Trigger disc orientation help.

[garfield at pilgrimhouse.com]

On Wed, 17 Jun 1998 15:00:12 +0100, Chris Wilson
<chris at maximum-bhp.u-net.com> wrote:

>Yes,there *is* a disi fitted,and 2 coils,(cos it's a twin spark,the disi 
>has 2 centre contacts in the cap,and eight distribution segments,each set 
>of 4 contacts firing a single plug of the 2 per cylinder.The disi solely 
>distributes the HT.

ALRIGHT!

Now for the "steppin in it". I justed wanted from the question above to
narrow my "wallowing space" for me conjectures (and mindya, thas all you
get at this point, but I'll finish off with a real world example that
might look AWFULLY similar).

OK, so starting with the "Duh" of "Duh, ray, me", since the Dizzy is
alone responsible for distribution and "commutation" of the spark
relative to the cam, we could start by observing that were you to design
a similar system, you could actually get away with just TWO teeth on the
crank wheel, 180deg apart, at the two TDC points per crank rotation.
This is because all you need to do is supply HV to the dizzy tower at
the right crank pos, and the dizzy does the rest. In this our
hypothetical design, you'd just fire a module (or two in this case,
since you have two coils) connected to the crank sensor.

Sooo, the fact that your crank wheel has 4, nay 5 !, teeth says that
THIER system isn't quite so simple. OK, time to anchor these
speculations in some plausible assertability. Take a look at:
	ftp://efi332.eng.ohio-state.edu/incoming/DIS_6cyl.jpg
Although what you see is a system for a 6-cyl, and so their wheel has 7
teeth, I suggest you mangle this in yer mind to your 5 toothed wheel for
your 4-cyl, and consider this a plausible explanation for what you have.

The fact that two of your 5 teeth are fairly closely together strongly
suggests that this 5th pulse is a "sync" pulse, and that the electronics
you've got actually driving your coils & dizzy is probably even capable
of wasteFire spark control. Now, here's how I figure this mystery. You
know them crazy Italians, eh? They'll put ANYTHING in a dish (even
Anchovies!!) if it makes the dish TASTE better (and don't forget there's
no such thang as TOO much garlic !! YES), so my guess is they took a
wasteFire controller from somewhere (cousin Marelli's Magnetti Spaghetti
Works) and are using it with a dizzy!

This would also explain the factory's somewhat "abbreviated"
instructions for aligning the wheel to ANY of the 4 teeth 90deg out from
each other, as long as you are at TDC 1&4. IF you had a wasteFire set of
coils wired to this IGN electronics, you'd probably need to align that
ONE pair of teeth (ya know, the one with the 5th tooth) with TDC 1&4, to
get the spark pairs commutated properly, but since you have a dizzy to
do that job, the coils don't care if the electronics thinks it's firing
1&4 or 2&3, it's all the same to it, they're both at TDC, and for just
the coil that's supplying the dizzy, that's all that matters. So, yeah,
it probably IS enough to just align to any of the teeth, MAYBE. Hmm,
wait no, that can't be right, cuz the spark doesn't fire on EVERY one of
the 4 teeth but on every OTHER one. Look at the DIS 6-cyl example again.
One thing you KNOW for sure is that you DON'T have 4 jugs firing each
crank rev. SO, I'd start with aligning the wheel to have the 90deg tooth
of that "special pair" of teeth be at TDC cyls 1&4.

OK, that's my scientific wild ass guess. BTW, you can see from an
economics standpoint why they might jury-rig up a system like this for a
dual plug engine. Cuz if they went to wasteFire, they'd need TWO modules
and 4 twinTower wasteFire coils. Lot more expensive parts wise than what
you have with the single module driving two coils and one twin-headed
dizzy. Hey, it's just a paste sauce; toss in some artichoke hearts and
some cilantro. Moto bene!

Gar Ignucci