Toyota COP vs NB2 COP
 

To replace the coil packs on 1999 running Megasquirt 3, what are pros and cons of going with (4) Toyota COP's vs a pair of NB2 COP's?

I'm wondering if the added complication of (2) more drive wires is worth the benefit of sequential spark.

Yes, seq spark is a great thing, and since you have a way to control the toyota cops as well as run seq I'd definitely recommend going that route. More smoothness, efficiency, power potential, etc.

Will the VVT "cops" even fit a non-VVT engine?

You'd need an MSM valve cover, but otherwise they would fit fine.

Why is sequential spark great? From what I've read, it doesn't make much of a difference.

Its great because the car idles smoother, combustion gets a little more efficient, and you output more spark due to having much higher recovery time per coil. more precision, more control.

If you're going to the trouble of fitting non-stock coils, why not go ahead and use GOOD coils? It's the same effort to slap on GM LS 6.0 truck or Ign-1a coils, both MUCH better than Toyota's and Mazda's coils.

I would go ahead and run any of the coil per plug options sequentially. It's better for the coils, and it's not like you don't have lots of extra outputs.

You have three things to consider about coils: available voltage, current and energy (in joules). The voltage is required to strike the arc across the plug electrodes. Once the arc is struck, the voltage plummets to a much lower value, but before any current flows, it must be high. The current determines the amount of fuel/air mixture that is ionized in the arc per second and the joules is the total energy available, which determines the amount of time the arc lasts.

The other thing to consider is that the waste spark system used in the Miata fires two plugs in series whereas the Toyota COPS fires one plug each. This is not as bad as it seems since the arc striking voltage is proportional to cylinder pressure, so the plug that is firing at the end of the compression stroke is paired up with a plug on the exhaust stroke. Also, any plug takes about 40% more voltage to fire if the centre electrode is positive than if it is negative and this is the real concern.

Let's take the example of a plug that requires 20,000 volts to fire at negative polarity at a 10:1 compression ratio. A COPS system would strike the arc at a fixed 20,000 volts. With a waste spark system, if the plug on the power stroke was negative centre polarity, it would take 20,000 volts to fire the operating plug plus 2000 volts + 40% or 2800 volts to fire the waste spark, since the plugs in series are connected in the opposite polarity. This is a total of 22,800 volts. If the operating plug was positive polarity, it would take 28,000 volts to fire the operating plug and 2000 volts to fire the waste spark for a total of 30,000 volts. The voltage does not build up instantaneously, so the positive-centre plug fires its cylinder later, giving a slight retardation to the ignition timing of two cylinders.

Once you add boost, the waste spark system in the Miata is relatively anemic, which accounts for the thread on COPS in miataturbo.net being over 1500 posts long!

Plug wires degrade from the time they are manufactured and heat and flexing do not help the situation. Like all Miata parts, plug wires have a stratospheric cost, both OEM and aftermarket - some builders have installed a complete set of used Toyota COPS for less money than the stock Miata plug wires!

The 40% increase in voltage requirement for positive-centre was standard advice from the '50's and '60's when most cars had distributors and separate coils and there were identical terminals on the coils. These terminals were marked positive and negative because tests had been conducted and the polarity was found to matter with the positive centre electrode requiring 40% more voltage. At that time, only a few cars had waste spark systems, such as the French Citroen cars.

Electricity can move in two ways with either the negative electrons or positive ions moving. A metal electrode has a work function, which is the energy required to strip an electron off a metal electrode. This energy is reduced as the metal heats up. The centre electrode gets much hotter than the ground electrode, so it emits electrons easier. It is very difficult (and destructive) to have entire ions moving - this causes electrode pitting because the metal is removed rather than just used as a springboard for electrons. The electrode that emits electrons is the one that is at a negative polarity. This is similar to vacuum tubes that heat the cathode up to orange heat to provide electron emission.

Coil polarity information has been around for more than half a century. You can do a search on google books and read the entire articles of which two are summarized below. There are a lot more articles that say the same thing and it follows the physics - only the electrons can move easily (ion movement causes pitting of electrodes because the electrodes themselves are getting relocated).

Popular Mechanics - Apr 1959 - Page 175
books.google.ca
Vol. 111, No. 4 - 294 pages - Magazine - Full view
Checking Ignition Polarity Reversing the polarity of an ignition coil by accidentally switching the primary wires will not prevent an ... From 35 to 40 percent more voltage is required to fire the plugs when coil polarity is reversed.


Popular Science - Apr 1978 - Page 178
books.google.ca
Vol. 212, No. 4 - 220 pages - Magazine - Full view
For example, a coil polarity test— an important one— is not shown. To make this test you set the selector to the "32 volts" position. Connect the red test clip to an engine ground (red is correct in this case).


One thing that I have found to help the Miata ignition is to remove the insulator connected to the coils (starting in model year 2001) and stretch the spring that makes contact with the terminal in the coil. Otherwise, the spring may collapse and shrink over time.

When cars had distributors and a single coil (before the GM HEI types that mounted the coil in the distributor cap), you had a separate coil with two connections to the primary side of the circuit and it was possible to wire a coil in backwards because it was just a transformer with no internal transistor switching. It takes more voltage to get electrons to jump from the ground electrode to the centre electrode than from the centre electrode to the ground because of the lower temperature of the ground electrode.

The waste spark system guarantees that one plug will be right and one plug will be wrong for each coil as far as polarity is concerned.