If you don't have any way to control fuel injection (via electronic
injection, or primary/secondary throttle system), controlling the
ignition time gives you "some" relative slack on the torque (followed by
throttle delayed control). In any way you "should" burn the mix "inside"
the combustion chamber, if not, as Mike already said, you can say
"goodbye" to the exhaust valves in a short time; "Cam is up, exhaust
valve is closed (from inside the chamber), explosion at the exhaust pipe
generate pressure in both ways, it forces the exhaust valves springs
opening the valves down the cylinder and it can bump the piston head
surface, returning it bumps the pushrods against the cam, so double
problem, or worse, if intake valves are open the explosion pressure
would go back all the way inside the injection system... not a good
thought, not even talking about dirt entering the chamber".

To control the ignition time, or you use a multi-slotted timming (opto)
disk, with one tick per degree or so, or use a software algoritm to
create the required delay. In this case, remember that this delay is
defined in "milliseconds" while rotational delay is defines as
"angular", so in degrees, minutes, seconds.  You need to measure the
actual time between two ignitions, divide that time by 90 (4 cylinders
engine), so you will have milliseconds per degree, then you can generate
the expected delay in "degrees" by delaying the relative milliseconds to
the next ignition time.

To "advance" degrees, you actually "retard less" degrees in a system
that runs with a fixed delayed degrees. Easy.

Processing power? at 10,000 RPM, 4 cylinders engine, you have as much as
1.5 millisecond per ignition time, this is not plenty enough for all the
math and control for a PIC running at 16MHz? You could probably run an
average of 10,000 instructions in this period of time.

Wagner

> Michael Rigby-Jones wrote:
>
> Skipping firing cycles will cause unburnt mixture to be pumped into
> the exhaust which could cause some nice loud back fires.  This is
> never very good for exhaust valves, or indeed the the exhaust system
> itself.  This method used to be used by add-on rev-limiters, but much
> better methods exist.  One traction control system I have seen
> actually uses a secondary throttle which is normaly wide open, but can
> be closed by a servo to back off power.
>
> Mike
>
>      -----Original Message-----
>      From:   Allen Demers [SMTP:farmbooy@IX.NETCOM.COM]
>      Sent:   Wednesday, May 03, 2000 2:53 PM
>      To:     PICLIST@MITVMA.MIT.EDU
>      Subject:        Re: Ignition timing, 2nd iteration....
>
>      Andy,
>
>      To avoid too much lag I wonder if this would work.
>
>      How about just skipping the firing of a plug... like an
>      interupt.  You
>      could detect rotation ratio front to back and if it is way out of
>      sink
>      you could interrupt the firing of the next plug?  Dont know if
>      this
>      mechanicaly would be detrimental.
>
>      <SNIP>
>      >I'm just thinking out loud here. Since you have maximum traction
>      when
>      >there's no wheelspin, maybe you could devise a system that
>      compared the
>      >rotation of the front and rear wheels and slightly backed off
>      the
>      >throttle if it detected any slipping, checked again and backed
>      off if...
>      >etc.