Re: flacky pretronix !

[jlandry@domain.elided (John A. Landry)]

The aliens commander decided Eric Hegstrom <ehegstrom@domain.elided>
would make a perfect specimen for dissection, and he yelled...

>Of course it has nothing to do with _when_ the coil fires, but if it's
>to far off won't the rotor be out of whack with the right spot on the
>cap.

This is caused "rotor phase"... and yes it's a concern.  Ideally the spark
should jump from the rotor to the plug wire terminal inside the cap just as
the rotor is perfectly lined up.  This rarely happens because of variations
in manufacturing, components, and advance.  Don't assume the rotor phase of
your breaker-points is somehow better or worse than the Pertronix.  You
have no way to know unless you actually verify it.  You'll notice how the
tip of the rotor is made a little wide... this is to help compensate for
less than perfect rotor phase.

>Let's assume TDC timing. No matter how we look at it the rotor is going
>to be pointed the same direction at TDC no matter where the sensor is
>placed. Is this correct. If not then just stop now ;)

Well your partially correct.  The distributor shaft is fixed in relation to
the crank... unless you physically reposition it.  However, the rotor is
attached to the distributor cam (or reluctor - if an electronic ignition)
which rotates separately on top of the distributor shaft.  The mechanical
advance thus moves the rotor.  But yes... if you eliminate the mechanical
advance from the discussion... the rotor does stay in one place.

>So  we have to rotate the distributor(and cap) to get whatever device to
>trigger when the rotor is in this position.

Correct.

>But if the triggers (pertronix, points, optical etc) are in different
>locations then the rotor will be in different locations when the coil
>fires. If the rotor is to far out of whack from the cap won't this miss
>the the little nub on the cap that feeds the spark to the right plug?
>(What's that nub called anyway?)

You're line of reasoning is right and it could possibly be a problem... but
not usually.  It gets sort of complicated, but assuming the rotor phase is
acceptable to start with, moving the distributor housing to adjust the
ignition timing "self-compensates" for the mis-alignment of the rotor tip
as the timing is adjusted.  That is, the rotor phase stays correct as the
timing is adjusted because as the triggering is being changed, the housing
(with the cap terminal) is being moved back in place.  But, if you move the
"trigger" by itself which leads to a rotor phase error, moving the
distributor housing afterward to compensate for the timing change will
further *exaggerate* the rotor phase error.  

>It seems to me that the geometry of the sensor placement on the
>distributor is also important (though it just has to be close).

Now that I think about this... your high speed miss could possibly be
caused by a severe enough rotor phase error with the Pertronix.  Let's say
your rotor phase is off to start with at idle TDC.  As the distributor RPMs
increase, the mechanical advance would move the rotor even further from the
terminal and at some point the spark will jump to the wrong terminal.  The
spark will take the path of least resistance, which is the closest terminal
to the rotor... or a ground in some cases.

Just so you know, the way to check the rotor phase is to take a spare
distributor cap and cut an open window into the side just under two or
three of the terminals.  You need to place the window in a spot where the
terminals can be seen when the cap is in place on the engine.  What I mean
is... don't cut the window so it's facing the thermostat housing where you
can't see inside the cap.  To check the rotor phase, you connect a timing
light to one of the plug wires attached to a terminal you can see through
the window.  With the engine running, point the timing light in the little
window and the strobe effect will freeze the rotor in relation to the
terminal.  You will be able to positively see the rotor phase at not only
idle, but throughout the rpm range (as the mechanical advance takes effect)
as you increase the engine rpm.

The rotor tip at idle should be just *past* the terminal in the direction
of rotation.  As the engine rpm increases, the rotor tip should move into
exact alignment with the terminal and then ultimately be prior to the
terminal.  The rotor phase error due to the mechanical advance (too early
or too late) should ideally be split on either side of the terminal.

I hope this helps.

John L.
-------------------------------------------------------------------------- 
jlandry AT halcyon DOT com      | 
Conservative Libertarian        |  "The road to  tyranny, we must never
Life Member of the NRA          |   forget, begins with the destruction
WA Arms Collectors              |   of the truth."
Commercial Helicopter - Inst.   |                     William J. Clinton
http://www.halcyon.com/jlandry/ |    10-15-95, speech at the Univ. of CT