older stovebolts dont have centrifical advance? uhhhh just older v8s.
here is more info from
web page How much advance do you need? Clearly, it depends on how fast the air/fuel mix burns, and how fast your engine is turning. Roughly speaking, if your engine is turning faster, you want more advance; this is the why distributors have mechanical advance in them, which puts out the spark earlier and earlier in the cycle as the engine rpm's climb. Once you reach a high enough rpm, the air-fuel mixture begins to whoosh into the cylinder with so much velocity that it becomes turbulent, and consequently the flame spreads very fast; increase the rpm, and the mixture becomes more turbulent in direct proportion, and the flame spreads even faster. This means that once you exceed a certain high rpm, the mixture tends to burn in about the same number of crankshaft degrees, no matter what the rpm. Now you no longer need the ignition timing to keep advancing with increasing rpm, so the distributor is designed to level off the advance above some rpm.
The somewhat large overall advance numbers seem too large to believe, and it freaked me out also. However, when I began to understand the role of ignition timing, I began to understand why this much timing is okay in certain cases.
Your distributor contains a mechanism (mechanical/centrifigal advance) of springs and weights that advances the spark timing as engine rpm goes up. This is to compensate for the fact that the engine turns faster, so it goes through more degrees of rotation in the time the flame spreads through the chamber. To compensate the flame is lit off earlier.
The distributor also contains a vacuum advance canister. It's role is to add a bit of advance during certain high vacuum phases of driving, like cruise, mainly for fuel economy.
The reason for vacuum advance is that the rate at which the air/fuel mix burns also depends on how much of it is packed into the cylinder in the first place - i.e., how dense the mix is. When the engine is driving around at part-throttle, the almost-closed carburetor throttle blades restrict the amount of air/fuel mix entering the engine (compared to wide open throttle). This low-density mixture burns more slowly. To compensate, the ignition needs to be fired off earlier when the engine is at part throttle (than when it is at full throttle). The clever solution is vacuum advance. When the engine is at part throttle, manifold vacuum is high, and this sucks on the vacuum advance diaphragm and advances the spark. If everything is set up correctly, the extra advance compensates for the slower-burning mixture.
Vacuum advance during WOT acceleration is a common myth. More than one mechanic has told me this one. Part throttle yes, but not all out acceleration.
When the engine is at part-throttle (cruising at constant speed on level road, for instance), the intake manifold vacuum is high and engine load is low. That is to say, the resulting air/fuel mix is lean. And it turns out the speed at which a flame spreads in an air/fuel mix decreases when the air/fuel mix is thin. So the flame takes longer to spread from the spark-plug and fill the whole combustion chamber. If we want the combustion process timed right, then, we need more ignition advance at part throttle, to compensate for the slower flame burn in the leaner air/fuel mix. And this is exactly what the vacuum advance does: the vacuum cannister measures engine vacuum, which is to say, it measures engine load. The lesser the engine load or need, the more the spark gets advanced. Exactly what the engine wants, and a very clever idea!
During acceleration, there is less vacuum signal. Vacuum advance is less when there is less vacuum, using either manifold or ported sources, as I explained above.
Most vacuum advance units are not adjustable - the factory figures out what works to meet emissions requirements, then presets that amount in the vacuum cannister design. Some Mopar units are adjustable from the factory. For Fords and GM cars you can buy aftermarket adjustable vac. advance units (Accel, Crane, etc).
VACUUM ADVANCE: If you didn't disconnect the vacuum advance when setting the timing, it is probably bumping up your advance as you hit third gear, since your vacuum starts to come up as you approach top speed. It dials in greater ignition advance, which is necessary when you've got a thin or lean air-fuel mixture such as at part-throttle; you have to start that mixture burning quite early in order to get all that fuel combusted by the time the piston is passing TDC. It greatly improves part-throttle fuel-economy. You can disable it temporarily with a golf-tee plugging the line to the canister. It also bumps your idle speed up. That's it. Makes little to no difference for actual driving, only for idle emissions quality. Bob Barry
The total amount of advance or overall advance depends on many things (compression ratio, head design, rear-end ratio, weight of your car, etc) but I've been told numbers of around 10° to 15° advance at idle, around 36° full mechanical advance (with the vacuum advance disconnected). At part throttle, high rpm, with vacuum advance, the ignition timing should be somewhere in the range of 50°. That number surprises many, but that's what's needed for maximum fuel economy at part throttle (cruising).
