In a response to one of my rambling posts, Barney Gaylord wrote:
> Given due consideration, the polarity of the spark plug is probably not
> very important.
> Many newer cars have distributorless ignition systems with multiple coils,
> where one coil fires two spark plugs at the same time. It's called a
> "waste spark" system, where one plug fires on the compression stroke as
> normal, and the other plug fires at the same time on the exhaust stroke
> (wasted spark). Older single coil systems pass the spark from the coil
> (via the distributor) through the spark plug to ground, and the high
> voltage circuit is completed by the body of the coil also being grounded.
> With the waste spark system the body of the coil is not grounded, but the
> high voltage side of the coil completes the circuit by being connected to
> two spark plugs, both of which are grounded. The spark goes from the coil
> through one plug to ground and then from ground through the other plug back
> to the coil to complete the circuit. If you pull one plug wire, two plugs
> stop firing. In this case the two spark plugs are opposite polarity, and
> it doesn't seem to affect the spark quality or the life of the plugs (at
> least not enough to bother anyone).
> And if you're worried about that wasted spark wasting energy or degrading
> the spark plug, it does neither. Since the two spark plugs are in series
> with the coil, the same current passes through both plugs. The plug firing
> on the compression stroke requires a lot of voltage to make the spark jump
> the gap under pressure, while the plug firing on the exhaust stroke takes
> very little voltage to fire. As a result, most of the energy of the spark
> is going to the plug doing the useful work, and very little energy is going
> to the idle plug. So not much energy is being wasted, and that little
> energy being passed by the idle spark plug causes very little wear on the
Even though I have a great deal of respect for Barney's opinion, I had the
temerity to say that I found that hard to believe. Not the "wasted spark"
part, but the "series" plug connections. Well, by golly, ole Barn' was right!
They are connected just as he described. I went to the library today, and went
through several automotive books and learned a lot about modern ignition
systems (as opposed to the antiques we use in our MGs).
> Just one of our modern day curiosities, but it does show that modern day
> designers have determined that the polarity of the spark plug is pretty
> much irrelevant.
>From what I read, modern day designers haven't as much determined plug
polarity to be irrelevant, as they have just created ignition systems so good
that it just doesn't matter. One of the books I read carried a warning that
the newer ignition systems can be FATAL, unlike ours, which will just knock
you on your keester. The biggest danger from an MG is knocking yourself silly
when you raise your head and hit the hood after a mild shock.
According to the book, some of these systems produce a spark with as much as
100 WATTS of energy!!!! The primary windings have a resistance, cold, of 0.45
to 0.65 ohms, as compared to 3 ohms in a Lucas sports coil. That's from 22 to
32 AMPS!!!! (ignoring duty cycle). The resistance goes up as the coil heats
up, but the books didn't give that value.
With that kind of energy, a piddlin' 20% inefficiency is of no concern. The
books didn't mention dwell time, but with this system, dwell could approach
180 degrees, regardless of the number of cylinders.
My apologies, Barney. And to think, I bought MGs and Triumphs so I wouldn't
have to learn anything.
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74