I thought I would try and compose this post to answer timing questions I see all the time on this BB.
I see alot of questions about what initial timing to run on a modified BBB, without regard to what the timing is as the RPM's increase. Everyone should know exactly what their total advance is, and at what RPM it's all in at. The right timing, at all RPM's can make a huge difference in the way your engine runs, and makes power.
There are 3 components to total timing. Initial advance, mechanical or centrifugal advance, and vacuum advance. Since vacuum will be at or near 0 at wide open throttle (WOT), initial advance + mechanical advance are most important to how your engine runs under race conditions.
Initial timing is simply how you have your distributor installed, and adjusted, in the engine. You attach a timing light, and with the engine running,(vacuum advance plugged), you turn the distributor until the timing mark lines up with the desired number on the timing tab. As you increase the engine RPM's, you'll notice the timing mark move upwards, and out of sight. This is the mechanical advance in operation. There are weights inside the distributor, that pivot against spring tension, and move the base plate so that the spark occurs earlier(advance).
The springs determine how quickly the mechanical advance increases with rising RPM. There is a pin that moves in a slot under the weight plate. This is what determines how much mechanical advance is built into the distributor. Aftermarket advance curve kits generally provide a bronze bushing that goes on that pin, and limits the movement of the pin in the slot, thus limiting total mechanical advance.
The only other way to modify the amount of mechanical advance in the distributor, is to disassemble the distributor, weld the slot, and file it until you have the desired amount of advance you need. This is why you can't simply buy a junkyard HEI or other distributor, and put it in the engine, and run whatever initial timing you like. If there is too much mechanical advance in that distributor(this is typical for all factory spec'ed distributors), you'll overadvance at higher RPM. If you need higher initial timing, you need to reduce the mechanical advance in the distributor to avoid over-advance.
Most Buick V8's run best at WOT, with a total timing of 30-36*, all in at 2500 RPM, or less. The easiest way to determine your total advance is to use a dialback timing light. You simply connect the light, plug your vacuum advance, and have a second person slowly rev the engine. With the dial back feature, you adjust the light to keep the timing mark in sight as it rises. When the timing mark stops moving, you hold the RPM's steady, adjust the dial until the balancer mark lines up with the 0 on the timing tab, and read your total advance off the dial.
To do this with a conventional timing light, you need to make a 30* mark on your balancer. The Buick 350, and 455 balancers are 6 3/4" in diameter. Circumference (360*) of a circle is pi(3.14) X diameter. 6.75 X 3.14 = 21.195"/12 = 1.76" (30*). Looking at the engine from the front, measure exactly 1 3/4" clockwise around the balancer, and make a second mark. This is your 30* mark. Connect up your timing light, and watch your 30* mark as you increase the RPM's. At some point, your 30* mark will stop rising, and move no higher. This is the RPM, where all of your mechanical advance is in.
At this same RPM, with the distributor loose, adjust it so that your 30* mark lines up with the 0 on the timing tab. You now have 30* of total timing. Line it up with the 2, 32* total, ect.
Keep in mind that a stock distributor usually has stiff springs in it, that don't allow full advance in until 4000 RPM or more. For best performance, you want your advance in at 2500 RPM, or before. The easiest way to do this is to purchase a Crane adjustable vacuum advance kit. It comes with 3 sets of springs to allow your advance in as early as 1600 RPM, or as late as 3200 RPM, or anything in between. For points distributors (Jegs part # 270-99601-1,) (GM HEI, 270-99600-1). What I did was purchase the kit, and install the lightest springs(2 yellow). I used these springs to adjust my total timing, that way, I didn't have to rev the motor very high to see my total. Afterward, I installed the springs that brought my advance in at 2200 (2 silver)
One important note is to make sure the timing is returning to the initial setting, when the engine is idling. So when setting your timing, pay attention to when the advance starts, as well as when it is fully in. Having the distributor in the advance curve, at idle speeds, can cause excessive rpm drop with an automatic trans, with some camshaft/converter combinations. Generally, the more agressive a camshaft you use, the more important this will be. Advance curves should generally start at around 1000-1200 rpm, when your idle speed is around 800 in Park.
Now for vacuum advance. Some people prefer not to run vacuum advance at all. Under race conditions it is totally irrelevant, because it won't function at WOT. BUT, on a street car, it can be used to enhance throttle response, increase gas mileage, and let the engine run cooler at idle and low speed.
The problem occurs when there is too much vacuum advance coupled with a modified mechanical advance. As mentioned before, stock distributors didn't allow total mechanical advance in until upwards of 4000 RPM. At your typical cruising speed of 3000 RPM, only part of your mechanical advance was in. The vacuum advance would supply an additional 14-18* for a total of around 40* or more. At light loads (cruising), an engine can easily tolerate this much advance. The result is better gas mileage.
With a modified mechanical advance, all your mechanical advance is in at cruising speeds. Add the 14-18* of vacuum advance, and the engine pings when you punch the throttle, and the advance from the vaccum cannister doesn't/can't go away quick enough. The answer is to modify the vaccum cannister to allow only 8-10*of additional vacuum advance. With 30-34* timing + your vacuum advance, you'll be at 38-44* which should be optimal.
As far as running your vaccum advance off manifold or ported vacuum, everyone has a different opinion. In most cases, OEM used ported. I use manifold vacuum. With a wild cam, you can use manifold vacuum advance to give extra advance for a smoother idle, and better low end response. Everyone's engine will be different, so you need to experiment with your combination. With the Crane adjustable advance cannister, there is a better way to limit the degrees, than what Crane suggests. I'll post some pictures to detail this. With the stock cannister, you'll need to fashion a block off plate. You basically restrict the pull pin travel to .086" for 8*, or .104" for 10*. Again, I'll post some pictures. Photos, courtesy of Dave Ray(The Ignitionman) Hope this post helps to answer alot of questions.
One other point for those using points. Before checking or adjusting timing, it is important to check point dwell and set it to specs (30* + or - 1*). Dwell affects ignition timing, but timing does not affect dwell. Make sure the point dwell does not vary much as you rev the engine. Bad distributor bushings will cause the the distributor shaft to wobble, and the dwell will vary a lot in this case. Have the distributor rebuilt or replaced it if this is the case.