Your vaccum can may be adjustable. With some, you can use an allen wrench thru the vaccum hose hole to limit total vaccum advance.
The main benefit of the vaccum advance is keeping the plugs clean and better mileage. As IgnitionMan says, it's supplemental and not power producing.
Many pre-EGR cars came with ported vaccum for the vaccum advance. I have several trucks that start at 1961 and up that are that way.
I believe the ported spark helps the engine idle down faster. With a performance engine with longer duration cam, and especially with an automatic trans, full vaccum advance at idle should help the engine run smoother against the torque converter.
If you tried to provide anywhere near the amount of advance provided by a stock vaccum advance distributor by using only the mechanical advance. You would have trouble starting the engine because it would kick back against the starter.
When you get into an aggressive mech advance curve, then you have to limit the vaccum advance, or you'll have too much. Also a heavier springed vaccum can is needed so when you punch it, it will drop the vaccum advance back quickly.
I would think the MSD distributor would come with the proper vaccum advance amount, and not be as bad as a stock distributor.
It's been a while since I've done a street type curve and maybe the fuels used now are making things a little different. But I think what I'm saying here is pretty close even today.
You can run just mechanical advance on the street, but you need to jet lean, have fairly warm plugs and plenty of mechanical advance that comes on quick, with a lot of inital advance. I think I ran over 20 degrees initial advance on mine.
If you can get the vaccum advance to work, you might have better mileage, and cleaner plugs. The horsepower is in the mechanical advance. But if the mech advance only causes the plugs to foul then you can loose power.
Check my web page for suspension info:
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer
[This message has been edited by davidpozzi (edited 07-21-2000).]