How do coils (Miata?) work?

This morning while sitting on the porcelain throne reading GRM I was thinking about the Model T. I think it's coils are bad. Am I right thinking that the Miata coils take a 12v "charging" voltage and a 5v trigger?  Wasted spark right?  The T has a basic distributor that has 4 leads. I think I could use them to trigger the coil and maybe a total loss ignition system with a 12 volt battery just for the coil feed. 

Generally 12v feed, ground controlled.  If there is a third wire, it's a monitor.

What are you going to read the timing with on the T?  Crank trigger and megasquirt?

In reply to Streetwiseguy :

The T has a distributor with 4 low voltage leads to trigger the coils. 

This thread is really confusing.

Ignition coils work by applying power to the primary winding, which makes a magnetic field.  When you cut the ground to the primary winding (points open, or transistor opens) the magnetic field collapses and induces voltage in the secondary winding, which has nowhere to go but through the spark plug.  Because the secondary winding has a lot more wires in it, the magnetic field multiplies the voltage massively, turning 12 volts into thousands.

What year are the coils from?  IIRC 90-93 have an external igniter, whereas 94+ are built-in.

In reply to Knurled. :

That helps. I think I can rig this up.

In reply to codrus. :

i just mentioned Miata because 4 cylinder and all the coils are on one bracket. Just so happens there's a 99 in my local you pull it. 

It might be helpful to mention that in terms of ignition coils an 'ignitor' is just a bunch of transistors wired in parallel. Basically a super fast acting version of a relay. Same thing that was called an 'ignition module' before it was called an 'ignitor' and before the term module implied it had some kind of logic or decision making power. Also called a coil driver. Just a transistor switching device. 

Generally if a coil has more than two terminals it is using a built in switching device (that may be triggered by a signal other than 12v, as mentioned) or if it's fairly recent it might even be using 'ionic monitoring' where you infer a bunch of things about combustion chamber conditions by looking at the waveform of the circuit. 

So you want to have miata coils triggered by the factory distributor? I think?

In reply to buzzboy :

Yes

The Ford distributor outputs voltage much too high use a trigger in any electronic circuit. If a second set of points can be added, they could be used.  

At rpm and power level of the T engine, the original distributor (in good working condition) should be just fine.

In reply to Vigo :

A lot of old school modules did have logic circuits however rudimentary.  Mostly in the interest of current limiting.

Model T ignition coils are seriously weird - here's a good article on how they work. The Model T used a distributorless ignition system with a separate points trigger for each coil and the coils designed to keep sparking as long as they receive a trigger signal. Using a Miata coil on a Model T would require some sort of circuit to control the dwell. Possibly a set of '90-'93 Miata coils triggered by Bosch 139 modules.

This might explain what I'm trying to do. What I've been calling the distributor is the commutator. I want to replace the coil box with the Miata coil pack.

Actually, I tried something similar on my neighbors 27 Model T.  Instead of Miata coils, I used  '96 and up Saturn coils.   I configured the modern ignition to act like the old buzz box ignition.   Long story short, it will work, however the Saturn coils put out enough energy to erode the Model T spark plugs fairly quickly.  

Here is the link to the thread  27 Model t..  The photos are hosted on photobucket and are blurry.  The Ignition stuff is on page 2.

In reply to Doc Brown :

Great build thread. I don't know why I've never found it before.