 means 20 degrees + initial of 14 degrees gives you a total of 34 degrees advance. Don't quote me for sure. If I can remember where I saw this information I will be sure and post it.