<VV> Vacuum Advance or Vacuum Retard

[jay m]

ok, c'mon-

If there's no vacuum to the VA unit, you get whatever the mechanical 
advance gives you.
If there is enough vacuum to the VA unit, it advances the timing.

That's vacuum advance.

There's a spring in there. Maybe the engineers found that mechanical 
advance was suitable for moderate to heavy load, but performance/economy 
is improved with more advance under lighter load conditions.

"Advance" exists so that the combustion event yields best results under 
a certain set of conditions.
Since the engine operates under a range of conditions, you need a 
variable advance.
That's why you have a basic curve set by RPM, plus and adjustment for 
engine load.

The air-fuel mix will burn faster when temperature and pressure is 
higher. But it burns according to time, not degrees of engine rotation.
That's at least part of the reason that you need more advance as RPMs 
increase.
I'll guess you max the advance at about the point where volumetric 
efficiency starts to drop off?
(that would be peak torque, I expect?)

Under light throttle cruise, I expect you have some noticeable manifold 
vacuum - which means the cylinders aren't getting a full load of 
mixture, and the pressure/temperature is less than under full throttle 
at the same RPM.

That means that the burn should be slower, so you'd want to start the 
burn earlier to get best results from that gas you just paid $4+/gallon.

I'm told that pre-ignition and/or detonation are affected by a lot of 
things, but many of them are addressed by a careful overhaul and 
blueprint job.
(equal cylinder volumes, equal piston heights in the cylinders, etc.)

That means you might want to run different advance than nominal factory, 
because factory settings allow for factory engine builds and likely 
use/neglect in the real world.

Starting with specs provided by experienced builders would be a real 
good idea.
But my above comments might help clarify the 'what' and 'why' of spark 
advance.


On 2/22/2013 9:09 PM, Matt Nall wrote:
>
>
> This doesn't sound right. The VA simply removes the spark advance that IT
> PROVIDED when the throttle is opened significantly. To call this a retard is
> a  play on words as I see it. It is still correctly called a vacuum advance
>   unit.
> Regards,
> Bob Helt
>   
>