V8 DIS revisited

[Shannen Durphey]

Hector wrote:

> 
> Sorry about that. I changed to plain text and wrapped text at 70 char.
> Hope that does it.

: )  Yes, thank you.

> 
> >From looking back through the archives it seems you have been studying
> this for a while.

* whistling absently* ahh.. maybe ; )


> 
> Yes, that's the system I'm dealing with. I understand there are two
> signals: one the module sees and one it sends to the ecm. My question
> is whether the different reluctor design would delay the signal or
> take the function of inverting it? Or, would it not matter and the
> module revert to the same signal it sent out before? If someone has
> inverted the signal, how? This seems the easiest and quickest way but
> I wouldn't know how. Is it some type of gate or other Radio Shack
> device?

You could change reference pulse timing in relation to piston TDC by changing
the position of the reluctor.  There is a tradeoff.  The DIS module delivers
spark at TDC during startup based on the position of the notches in the wheel. 
If you retard the wheel, you retard startup and bypass spark timing.  This may
work for a summer car or a warm weather vehicle.

It may be possible to invert the REF signal by machining the reluctor to have
seven high points rather than seven notches.  I do not believe this will work,
but I haven't tested the possibility.  I'll say the module  will get confused
until someone proves otherwise.

> 
> > The difference in wave shapes can be attributed directly to dwell
> control.
> > Since this DIS system handles it's own dwell control, the ecm simply
> cuts the
> > time availabe between spark plug firings in half, and provides a
> low/high
> > signal.  Single coil distributor signals are calculated to rpovide
> optimum dwell
> > times, hence the varying width on/off portions of the signal.
> > ****
> 
> OK, so by this statement you're saying it won't work. In order to run
> DIS you have to run a DIS bin. 

No I'm not saying that.  I'm explaining the differences in the EST signals
between DIS and dizzy calibrations.

> Is this possible: run a 730 4 cyl DIS
> bin with two coil packs. I understand you still end up dealing with
> the two coil pack problems and of course the mask may not be available
> for rookies like me.

This does not address the need to fire the correct spark plug at the correct
time.  

> 
> > Timing and consistency are the most important parts of the DIS crank
> sensor
> > signal.  Any missed pulses from the crank sensor can result in the
> module
> > "resetting" #1 cylinder.
> 
>  Understood. If I pursue it, it'll have to be an exact thing.

The main concern is with stamped steel pulleys as reluctor wheels.  I was using
a double groove power steering pulley with ground in  notches for bench
testing.  At 2500 rpm or so, the spark would shift from 0 degrees BTDC to 90, or
to 60, because the .006" variation in pulley diameter was enough to cause the
module to reset number one cylinder.
> 

> 
> I should have taken a closer look when the engine was being swapped
> out. Next time one comes in I'll inspect it a little more.

Do you have access to Alldata?  That would provide the information you need.


> 
> > ****
> > You need to merge the output pulses from the modules, since each
> would only
> > provide 1/2 the needed pulses.  You'll need to separate the output
> pulse from
> > the ecm to be delivered to the correct module, since each handles
> 1/2 (and only
> > 1/2) of the spark firing chores.
> 
> Wouldn't a constant single signal to the modules be OK? I mean, when
> they are ready to fire, the signal would be for the right one, no?

With the dual DIS design, module 1 fires, then module 2 fires, then module 1
fires, back and forth. If module 1 handles cylinder 1 of a smallblock Chevy then
it also sparks 4, 6, and 7.  Module 2 would fire 8, 3, 5, and 2.  If module 1 is
ready to fire #1 cylinder, and gets a pulse from the ecm, it will fire #1
cylinder.  What cyl will fire when module 1 gets the next pulse?  Number 4.  But
what cylinder do we need to fire next?  Number 8.  So if the same pulse goes to
both modules you will deliver spark to 2 cylinders at once, numbers 8 and 4. 
You need a way to direct every other pulse to module number 2.

> 
> > You'll want to synchronize the pulses from the
> > ecm to the correct module to prevent firing the wrong plug.
> 
> I thought the ecm only handled timing duties, not cylinder selection.
> A HEI bin doesn't know which cyl. is firing. Nor would a DIS bin
> unless it had a cam sensor, or so I think.

Correct.  The ecm for the distributor doesn't need to know which cylinder it's
firing because the physical position of the rotor determines where the spark
goes.  Doesn't matter whether or not the ECM has a clue which cylinder it thinks
it's firing.  The spark goes where the rotor points.  Now the ecm for the DIS
system we're describing doesn't need to know which cylinder it's firing either. 
Why?  The double notches in the crank provide a synch pulse which the DIS module
uses to determine which cylinder to fire first.  The ecm doesn't care which
cylinder it's firing because there's a component outside the ecm handling the
job of delivering the spark to the correct cylinder.  But when you create a twin
DIS system there's no way to ensure which module will fire first and there's no
way for the two modules to communicate to determine which one should get the
next pulse.  Since you cannot just throw EST pulses at both modules you'll need
to direct the first pulse to the first module, second to second, third to
first...  


> 
> >Remember that
> > startup can cause either module to fire first.
> 
> Yes, it can but I don't see that as a problem as the spark will be
> sent to the right plug regardless. The TDC mark will travel across the
> crank sensor for one and 90* after, it will pass the other. The
> correct module should fire the correct plug.

Until you switch to EST mode.

> 
> >Also, just for reference, the
> > DIS systems that we're speaking of do not fire #1 cylinder first.
> It makes
> > little difference at this point, but it's somthing to keep in mind.
> > ****
> 
>  That I didn't know but I guess it could be worked around. Which one
> does it fire first?

For the 4 cyl the 2/3 coilpack fires first.
Shannen
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