MSD 6A firing two coils simultaneously.
 

Folks,
There was a thread I read here from a few years ago entitled 'MSD ignition exposed'. In that thread at least one member was firing a twin plug engine with a single MSD ignition box. I did a quick test today and I certainly do not recommend doing this. It works, but not very well, and that is provided the coils are in parallel with the MSD output and not in series. In series the impedance is doubled and the energy output from both coils is so low that it isn't worth considering. Here are the test results. By the way, some of you may be surprised at how low the voltage output is with MSD given the advertising. The unit I tested was a 2009 model MSD 6A. It is basically the same design that they used in the 1970s. The output is completely dependent on battery voltage as it does not use an oscillator type power supply to charge the discharge capacitor. Instead, the emitters of two power transistors in parallel, are connected directly to the battery by the thick red wire. When triggered 'on', the transistors attempt to charge the 1uF discharge capacitor in one half cycle through a massive transformer (at least that's how the circuit appears to operate) The spark duration is short too (about 35 to 40 uS) and much shorter than you would expect for a 1uF capacitor which should give a spark duration of at least 100uS with most CDis. The time between individual sparks is almost exactly 1mS, which I think is far too long after the first spark to be of any use (my opinion).
Results of quick test using two more or less matched coils.
1500rpm (6cyl)
-Maximum voltage at calibrated spark gap 28kV with single coil (4 sparks separated by 1mS).
-Maximum voltage attainable at calibrated spark gap 20kV with two coils in parallel. Still able to produce 4 sparks.
2000rpm (6cyl):
-Maximum voltage at calibrated spark gap 28kV with single coil (3 sparks separated by 1mS)
-Maximum voltage attainable at calibrated spark gap 20kV with two coils in parallel. Still able to produce 3 sparks.
4000rpm (6cyl):
Maximum voltage at calibrated spark gap 26kV with single coil (2 sparks separated by 1mS).
Maximum voltage attainable at calibrated spark gap 19kV with two coils in parallel. Still able to produce 2 sparks.
6000 rpm (6cyl):
Maximum voltage at calibrated spark gap 24kV with single coil. ( 1 spark)
Maximum voltage attainable at calibrated spark gap 16kV with two coils in parallel. Single spark.
The above testing was done at 12.5V applied. Charging voltage near 14V would make the values higher. What I didn't check for, but perhaps will do another time, is check how much shorter the spark duration is with two sparks plugs firing at once. Certainly the energy of each spark is considerably lower than if the MSD 6A were driving a single coil. One thing that did stand out on the scope was the massive voltage overshoot in comparison to the actual attainable gap breakdown voltage when two coils are used. So despite the lower energy, the spark plug wires and coil secondary windings are still taking a beating from voltage stress with the CD output split between two coils. Fred

Good info! Thanks.

I did a quick measure of the spark duration with two coils compared to one. These numbers are variable depending on the coils used and the plug gap. However, the duration with two coils in parallel with both gaps firing is approximately 1/3 the spark duration for a single coil in my testing. So, that makes the spark duration extremely short for the single polarity MSD output firing two coils in parallel. Only 10 to 15uS or possibly a little longer with high efficiency coils and a narrow plug gap. Fred

When I was building my 3.0L twin plug motor I already had one MSD running my single plug 2.7L. I talked to my engine builder Mike Bruns and also talked to the manufacture of the Mallory and the other MSD I was using, don't remember the name and can't see it on the car.

The consensus was there would be a slight difference but you would be leaving horsepower on the table. So I installed the new Mallory with the old one, one running the top and the other the bottom plugs. Then, from my aviation background, I installed an off/on switch to turn one off. This allowed me to check every once in a while to make sure they where both working.

This is great information, adding science to a technical forum is always a step in the right direction.

This also tends to be a pretty transparent place, in the interests of disclosure, is this the same Winterburn whose father developed an ignition system for porsches and others, and now his son has developed that and marketing it? In Canada?

http://www.capacitordischargeignition.com/

Yes, I build these in small quantities as a hobby and sell 3 or 4 per month. Most of my test equipment is recycled from the same old equipment my father used. Low budget R&D, but it works. Fred

Did you perform the same tests on your equipment, or have an alternative approach for firing 2 sets of plugs?

John, I ran the test on the spark test machine which consists of a variable speed motor driving a 1959 Ford V8 distributor with a calibrated variable spherical spark gap that my father built in 1961. It still uses the same motor and the same SCR controlled variable drive he designed for it back then. ( It was built in a real hurry and unlike some of his other homemade test equipment looks quite crude, but is an indicator of how fast he was moving with his ignition design). I cobbled up a second spark gap just to run the experiment that wasn't calibrated but close enough to get a feel for what would happen in real life. The spark test machine is actually better than a real engine for comparing ignition systems. With a calibrated gap (not a zener string which is a very poor substitute for a few reasons) one can remove one variable from the process and measure other parameters more accurately. Fred

http://forums.pelicanparts.com/uploa...1466031509.jpg

Very interesting! Cool to have pics of your father at work. I have a great old one of my Pepere working over a lathe...

I have only a rudimentary understanding of electronics...I see reference to the triggering method on your site, and if I understand you that triggering method has an impact on the final plug spark quality?

On this site, and especially in the Supercharged/Turbocharged and Engine rebuild sections as well as here, it is quite common for people to have removed points and replaced with pertronix or competitive optical triggering or flying magnet or missing tooth either in the distributor or on a crankwheel.

I will go ahead and ask about one specific example, mostly because it is one motor I am finishing up and had planned to use a couple of spare MSD boxes that worked in a previous installation...

Triggering will be handled by a missing-tooth wheel on the cranknose (EDIS or Clewett.) THe ignition curve will be managed by Megasquirt. The outbound triggering signal would typically be spliced out to the 2 MSD boxes, then in turn to 2 coils and then those to a 12-plug distributor (JB Racing in this case). The distributor has been locked down to remove any curve.

Components of this above are pretty common here, with people sometimes swapping in different 12-plug distributors, adapting later twin-dizzies, or using Motec/Adaptronic/SDS/other ecus.

Would you recommend your boxes in this instance, have you tested a similar setup and any suggestions or questions you might have?

John

John, The unit I make wouldn't be suitable for your application. It is meant to be triggered by points but can also be triggered by pertronix or other ignition systems that mimic points. It's meant for old technology points distributors, and for that it works really well. Fred

If you read the latest thread/battle of this on this forum it has turned into a slaughter house. Fred "good Job" as this is the kind of post we need to help people. Thank you for the contribution. I would like to move your findings in the start of this post but it in not my work or place. Also I do not want to hurt anyones feelings as we live in a day where even if you are wrong you get the biscuit too. Thanks, William Knight