On Sat, 8 Aug 1998 13:29:10 EDT DANMAS@aol.com writes:
>Barney, Bob, Charlie, Lawrie, and others,
>
>Some random thoughts on traditional ignition systems:
>
>1) For a given igntion coil, spark intensity drops off as RPM goes up.
>
>2) If a coil has sufficient capacity to provide a sufficient spark at
>5000
>RPM, then there is an excess capacity at 3000 RPM.
Assuming there is suffecient capacity to operate the plugs at 5000 RPM
(which in too may cases with original equipment is not true), and thus
"excess" capacity at lower RPM, then the whole reason for High Capacity
coils (such as the Lucas Sport coil) no longer exists. Yet, listers and
others who have installed such a coil do report advantages. Stronger
spark, allowing wider gaps etc, etc etc.
>3) The excess capacity at 3000 rpm is enough to overcome any reduced
>efficiency due to reversing of the plug polarity, if indeed there is a
>reduction in efficiency.
If by simple act of checking the connections of the coil (a 10 second
operation) you could avoid any suppoised inefficiency, why would any
thinking person do so?
>Most "experts" claim there is a reduction in efficiency from using
>the
>incorrect polarity on the plugs. I personally don't know.
You can see the results on a Dyno. It's real
>5) An ignition coil that will fire an eight cylinder engine at 5000
>rpm will
>fire a four cylinder engine at 10,000 rpm, all else being equal. That
>is, the
>spark energy *produced* will be the same. Spark energy *required* may
>be
>another matter.
>
>6) Few of us routinely operate our cars in the 5000 + rpm range.
>
>7) Those of us that do, usually will have our ignition system in
>tip-top
>order, including correct polarity of the coil.
Just because you never use the engine to the max, is no reason to not
have the ignition system in tip-top order, or indeed any system on the
car.
>8) The ignition coil windings are not grounded to the case.
>
>9) There are three return paths for the spark current to the
>secondary, and
>one of these three paths, albeit a minor one, is indeed the air under
>the
>hood. This path is called "distributed capacitance". We live in a
>world of
>distributed capacitance. That is how the "touch" actuated buttons on
>an
>elevator, for example, operate. The mere presence of your finger near
>the
>button changes the amount of capacitance, as part of the distributed
>capacitance normally surrounding the button, seen by the control
>circuitry.
>The solid state control system detects this minute change, and
>triggers the
>doors to open or close, or selects the appropriate floor.
>
>10 )The other two paths are the battery and the ignition condensor. I
>have
>drawn up a JPG file of the complete ignition circuit, if any one is
>interested.
>11) I don't fully understand items 9 and 10.
Nor do I. But then again, I don't have an elevator in my GT :>)
>12) Race car mechanics go to the trouble of using washers of varying
>thicknesses to index the spark plugs such that the electrodes are all
>pointing
>in the most efficient direction.
If they use washers, they aren't mechanics on the cars up front.
Besides, washers won't work on tapered seat plugs, which are found in
most modern cars.
The proper way is to test each of a handfull of plugs in each plug hole,
selecting the one that places the side electrode in the proper position.
On a 4 cyl engine, I've rarely had to use over 7 plugs to get a set.
Usually it's 5 or 6. Got lucky once and did it with 4.
If they go to that much trouble, it
>seems
>that plug polarity would be high on their list of things to do as
>well.
Hooking up the coil correctly is one of those things that is done,
like hooking up the battery correctly. It's just routine to hook it up
correctly. You don't even think about it.
>13) Last week, at the local magazine store, I read article in two
>magazines
>about spark plug indexing. One had a how-to article, and the other
>said that
>indexing was nonsense, and gave no performance gains. Go figure! I
>haven't a
>clue.
Indexing of the plugs will give a marginal performance gain, which
will vary with a particular engine, but usually (assuming a 100 HP
engine) of 1 HP or so. Granted, the gains generally are in the upper RPM
range, and for the driver/owner who never gets his MG above 3500, no real
improvement. There is one other advantage to indexing the plugs, and that
is the placement of the "open" side of the plug facing the intake valve,
results in a plug that will stay cleaner, longer as the incoming charge
sweeps across the center electrode with the least interference which
tends to scour the electrode faces, keeping them somewhat cleaner. Also,
the incoming charge, being relatively cooler, will lower the temperature
of the electrodes, which (as I understand these things) reduces the
resistance, and thus making it easier for the spark to bridge the gap.
>14) Barney says the multi coil ignition systems fire two plugs in
>series. Past
>experience with Barney says believe him. Past experience with igntion
>systems,
>and my electrical engineering background, says don't believe him. I
>think the
>operative phrase here is "trust, but verify." I will look into this
>some more.
>Any reference material you can direct me to, Barney?
I don't know what Barney will point you to, Dan, but, for starters
check out any service manual for any GM car with dual lead coils (Grand
Prix, Beretta, etc)
Then just for funsies, look under the hood and trace the coil leads from
any dual lead coil to their respective plugs. You will notice that the
two plugs are exactly 180 degrees apart on the firing order. Ie. one is
on compresion/firing stroke, the other is 180 degrees different, on the
exhaust stroke.
Finally, check out a coil itself. You will notice that (1) there is no
switching system to direct the coil output to one or the other tower and
(2). the wiring diagram (from either a Haynes manual or other source)
will show the two HT towers are internally wired together.
>Enough is enough, so I'll quit now. Have a nice day.
>
Rick Morrison
72 MGBGT------- 750-1000 mile per month daily driver.
74 Midget -------- Auto Xer when not being worked on.
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