I had the new motor in the 68 Barracuda up and running last November. I had basically carried over the MegaSquirt setting from the prior slant six motor. The new motor should be 100+ HP more, has 10.3 to 1 compression, a much racier cam, fully ported head, long rods, 2.2 L pistons, but I am using the same fuel system, injectors, rail and the same ignition components, MDS box and coil and a lean burn distributor. The car started and idled ok and I had driven it around the block but did not lean on it at all. Then it sat till a week ago.
Got plates and insurance in effect and took the car for a short drive. With the car setting still it would easily rev to 4500, however with the car pulling its own weight, it would accelerate to about 3000 RPM then mis fire like crazy. not backfiring out the exhaust or snorting through the intake, it was like it would just cut off then back on.
My first though was to adjust the AF and VE tables, that may have helped a little, backing the ignition timing down and closing the spark plug gaps down to .030 also helped a little. But it certainly was not fixed.
I did a lot of internet searching and found an incredible number of posts where folks had issues with MSD boxes mis firing above 3000 RPM. Fixes included swapping boxes and coils and working to eliminate EMI interference and current jumping spark plug wires. Also found several threads where folks also had MegaSquirt issues, again the fixes were eliminating EMI interference, getting the rotor phasing right, getting a stable timing signal, switching trigger polarity from high to low to low to high. I found one post where a person had mentioned that they resolved the miss above 3000 RPM by reducing the coil duty cycle from 50% to 25%. The Megasquirt manual recommends that when running a MSD box to set coil duty cycle to 'fixed' that is easy to do with the drop down menu. But they do not mention adjusting the coil duty cycle, which the default appears to be 50%. I changed the coil duty cycle from 50 to 25% with the engine running. With a MegaSquirt making that change was as easy as changing numbers in a text box on a computer screen. Immediately the idle smoothed out. I took the car for a short ride and the engine was absolutely singing. The mis fire above 3000 RPM is fixed.
I contacted the MSD tech line to get information on how reducing the coil duty cycle could resolve this problem. The Tech said it sound as if the coil was not getting enough time to fully charge, and by reducing the coil 'on' time the coil had more time to charge.
Passing this on as this simple fix got my motor running great.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

That doesn't make sense to me. Duty cycle is normally stated as "on" time with respect to "off" time. If you reduce the duty cycle, you reduce on time. If you reduce "on" time, the coil will have less time to charge, not more.

I'm wondering if it didn't have something to do with the multi spark. A lot of people are not aware that the (at least the anolog) boxes only have multi spark up to 3,000 rpm. After that it is just single spark. It would be interesting to see the spark pattern with a scope, and see what the difference is with the change in duty cycle.

_________________
Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Charlie,
I really don't know why reducing the Duty Cycle in the MS3Pro with a MSD box had such a positive effect, but it did for me and for at least one other person, as it was not an original idea by me but I did see it mentioned in one post out of the hundreds of posts that I looked at. I spoke with Dennis S on this he also mentioned that it is above 3000 RPM that a MSD box shifts to a single spark.

I like to know the answer why it fixed it, I have posted the same question at the MegaSquirt forum but have not received a response. Hopefully Matt C (MadScientistMatt) will weigh in. I know there are others here at .org that run MS ECU's but I don't know if they run MSD boxes.

Also did some looking on the internet found this article by Fluke

https://www.fluke.com/en-us/learn/blog/ ... er%2040%25.

They describe Duty Cycle as 'on' time, with an ignition coil that can be interpreted as time allowed for the coil to spark, with the remaining time being time for the coil to charge.

and this article talks to Dwell and the time it takes for the magnetic charge to form. Indicating that if the charge time is too short the magnetic field will be weak and the resultant spark will be weak.

https://www.motortrend.com/how-to/ignit ... explained/

and another bit more technical Coil article

http://dtec.net.au/Ignition%20Coil%20Dw ... ration.htm


Again, I don't have the answer and would like to know why reducing Duty Cycle had such a positive effect.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

FYI on the single spark:

It's 3000 rpm for an 8 cylinder engine.

It would be 4000 rpm for a 6 cylinder, 6000 for a 4 cylinder.............


I think they also state that the multi-spark occurs over so many degrees of crank rotation.

Again, that's for an 8 cylinder engine. It would be greater for a 6 and 4 cylinder engines.

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I know that the engine performance really went south above 3000 RPM,, I did not realize until I had reduced the coil duty cycle how bad the engine was running even below 3000 RPM. I made the change in the coil duty cycle from 50% to 25% with the engine idling. The change in the idle smoothness was immediate and significant.
The AFR meter stabilized, it was a big-big change.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

Is duty cycle the time the coil is charging or the time the coil is sparking?

Then with a MSD, duty cycle would be before the MSD box. A capacitive discharge spark is completely different than an inductive discharge where duty cycle will affect the quality of the spark.


Sounds like the MS3 confuses the MSD box with the duty cycle.

_________________
Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I would think that the MSD Box Just needs a High / Low square wave type signal to begin the sparking / charging / coil saturation process.

I bet if you set the duty cycle low in tuinerstudio on the MS3 PRO to like 1 or 5 % it would still work possibly..

