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Re: digital ignition



At 01:29 AM 12/29/98 PST, you wrote:
>>  That wouldn't happen to be the DDIS system using the 68HC11 designed 
>by
>>Tim Drury, would it? 

>That's the one.  Don't tell me its a POS!

No, its generally a pretty good system. I went to school with Tim, and we
worked together a bit on a fuel injection system. Its got several good
ideas going, but there are some things that could use some improvement. The
trigger conditioning circuit is one of them. It will allow for several
degreees of variation of trigger point due to RPM, sensor age, sensor gap,
etc. Using the stock Hall sensor gets rid of that. And will simplify your
circuit design.

>Problem with using the stock distributor is that, even if I disconnect 
>the vaccuum advance,  it has built-in centrifugal advance, does it not?  
>I suppose though, that it might be easy enough to remove the advance 
>mechanism.  It still doesn't solve the problem of sensing TDC since I'm 
>sure that the trigger plate for the hall sender has four equally spaced 
>'lobes' so you can't tell where in the cycle you are.

  I haven't gone through and timed the lobes of the Hall output, but
generally, this is all you need. Harleys use the same setup. The leading
edge of one lobe occurs a bit before the earliest possible firing
point(most advance), the other occurs at the last possible firing
point(least advance). You don't need to know TDC. If you get the leading
edge signal from the lobe, that is a fixed point in terms of degrees BTDC,
and thats all you need to know.
   If you use the distributor to also correctly send the coil output to the
proper plug, you don't need to know where in the cycle you are, the
distributor takes care of that for you. If you have your heart set on going
to a coil pack type system, you could always affix a second sensor to the
distributor, and sense the notch that is cutout for the rotor. This will
tell you exactly which coil needs to be fired next.
But letting the distributor handle this would be much easier. 

Brad


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