Ignition timing ref points

[Kalle Pihlajasaari]

Hi Todd,

> I want to do the timing with no interpolation so that the counter
> can be very dumb and just count pulses ant any RPM and then go blip.   >>>
> 
> Good point; but I think you may find that a ref mark of say 50 deg BTDC for 
> each cylinder combined with a *simple* interpolation will do the job within a 
> few degrees as well, plus drastically reducing the interrupt overhead. But 
> let's see, I guess after every firing event you could determine the current 
>rpm and load; consult a lookup table for the correct "tooth count" of the next 
> fire event and initialize the count; when the count is up again it's "Fire in 
>the hold! <*ka-boom*>" time, plus set up the count for the next event. FYI the 
>spark map in one of the GM PROM's only has a granularity of 400 rpm increments 
> (Y axis) (if memory serves) vs a load (X axis) granularity of something like 
>12 or 16 or so columns. But what about dwell time for your coil(s)? Kalle, can 
> you discuss any of your plans for this with the list?

I can discuss anything that people are prepared to listen to :-)

In the limiting factor I would have 4 coils but My friend with
the big V8 Mercedes block says a quality V8 ignition system will
always have enough dwell for a 4 cylinder system.  I will try 
with a single coil first.

My ideal sollution was to have a optical setup that would
generate 5 outputs, one a pulse train at say 1deg per pulse and 
the other 4 are earliest timing marks for each of the individual
plugs which would then allow for real simple stuff that cannot go
wrong and would fire on the second plug when you crank.

For dwell in the 4 (or 2 coil wasted spark) system I could easily
turn on the next coil as I fire the current one.  This would
give 90 deg dwell which would be loads of time but a bit crude and
not constant with RPM.  With a single coil I would have the option of 
adjusting dwell but I think that after TDC the spark can be quenched
anyway even if it is still on.  This would give a dwell of 90 deg
minus timing angle which could be 70 deg that would suit me.

Could start to use a timed dwell but this would them be a variable
number of degrees before the firing point depending on RPM.  If I
put in real high performance stuff and don't want the coil to melt
I could d othis and have a constant dwell time.  The thought of
putting a current transducer onto the coil supply to monitor the
build up would also be an idea to evaluate the dwell angle for the 
next plog so I dont have to worry about time, just track the required
saturation point for the coil on a spark by spark basis, could then use
a 6 V coil with no ballast to get real short dwell times at high speed
but on a 4 cyl this is all over kill.

Cheers
-- 
Kalle Pihlajasaari     kalle at data.co.za
Interface Products     Box 15775, Doornfontein, 2028, South Africa
+27 (11) 402-7750      Fax: +27 (11) 402-7751