Do cylinder #2 and #3 fire at the same time?

Yes, this is correct. Except, keep in mind, one of those cylinders is in compression when the other is in an exhaust cycle. It's like comparable motorcycle engine configurations. Only one of the cylinders actually fires at a time.

Dana

I get it. Thanks. Any idea why Mazda switched the order of the two ignition coils (e.g., #2 & #3 vs #1 & #4) post 93 vs per 93?

Gus

Just a different design teams, probably...

Also, I've got some info on my website if you'd like to know more about the Miata's wasted spark ignition system...

-cliff-

Cliff,

Thanks for the info. Your thoughts do raise a couple of questions. Would periodically reversing the #1 and #4 and the #2 and #3 plug wires extend the life of the wires and plugs (e.g., reverse the current flow)? Second, are there other differences in the two sparks other than the current direction (e.g., #4 slightly before #1) that would cause differences in performance depending on the direction of current? Third, in a proper plug wire configuration, each wire runs straight from plug to ignition coil with out crossing one of the other wires. This is not the case when the connections are reversed. Would this effect performance? Finally, does current really flow from the #4 to the #1 plug or does it go from #4 to ground and from ground to #1 (the cylinder head is grounded)? Thanks for the great learnings.

Gus

See my responses below...

-cliff-

The wires wouldn't be affected, however the life of conventional could conceiveably be prolonged by reversing the erosion of the side electrode in the "reverse current" plug. It is unlikely that any significant extension of their life would be accomplished, and using double platinum plugs alleviates this problem anyway.

The speed of the current flow through the first plug, to the second is at nearly the speed of light. If we assume for the sake of argiment that that distance is 1 foot (through the head), then the timing difference between the sparks would be about 1.0 nS (billionth of a second), or about 0.00004 degrees of crankshaft rotation at 7000 RPM.

connections

Yes, I think that this is one of the failure modes of plug wires in this sort of system, as their insulation breaks down the path of least resistance for the spark could be to "jump" to the other wire.

Finally, does current really

The fact that the cylinder head is connected to the vehicle's "ground" (see note, below) is not really relevant to the current flow from the #4 to #1 plug--the head is aluminum and therefore would conduct electricity between the plugs whether it was "grounded" or not.

Note: Calling a vehicle's chassis "ground" is somewhat of a misnoner, it's really just an uninsulated common return path to the negative battery terminal.

Thanks for the great

Why would switching the #2 and #3 wires reverse current? As far as I know, all the plug wires carry current from the electronic ignition module to the

• end of the spark plug, where it sparks, and the current is carried away to the - terminal of the battery via the "grounded" engine block. Current flows the same direction in both wires.

??? Aluminum is an excellent conductor of electricity, very nearly as good as Cu and Ag. It was used, during a significant fraction of the early part of this century, in house wiring, at least in the US. The current does not flow from the #1 plug to the #4 plug because the engine block is on the negative (ground) side of both plugs, and serves as the drain to battery ground. Current flow in both plugs is ignition --> wire --> plug + terminal --> spark gap --> engine block --> battery - terminal, at least as far as I understand car ignition wiring.

Eric Lucas

I seem to recall that the current indeed flows the other way in one of the plugs firing at the same time. I will try to remember the reference. Also, the way I understand it, "current" practically speaking means electrons, which flow from negative to positive. This is likely to make a difference across the spark plug gap.

Leon

Nope. The electronic ignition model delivers a pulse to the coil pack. The output coils (there are two) are connected to a spark plug tip on each end. Current flows in opposite directions in each wire.

The discharge from the coil is not a single pulse. It is not direct current either. The coil discharge is a high voltage alternating current. It is oscillatory in nature. It does not have a polarity. The primary winding of the ignition transformer when activated by the 12 volt direct current from the battery causes a iron core to become magnetized. When the 12 volts to the core is turned off, the magnetic field in the core collapses. The collapsing field passes through the secondary winding of the transformer. Because the secondary winding has a high number of turns a high voltage is generated. The inductance of the coil resists the collapse of the high voltage and the polarity reverses. The results are a declining oscillatory AC wave.

A four cycle engine fires every other stroke of the piston. On the first stroke you have Intake and Compression. On the next stroke you have Power and Exhaust. A V8 with a single ignition coil fires the coil 4 times for each revolution of the crank. Mazda engineers decided that two coils would be used to fire the four cylinders. By using two coils, each coil has to fire only once per engine revolution. This gives a longer charging time for each coil and produces a higher voltage at high engine rpms where it is needed. By wiring the same coil to two cylinders that are 360 degrees out of phase, no switching is required. Both plugs fire at the same time but one cylinder is at the end of the compression cycle while the other is at the end of the exhaust cyle.

The coils on the Mazda coil packs do not like to be be run unloaded. When a spark plug wire goes bad, the reduced load on the coil results in a substantial increase in voltage. The insulation on the high voltage winding of the coil can break down and the coil starts shorting out. Plugs with excessive gap and failed wiring can result in damage to the coil pack. The car starts missing and the unburned fuel damages the O2 sensor. Failure of the O2 sensor can result in damage to the catalytic converter.

I found a reference in Norm Garrett's Performance Handbook p70. The number 2 and number 4 plug wires have the current running opposite from normal.

The way the process works, as well as I can make out, is that the PCM creates a magnetic field in the coil bar by allowing DC current to flow through the primary coil wire.

At the time the ignition is desired to start, the PCM cuts the current. That causes a very high voltage difference to occur between the ends of the secondary wire, having much more windings around the coil bar than the first. One end of this wire is attached to, eg, the number 4 plug wire and the other to the number 1. The electron stream runs then from plug wire 4 over plug 4 to its side electrode and hence to the engine, and from the engine over plug 1 back to plug wire 1.

As soon as the electron stream, which is of course DC, bridges the gaps between the electrodes, the voltage drops down to a fairly constant, lower level:

The figure shows that *after* the sparks terminate, the voltage across the spark plug is actually decaying AC. I do not know why; maybe when the spark plugs are no longer sparking, they act as capacitors, creating an LC circuit with the coil.

Leon

Two conductors with an air gap; textbook capacitor.

Excellent job Larry,

Thank you for the explanation. I almost had a cow when I found #2 and #3 switched on my '91.

I have only had it a short time but knowing what the firing order is suppose to be and seeing something else... Well I think that I can wash the mess out of my shorts.

For Gods sake someone please post this on Miata.net.