A potential problem is the voltage also has to have enough
current when the cylinder is at the top of the compression cycle
and a voltage reading under "no load" might not indicate the
voltage when the spark plug is under maximum compression.
The Leslie unit actually indicates voltage/current flow
through the spark plug via the arc.
Proper high voltage will make the neon bulb light up rather
brilliantly, weak high voltage/current will give a very weak
light output.
I considered the style indicator you suggested but a mechanic
told me to spend a bit more for an indicator that actually shows
you what is going on.
The Leslie costs about $20 and worked very well for our
needs.
Most modern vehicle ignition systems have "coil packs" per
spark plug or for two spark plugs.
The later doesn't ground the one end of the high voltage
winding, it is double ended and feeds one output to one spark
plug and the other output to the adjacent plug. This was more
common in the 1995~2010 time frame or with inexpensive [cheap]
cars today.
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My wife's 1998 Ford Escort ZX2 used this scheme and called
the spark in the non firing cylinder a 'waste spark.' I can post
a diagram of the spark plug wiring scheme in the files section
if there is any interest.
While we got rid of her car when the rear end rusted out, I
found I didn't quite delete all the service data like I thought
I had.?
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I always wondered, still do, what effect the "wrong" polarity
voltage on the spark plug tip will have.
Based on what I learned in a physics class a lifetime ago,
you'd want the center terminal of the spark plug to be positive
because it will arc at a lower voltage.
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But the ZX2 had plenty of horsepower, really way overpowered
for the weight of the car, it'd get away from you if you had a
heavy foot on take off. The cops in central Kentucky love to
write tickets for "improper acceleration" and if you lay rubber
or scratch off around a cop, you will get a ticket. I always had
to be extra careful to use a light foot when driving her car.
And driving that car in snow was an interesting experience.?
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So the ignition system in the ZX2 had more then enough
voltage/current to fire even a negative center terminal spark
plug.
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I saw the spark plugs in another ZX2 that hadn't been changed
since the car was new, the ground electrode had been eaten away
by 280K miles of "sparks" and the car still ran fine. The center
terminal was also eaten back but not near as bad. Frankly if
you'd shown me the spark plugs I'd have bet the engine they came
from would not run. The ceramic cone looked nice and grey like
they are supposed to. Replacing the plugs didn't make any
noticeable difference in performance horsepower wise, but did
improve gas mileage about 10%.
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So I guess the ZX2 had plenty of high voltage.
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Back in the late 1960s Champaign offered an odd plug with no
bent over ground element. The spark traveled directly across a
ceramic plate. My father tried them in his Chrysler Valiant.
They seemed to work OK.?
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To check polarity of voltage feeding center element of spark
plug
"use
a soft graphite pencil (the softer the better) sharpened to a
fine point. Disconnect a spark plug lead and remove the
resistor plug cap so you have the bare HT lead conductor
exposed. Hold the end of the lead close enough to the cylinder
head for a spark to jump. Put the tip of your graphite pencil
in between the lead and the head. Now with your 3rd and 4th
hands (you will need helper) switch off the workshop lights
and crank the engine. A spark will jump from the lead to the
engine via the graphite pencil. In the darkened room, you
should be able to see a flare (of ionised graphite) from the
pencil tip. If the flare is toward the cylinder it indicates
correct polarity. A flare toward the HT lead indicates reverse
polarity."
From
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He
notes that with the one ignition pack for two spark plugs one
will be fed reverse polarity.
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Enter "what effect does pressure have on an electrical arc?"
into Google and the AI bot has a nicely detailed explanation of
the relationship between air pressure and the voltage to strike
an arc.
I'd enter the direct link but is way too long and unwieldy
and you can'c chop any of it out.
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Most gasoline engines have at least a 6:1 compression, most
car engines probably have 8:1 and motorcycles have up to 12:1
for high performance "donor cycles." The Honda CB/C: 250/350 had
a compression ratio of 9.3:1, the higher the compression ratio
the higher the octane required. That's why racing motorcycles
often use aviation gas...it has a much higher octane.
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Thus ends today's lesson on ignition systems, which I'm sure
bored everyone to tears.
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