Posted by MarkB_MI on December 31, 2012 at 03:03:58 from (75.219.145.82):
In Reply to: Re: Vacuum advance posted by B-maniac on December 30, 2012 at 17:16:48:
Not true.
First thing to consider is "why have timing advance at all?" Why not just fire the spark plug at top dead center and be done with it? Well, the answer is that things don't happen instantaneously in the combustion chamber. It takes time for pressure to develop after combustion starts, and you want to time the spark so maximum pressure in the cylinder occurs roughly at the middle of the piston's stroke, where its velocity is greatest and the mechanical advantage of the crankshaft is greatest. And it turns out that to do this you need to fire the spark plug in advance of TDC. How far in advance depends on how fast the engine is turning. Understand that the flame front in the cylinder doesn't care about the position of the crankshaft; all the flame responds to is time and pressure. So the faster the engine is turning, the more degrees of advance are required to give the proper advance in milliseconds.
Pressure is dependent on throttle position and elevation; as pressure rises the flame travels faster and the advance must be retarded to compensate. What we call "vacuum" is really manifold absolute pressure, zero inches of vacuum is really 30 inches MAP. As MAP decreases (vacuum increases), the timing can be advanced. Anyone who lives at a high elevation knows to increase timing advance to compensate for thinner air; vacuum advance does the same thing, only it's responding to throttle position rather than elevation.
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