Anyone else here using an MSD ignition?

Good day Adrian,

But does not the MSD go in front of the ignition coil (i.e. drives the ignition coils primary windings)? I am pretty sure mine does on my BB and so would it not be true that the plugs are actually driven by the secondary Ignition coil and thus Inductive. I always thought that MSD uses their capacitive nature to create a much higher energy/voltage pulse (and multiple ones at that) on the coil's primary windings? Please correct me if I am wrong, as it has been a while since I last examined it.

Cheers,

Sam

 

Hello Sam,

the MSD or Dinoplex in your BB is indeed in front of the ignition coil, but it deploys the coil just as a step up transformer, sending around 400-500V into the primary terminals at each ignition trigger. The secondary output voltage roughly depends on the input voltage multiplied by the wiring ratio. Your BB's Dinoplex supplies around 40KV to the spark plugs.

Capacitive ignitions are much faster in creating a spark than inductive ignitions as charging a capacitor takes less than 1/10 of the time creating a magnetic field in a coil (even with modern high inductance low resistance coils) but unfortunately the spark duration is also much shorter. That was the primary reason to introduce the multiple spark feature. A high power inductive ignition can easily create sparks with a 1-2 ms burn time, whereas capacitive ignitions have a spark duration measured in microseconds.

Best,
Adrian

 

Hi Sam and thanks for joining Adrian.

You guys are taking me back a ways - AND- forcing me to think


" (1/2 x primary coil Inductance x current squared or 1/2Li^2) " Ok ------what subject / section & chapter would I have seen this?

Your 6kV / mm sounds right, as I recall 50kV / cm. (never seen this proven and don't think it's used much no matter how important we feel it is ) I also recall a biq qualifier such as conditions. Humidity plays a larger role than ionizing N2 & O2 - not to mention gasoline which has a higher dielectric constant (Air = 1; Gasoline = 2; Water = 80) - thanks internet. So a gas soaked plug is twice as hard to fire -or- takes twice the voltage (sound right?).

Also, I'm a fan of not going huge with the gap, and I get that sense from you as well in your second paragraph - Do I read correctly? Sam, what gap are you running with your MSD equipped car? I'm running .025 - .032", and I'm sticking w .020 for non MSD cars.

I'm not falling asleep. I'm always looking for the part where the theory help us get our cars running best. So far it's served me well.

Warmest regards,
Bruce

 

Good day Adrian,

I think I did not make my point very well. Regardless of what is driving the primary side of the ignition coil it is the secondary part of the coil that is driving the spark plug. Because of this it is the magnetic field (which is created passively like breaker points or actively from MSD, etc) that is actually storing the energy necessary for driving the spark... Correct? Effectively what I am saying that regardless of where the energy is coming from (Dinoplex, MSD, breaker point opening/closing) it is the magnetic field within ignition coil that is the source for the plug energy... hence why I still consider the system inductive. Perhaps my view is incorrect in the context of these ignition systems, as I am looking at the end result... and not what has actually created the initial energy source?

Cheers,

Sam

 

Good day Bruce,

This formula is for the energy storage (magnetic field) within an Inductor given a current though it.

I will have to check what gap I am using... I suspect that given that I am/was using Iridium plugs I may have left them with the stock gap size. Since the car ran very well I did not have to play with the ignition system too much... other than replacing the wires, extenders, etc.

I am glad that our discussion is keeping you awake ... is that a good thing or a bad thing ?

Cheers,

Sam

 

This is a good spark plug wire article to add to the ignition system information.

Getting Wired: Spark Plug Wire 101 - Dragzine

DRG
Jerry

 

Hello Sam,

i think you have to look at the two types from a system perspective. The fundamental difference in a capacitive discharge and inductive discharge ignition is where the energy is stored and released, and this directly influences the end result/spark output characteristics. In an inductive discharge setup, a high inductance coil stores the energy in a magnetic field, charging and releasing the energy is relatively slow (~0.5-1ms until saturation) and depends on the coils primary resistance/inductance and the materials used for the core. In a capacitive system, a low inductance coil just transforms the energy supplied by a capacitor, here the energy release is extremely fast as the coil only acts as a pulse transformer.