It would be interesting to experiment with, since the Ms3 Pro is not actually sending any signals that control or saturate coils.. just a trigger for the MSD in this instance.

...At least that's how i perceive this interfacing of the 2 boxes in my mind.


Greg

_________________
http://www.youtube.com/hyperpack

Section 5.3 Ignition Settings of the MS3 Pro Manual:
Dwell Type:
1) Standard Dwell: used for most installs, i.e. regular coils that are fired by a module, or internal drivers or logic driven COPS
2) Fixed Duty: locks the spark output to a constant duty cycle instead of controlling dwell. Used for modules that control dwell directly; Ford TFI modules, Bosch smart ignition modules, MSD or other capacitive discharge boxes
3) Time After Spark: for Saab Tri-Ionic DI
4) Charge at Trigger: highly specialized mode intended for driving a CDI thyristor.

Dwell Duty: for Fixed Duty, this specifies the percentage duty cycle of the output signal.

I have selected "Fixed Duty" that brings up a text box where Dwell Duty values can be entered. From the text description of the Dwell Duty, it appears that the duration of the output signal from the MS ECU to the MSD box can be controlled via the Dwell Duty setting. I know that my engine had a misfire issue with the Dwell Duty cycle at 50%. That mis fire issue was eliminated when the Dwell Duty cycle was set to 25%.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

received this response from Matt Cramer via the MS support forum where I asked what is the function of the Duty Cycle selection.

"This is the duty cycle of the square wave that is triggering the MSD box. The MSD box should spark on the rising edge of the wave (note - this isn't necessarily the rising edge setting in TunerStudio!) but normally an MSD box would not be affected by the location of the falling edge. MSD boxes do not have dwell as such; they dump all the energy in the capacitor through the coil instead. So it is a bit odd that the duty cycle is having an effect here."

He asked if the timing at the damper is changing when the Duty Cycle is changed. My non dial back timing light recently died, I have a new one on order, will check that when it arrives.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

Checked the ignition timing at the damper and the ignition timing did change (advanced) when the coil duty cycle dwell was increased from 25 to 50. That was with the MS3 Pro Spark Output set to Going High. Changed the Spark Output to Going Low and changes in the coil duty cycle did not affect timing at the damper.
My interpretation of what was happening is with the Spark Output on Going High the spark was being triggered off the leg of the square wave that changed position when the length of the duty cycle changed. Moving the Spark Output to Going Low triggered the spark off the leg of the square wave that did not change position as the duty cycle changed.

Thanks to Matt C for help in resolving this. And Greg O whose info was close to being on track.

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8

Just my random experience, the way the wires are hooked up to a stock electronic distributor changes whether the MSD is reading the leading or trailing edge of the reluctor. It will change the timing by 30° or so.

_________________
Official Cookie and Mater Tormentor.

I do electronic engineering for a living. I have spent a bit of time working with an ignition timing controller for go-karts and other single-cylinder small engines. I spoke with a fellow engineer who does work with MSD and Holley about controlling ignition coils. When he told me that MSD & Holley vary the activation time by calculating "Duty Cycle", I was just blown away!! How archaic!!! That's how points-&-condenser ignition systems work!

Ignition coils are inductors at their core. Electronics text books provide the RL time constant formulas to determine when an inductor reaches saturation. Extending activation beyond saturation simply works to drain the battery (at best), and/or reduce coil recovery time at worst (which is what I believe you were experiencing). I've been working with a cheap $23 Jegs coil that operates flawlessly at 2.1 milliseconds of activation time. My algorithms calculate when I want the spark to fire, then I subtract the 2.1 ms charge time from that. Software triggers the coil based on that math. How long the coil is activated should not vary with engine speed, multi-spark, or any other parameters. Using "Duty Cycle" to trigger a coil -- in my opinion -- is sophomoric engineering.

If you can select a predetermined amount of time to charge the coil, take that route. That amount of time will be the same regardless of RPM (or multi-spark). This is working with the electronic principles of the Laws of Inductance governing the ignition coil.

_________________
Recognized by US Federal Courts as a Fuel Economy Expert!

That makes a lot of sense, Mike. I'll file that away for when I fire up my MS3X w/ign control (hopefully soon)... Thanks also for your experience, John.

Lou

_________________
Home of Slant6-powered fun machines since 1988

The help menu in the Ignition setting page of the MS 3 Pro recommends that with MSD ignition boxes that 'Fixed Duty' be selected for Dwell Type. Standard Dwell and Time After Spark and Charge At Trigger are the other options.

When I enquired at the Mega Squirt Msextra forum as to what the Dwell Type selection did with an MSD box, copied below is the response:
I take that to mean that the MSD box does not charge the ignition coil for a 'length of time' as is normally associated with the term coil dwell as you were suggesting. The Duty Cycle mentioned is actually the wave that is triggering the MSD box to fire the coil. My issue was the MSD box was triggering off the back leg of the wave. This is what Greg O was mentioning in post dated 8-2-21 when he suggested shortening the wave

In my case swapping the spark out put so the MSD was triggering off the leading leg of the wave resolved the ignition timing being affected by the length of the wave

_________________
Doo Ron Ron and the Duke of Earl are friends of mine.....
https://www.youtube.com/watch?v=uX8Nj8ABEI8