[Diy_efi] Timing Calculations

[cobraman at insightbb.com]

With a 60 tooth wheel, aren't we measuring RPM 60 times per revolution? 
(weighted average of last x measurements? where x is < 10).  I would think we 
could measure the crank acceleration - maybe even the crank harmonics. 

Engines under rapid acceleration (1-2 gear) can use several deg. more advance 
than under slow acceleration conditions (3-4 gear), hence the old retard 
switch. 

With the current processing power available, we should almost be real time with 
our calculations.   TomS


> > I was thinking of checking the trend of the last x number of 
> > cycles and 
> > trying to anticipate whether the engine is accelerating or 
> > decelerating.  It would never be perfect, but it would average the 
> > timing out better.  Or maybe using the throttle sensor to anticipate 
> > accelerationg/deceleration, kind of like a throttle pump, but 
> > for timing.
> 
> Say your engine accelerates in first gear from 1000 rpm to 5000 rpm in 2
> seconds.  That's an acceleration of 2rpm/ms.
> 
> Revolution time at 1000 rpm is 60 ms
> 
> If you start at 1000 rpm, at full acceleration, you will finish that
> revolution at (60 ms * 2rpm/ms)=120 rpm faster, or 1120 rpm
> 
> Revolution time at 1120 rpm is 53.5 ms
> 
> Under full acceleration, you will begin the revolution at 1000 rpm and
> end the revolution at 1120, with a revolution time of (60+53.5)/2=56.8
> ms.  The error comes in not anticipating the acceleration, and
> calculating a spark timing based on a 60 ms revolution when the motor
> actually finished in 56.8 ms.  Or, to put a different way, your spark
> will be (60-56.8)=3.2ms too late.  3.2ms, at 1120 rpm, is
> (3.2/53.5)*360=21 degrees.  That doesn't seem right...  maybe I made a
> math error above.  Anyway, you get the idea.  As the rpms increase
> there's less time for acceleration from rev to rev, so the error gets
> smaller.  This'll give you an idea how important rpm prediction is.
> 
> --steve
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