There is no correct. In the 60's, most engines used manifold vacuum. In the early 70's, and beyond, ported vacuum was used for emissions reasons. At part throttle, vacuum advance will be EXACTLY the same. It is way more important to get the mechanical advance curve correct. Bringing in the mechanical advance sooner than factory has performance benefits. When you do that, you must reduce the amount of vacuum advance added. Vacuum advance doesn't work at WOT. Read my Power Timing thread. It also applies to Chevy motors. http://www.v8buick.com/showthread.php?63475-Power-Timing-your-Buick-V8
Use whatever the engine likes best if you have ports available for both. Some engines like the extra timing at idle and some get nervous (hunt or miss).
I use ported vacuum simply because mine hunts up and down at idle if I used manifold vacuum. Once you crack the throttle, it doesn't matter which you use.
I use ported. Only difference is really at idle and coming off idle. If your engine doesn't need the extra timing at idle, ported.
This all greek to me but how does one know if they need extra timing off idle? Is there a certain setup, compression or need that would require it?
Typically if the engine lopes a bit at idle more timing helps stabilize the idle. It also tends to raise the idle rpm a bit too. If the engine idles smooth at a decent rpm (like 600-800rpm), then it probably doesn't need extra timing at idle. The thing I don't like about manifold vacuum is typically when you come off idle to accelerate the drop in vacuum pulls the timing down because vacuum drops, versus ported where it starts to rise because it goes from zero to something, and that reduction in timing from manifold vacuum can sometimes cause an off idle hesitation feel in power. General Motors over the decades had some cars come off the assembly line with ported vacuum and some with manifold, just dependent on what application/drivability their engineers were targeting with that car/engine package. Which means there really isn't a right answer, try both and use the one that makes the car drive nicest for you. Going down the road and WOT, both ported and manifold vacuum read identically, it's just at "no throttle" and "coming off of no throttle" are the differences.
All of it is explained in my Power Timing thread. http://www.v8buick.com/showthread.php?63475-Power-Timing-your-Buick-V8 If it is Greek to you, it means you have to get familiar with basic ignition system terms and function. The ignition section of the Buick Chassis manual is a great place to start. Once you understand the basics, then read through the thread (I know it's long), and it will make sense to you. Some distributors had a lot more mechanical advance built into them. As a result, the initial timing is relatively retarded. Retarded timing at idle will make an engine want to run hot in stop and go traffic and with extended idling. Manifold vacuum provides vacuum advance at closed throttle. That boosts timing when you need it to stay cool. That is one reason to use manifold vacuum. Ported vacuum is non existent at closed throttle, but increases rapidly off idle. It is better for emissions. I tell everyone to try both and see which they like better seat of the pants. At part throttle there is no difference.
