I’ve been trying to write up a post going into excruciating detail on the concepts and theory of electronically controlled induction ignition and how it can be applied to our ignition system when I decided it would be much better as multiple posts.

I’ve simplified the following quite a bit because nobody wants to read the dry, technical, rambling wall of text that would turn into. Not even me. If you have questions or want more details, post below and I’ll do my best to answer.

Without further adieu, three lessons learned from building a DLIDFIS setup with HEI modules:

1. A V8 fires 4 times per revolution and a 2 rotor only fires 2 times. This means any V8 ignition parts rated for X rpm are good for twice that on a rotary.
2. There are 3 basic kinds of 4-pin GM HEI ignition modules, and they don’t play nice with each other in a DLIDFIS setup
3. 7-pin and 8-pin GM HEI ignition modules have a built in timing curve and probably shouldn’t be used for DLIDFIS because of this.

Details:If you search for information on GM 4 pin HEI modules long enough, you’ll find claims that their power drops off above 4000 rpm. They’re right, the stock coil doesn’t have enough time to fully charge above 200 Hz, which is 5ms between sparks, which is 3000 rpm on a V8. Lucky for us, with only 2 sparks per revolution, 200 Hz is 6000 rpm. That means if we were to retrofit the entirety of a GM 4 pin ignition setup onto a rotary, we would not see a drop in spark energy before 6000 rpm. See page 6 of the datasheet for the 4 pin GM module's control chip for a graph showing expected coil on / charging time with its stock 8.0mH coil.I originally built my DLIDFIS setup with one OEM and one Aftermarket(MC3334) module and it ran horribly until I swapped the aftermarket one for another OEM module. This site gives a fairly thorough background on these modules and their differences. All of them work in the same general way to control dwell. I plan to write a much more thorough post going into the way GM modules alter dwell compared to the way J109 modules do and how those differences may be causing issues, but the relevant details are:

• The distributor pickup produces a roughly sinusoidal signal to the ignition module when it spins, one cycle per spark.
• The ignition module uses this signal to switch on and off the coil.
• The module has a set voltage where it turns the coil on and a set voltage where it turns the coil off
• In order to alter how early in the cycle the GM modules turn the coil on, they provide a bias voltage from the ‘W’ terminal that positively offsets the entire waveform it gets back.
• This bias voltage changes based on rpm and if the coil is hitting current-limiting.

See the 3 modules' details below:

OEM: 0.77V Bias Volatge @ 0rpm, 1.30V Coil On, 0.86V Coil Off

Aftermarket (MC3334): 1.25V Bias Voltage @ 0rpm, 1.65V Coil On, 1.43V Coil Off

Aftermarket (Other): 0.00V Bias Voltage @ 0rpm, Coil On and Coil Off unknown

So if you mix and match your modules, you’ll have them trying to provide different voltages to the distributor pickup, turning on at different times and turning off at different times. This likely ends up causing both modules to misbehave badly.This is pretty self explanatory. From what I can gather, 7-pin and 8-pin modules are essentially identical except for the number and layout of connectors. These modules have a “Limp home” or bypass mode for when they are not receiving a computer signal. This bypass mode has a built in electronic timing curve to allow the vehicle to remain driveable without a computer signal. This patent, which I believe is for the OEM GM 7 pin modules details this timing curve. I’ve also confirmed the presence of a built in timing curve on my truck which is equipped with an 8-pin module by putting it in bypass mode and using a timing light while changing the RPM.

It is very unlikely that this timing curve will coincide with the timing you want for your rotary, especially since the curve was most likely set up for a V8. That is why I would recommend against using these modules.

 

Interesting post, looking forward to the HEI vs J109 thoughts.

Edit: OK, so apparently I enjoy your writeups, I looked in my bookmarks and found your post from 2018, which, incidently earned you a couple of "likes" :P.

https://www.rx7club.com/1st-generati.../#post12321986

 

yes very interesting!

the next car in the pipe line for me is a low budget beater with a HUGE bridge port, and its going to want more spark. i was in the early stages of thinking about that, so this is timely!

 

I have to mod some distributors next in the pipeline. I think we'll throw a couple of GM HEIs at one of them. Wouldn't be the first time.

 

Wow so believe it or not, I completely forgot that I posted that. I think in the next few days I'll add explanations to the recommendations I made in that post, add a couple more, and post it either in here or in its own post.

 

Very good details and all true for sure. I kind of understood this on my own way back when I was reading about all of these things while doing my own DFIS write up. Point #3 I always suspected but never tried, now I won't waste my time.

Don't forget the FB dizzies are the best timing source for HEI sighals. The 80 dizzies wave form doesn't work well at all with the OEM modules.

Throwing this out there as well, I think the solid state coils are much better than the canister ones for a number of reasons.

 

Very interesting stuff there. I recall you posting about this a while back; nice to see it all wrapped up in one place.

This is quite timely for me as well, as currently I'm trying to sort out why my DLIDFIS setup is misbehaving.

 

What does DLIDFIS mean?

 

Dual Leading Ignitor Direct Fire Ignition System.


It is funny how sparsely documented that is. It took me forever to figure it out.

 

Thanks.

So this is kind of like installing a direct fire FC coil, without all the hassle and mystery?

 

@Joekaistoe

I measured my HEI modules for consistency per your method (disconnected the dizzy signal wires (W and G) and energized the ignition but did not start the car). I measured W vs ground for each of my two leading modules and got 1.300V and 1.299V.

Pretty consistent! I am thinking they must be the "1.25V" type. And in your message from 2018 you state:

Since that was 2018, have you tested that tweak at all? And if you found it beneficial, any thoughts on how/what circuitry required to provide a regulated 0.5V positive bias to the W pin ?

 

Too many of these conversions done over many years for this to be a “real” problem.

Would put my own # at over 18 that are still running to this day(several10+year’s)using same components DFI was put together with.
Only increased spark plug wear,using Iridium plugs resolves that issue.

One piece of advice,stay away from cheap parts store ignition modules.

 

Now I'm curious what modules you and others use. I picked up my two from AutoZone for about 15 bucks a pop. Didn't realize some were notably inferior.

 

Mine are OEM from the picknpull via my socks! LOL