gcc, 68K and ignition advance

[Mike Gruber]

> My thoughts are as follows:
> 
> * The air/fuel mix takes a constant _time_ to burn.

        This is incorrect. A dense mixture burns faster than a mixture
        at idle or cruise, which was inducted at low manifold pressure.
        This is why we have "vacuum" advance, to provide additional time
        for the mixture to burn when it is not dense.  Under full power,
        with atmospheric pressure in the intake manifold, the mixture in
        the cylinders will be dense, and take less time to burn.  The 
        density of the mixture will also vary across the rpm band of the
        engine, even at a fixed throttle setting, due to the change in
        volumetric efficiency across the operating range.  This is one
        reason that engines don't require as much extra advance at high
        rpm as you thought.  They burn the mixture quicker at high rpm
        than low rpm, because they fill their cylinders better at 
        6000 rpm than at 1000 rpm, for the most part.

> * In the same way, at 6000 rpm the advance should be 110 degrees (!).

        See above...

> * And the next tricky point - vacuum advance. The vacuum is an indication
>   of engine load. But why is it a good thing to advance the timing when
>   the engine is loaded? By how much should the timing typically be advanced
>   for a given load? (And I mean _typically_, I know that the Bosch systems
>   amongst others use complicated, dyno-derived maps. However, I am more
>   interested in deriving the kind of curve a standard distributor gives.)

        Vacuum advance gives more advance when the engine is under *light*
        load, ie., high intake manifold vacuum.  When the engine is loaded,
        the vacuum drops, and advance will *drop*.

--
Mike Gruber

'88 Supercharged MR2  (ASP)
'72 Datsun 510  (In progress ... perpetually!)