Best,
Adrian

 

Good day Adrian,

I fully understand, but I am/was looking at only one aspect in that the plug is driven by the secondary ignition coil which is from the Magnetic field within the ignition coil. Indeed, the magnetic field is initially generated by whatever (breaker point open/closing, MSD Capacitive, etc)... but it is the coil's magnetic field that actually does the transfer of energy to the secondary system (i.e plug). As soon as we have a transformer in the system the final energy transfer will be via the energy contained within the magnetic field. Now, I am not saying that the output energy discharge will be all the same... In fact that was the basis of the original discussion in terms of what does the actual energy discharge look like via MSD, etc and why would it be better at the physical level... ... at least that is what I originally though we were chatting about.

Cheers,

Sam

 

Hello Sam,

yes, exactly! I had the impression that the discussion was starting to focus on inductive discharge ignitions as it was recommended to watch out for the correct coil resistance, so wanted to point that out. I am not a native speaker so my descriptions were probably a bit incoherent

Best,
Adrian

 

Good day Adrian,

Whew... I was beginning to question my own noggin ... You see since I have been ramping up and studying more and more on EMI and transmission line theory I thought I was starting to scramble my own neural electrons

Cheers,

Sam

 

Wish I spoke German as well as you speak English

And Sam, it's not the staying awake part, it's the thinking part that's tough Thanks for pointing me to that equation. I will look it up, an try and get back on top of physics. I have better recall for electronics (at the moment anyway).

So, someone was speaking to the response time of the coil being energized by a discharging cap as faster. I would agree, and also think there's limitations (time constant) to getting the cap charged, and this is a reason why the MSD stops being a MSD above some RPM.

I was concerned about the circuit current available to drive my 7-AL, and for 12 cylinders, the current required jumps up sooner in the rev range, and we lose MSD sooner

By the way - Do you think we helped the thread originator ?? His concern about the MSD being harder on ignition components ??

 

The capacitor charge time is quite short, around several µs. The turn-off time of the thyristor is longer than that, but even at very high RPMs this is not the limiting factor. The multiple spark feature is simply a work around at low RPMs to make up for the very short spark duration time typical for a CDI ignition. At very high RPMs the CDI spark duration eventually is long enough to cover the burn cycle time so an extension using multiple sparks is not required anymore, and there is simply not that much time left for another spark.

I am not sure our discussion helped the thread originator The MSD 6A series is not harder on ignition components if they are clean and free of defects from my experience. An exception is fine tip spark plugs, which help considerably in transistor/points ignition setups to lower the voltage required for the spark to take place but don't do much for capacitive ignitions except that the tip wear seems to be substantially higher with CDI setups.

Personally i would refrain from installing a MSD 7 setup in a standard compression (e.g. below 12:1) engine setup, the brutish 7 series puts out much more power than the 6A series, as required by high compression racing engines. An MSD 6A is quite sufficient even for a 12 cyl setup, it has about the same output as the F1 proven Dinoplex AEC104

Best,
Adrian

 

I install quite a few MSD units on Ferrari's and other vehicles. You, more than likely, either need to do one of two things. Either use a MSD 5a (less punch) or drill out (vent) your distributor cap. Good luck.

 

p.s. this sounds like you are actually enjoing EMI theory? I did not get very far with the theory when doing new designs so i'll try to apply what i've learned in previous designs and then have an EMI specialist do the reviews as well as the later EMI testing.

 

Good day Adrian,

In actuality I do not mind it. I have an undying thirst for knowledge and the overall understanding of the physical universe. Sadly, one has but a single lifetime to try and absorb it all. What is very humbling to me is that I am reading, re-reading, etc theories/principles that were developed almost 150 years ago... I, at least, have the benefit of books, Internet, easy access to schools, technology, etc where as the scientific originators had little more than the very basics...Like I said... a very humbling experience for me.

As for my need to refresh and understand this particular area... I find that by having a firm grasp of these fundamentals that this substantially minimizes the time/cost of having to sort out these issues later (in both prototype and production). Further, implementing any of the newer technologies makes it absolutely critical to knowing all of this in order just to have a working prototype. Interestingly, and sad as well, is that the more I learn the more I discover what I do not know ... Anyway, I digress and no doubt I have bored other readers of this thread.

Cheers,

Sam