I've been poking around with the ignition electronics on and off and there seems to be some interest in getting into some of the details of how things work. So here's a place to start.

Here's a pic of the ignition module signals. The input signal from the VR distributor is in blue, and the output signal that drives the coil is in yellow. Note that when the yellow output signal is low, the coil is charging (dwell), and when that output signal goes high, the coil would fire a spark.

Spinning the distributor (by hand) the signals look like this:

First think to note is that the ignition module (1977 style) fires the coil on the negative-to-positive (N-P) transition of the VR input signal.

Also noteworthy is that the triggering N-P transition VR signal is a steep, almost vertical, slope. This is important because the steeper the slope here, the more consistent the timing will be with less ambiguity and spark scatter.

I'd be really surprised to find there was a rev limiter in the HEI module set for 4500 RPM.

The Datsun modules I've looked at in depth (77 and 78) have no built in limit and eventually crap out simply because they can't react fast enough. But that point is way above normal operating frequency. Way above.

I don't have any of the aftermarket stuff here for analysis, but I'm hoping to get my hands on a couple of the GM modules to poke around with.

It's not a "rev limiter" it's a deficiency in the quality of the spark, according to Vizard.  I remembered that I had found and copied some pages from an old article by Vizard and managed to find them again.  His whole issue was that the stock HEI system's spark quality, aka voltage, dropped off too soon (for his desires).  As I recall now, the reason it bothered me was that he was bashing the system based on race engine needs.  Typical bench racer stuff.  (p.s. actually, in looking at what he wrote, I realize that what's bothered me over the years is how this limitation shown by Vizard has been spread around the gear head world as gospel, even though it's basically nonsense for a typical car hobbyist.)

In looking at the pages I even see that his "limit" was actually 5,500 RPM.  His recommendations are for ways to get the RPM spark quality up to 7, 8, or 9,000 RPM.  

Here are a few excerpts from those pages.

Edited September 26, 20222 yr by Zed Head

Another example of a writer/"racer" focusing on writing, not racing.  The dyno results are what people really want to see.

 

The "energy starts to fall off" mention is kinda interesting. At low RPM there is plenty of time for dwelll and plenty of time for spark. At high RPM however this is not so true. But the issue is it doesn't matter which ignition system you are running, you are still bound by the same cycle times.

By that, I mean... On a six cylinder engine spinning at 6000 RPM, there are 3.33 ms between firing events. In that 3.3 ms you have to charge the coil and then spark that coil. If you want enough spark time, you have to be able to charge the coil fast. And that's where a no-ballast system that uses a current limit in the module would have an advantage. You can charge it faster so you have enough time for adequate spark time.

I bet all the systems "fall off at higher RPM's". Unless they are using the same dwell across the entire RPM range, then they would all fall off as dwell decreased at higher RPMS. I haven't measured any of the aftermarket stuff with built in dwell control to see if they use the same dwell time at idle and 6000 RPM. I suspect not.

I had all those measurements done for the 77 and 78 modules, but I was just using a signal generator as a simulated input. But after seeing how the modules can react different using the real VR pickup, I'm rethinking the validity of those measurements. I'm not sure using a sig-gen is valid.

I guess the focus here is the std distributor/single coil setup? multiple coils of course allows longer dwell per coil and stronger sparks. 

 

16 hours ago, jonbill said:

I guess the focus here is the std distributor/single coil setup? multiple coils of course allows longer dwell per coil and stronger sparks. 

 

It's a good starting point since that's what most people here are running, but it would be interesting to learn more about ignition for the coil on plug systems.

Just from talking with @Captain Obvious on the phone and looking at some videos he suggested, I was able to detect a potential issue with my 240Z by using my oscilloscope. The scope indicated there could be some extra arcing (having to jump more than the gap between the rotor & cap and spark gap) at the firing line of the spark at different cylinders. When I looked at the cap, wires, and coil, I found significant corrosion at the coil. 

Before changing the coil and wires (yellow wire is the voltage at the negative post of the coil):

After the coil and wires were replaced (again, yellow trace):

Next up is for me to get a current probe for my scope so I can chart the current limiting effects of some of the ignition systems.

Edited September 28, 20222 yr by SteveJ

It would be interesting to see how a broken magnet vs an intact magnet affects the performance of the induction pulse. Does your 83 distributor have an unbroken magnet?

5 hours ago, jonbill said:

I guess the focus here is the std distributor/single coil setup? multiple coils of course allows longer dwell per coil and stronger sparks. 

For me, it is. Everything I have is stock parts. Spectrum of different years, but all stock. I'd be happy to look into other systems, but I don't have anything here.

It all started for me wondering what they did in 78 to allow the removal of the ballast resistor. And the answer is they started including current limiting inside the module in 78.

Then av8ferg wanted to be able to mount two different style modules (ZX matchbox and GM HEI) on his car and be able to swap between the two if one of them failed. But when he tried it, he noticed a timing difference between the two. So I thought it might be interesting to investigate that.

That's how I got here. I've been poking around inside the stock modules for some time now but figured nobody (other than me) really cared, so I didn't post about it. 

1 hour ago, kickstand80 said:

It would be interesting to see how a broken magnet vs an intact magnet affects the performance of the induction pulse. Does your 83 distributor have an unbroken magnet?

I think the ZX distributor I've been messing with has an unbroken magnet. I think I have one here with a magnet in pieces too, but it may be in too many pieces.

I'll look to see what I have.

I came across another variable reluctor system.  AMC.  Very similar to Nissan's in concept but not so bulky.  Also switched to a fancier module as it progressed.

https://www.rockauto.com/en/catalog/american+motors,1976,hornet,4.2l+258cid+l6,1004500,ignition,distributor,7108

https://www.rockauto.com/en/catalog/american+motors,1976,hornet,4.2l+258cid+l6,1004500,ignition,ignition+control+module+(icm),7172

 

https://www.rockauto.com/en/catalog/american+motors,1978,gremlin,4.2l+258cid+l6,1003936,ignition,distributor,7108

https://www.rockauto.com/en/catalog/american+motors,1978,gremlin,4.2l+258cid+l6,1003936,ignition,ignition+control+module+(icm),7172

Here's a sketch I whipped up showing a simplified input section of the 77-78 ignition module*. The topic and concept of "polarity" keeps coming up, and I'm going to explore that some more:

So take a look at the input pickup coil connections... The red and green. If you know what you're looking at, it's clear that the red is the "controlling" connection while the green connection is pretty much tied to ground. If you connect up a voltage source between the red input and ground and wiggle that voltage around, the output of the module will switch. However, if you connect up the same voltage source between the green wire and ground, the output will never change regardless of what voltage you apply to the green input connection.

In fact, if you connect up a voltage source between the green input connection and ground, you will forward bias one of the diodes and pass a bunch of current through the 1K resistor. The green input is tied to ground through diodes and will pass significant current if you connect up a source to it.

The red input does the switching and the green input is the "ground" side.

And I know from looking at the rest of the module that when the red input is negative, the output charges the coil. And when the red input goes positive, the module releases the coil and causes a spark. So for the early modules, the spark occurs at pretty much the zero crossing on the rising edge of the switching input. On this pic, the spark would occur on the steep rising edge when the red wire goes from negative to positive:

*I assume that 74-76 is pretty much the same as 77 and 78

4 hours ago, Captain Obvious said:

when the red input is negative, the output charges the coil.

I wonder if Nissan was changing the shape of the curve to get more charge time with the ZX reluctor setup.  Also still wondering about the ZX turbo ignitor swap with the GM HEI module.  The 5 volt reference never goes negative.

Anyway, neat stuff.  Different from the usual.

Here's your other post with the ZX wave form.  Post #261.