I hope this is self deprecating humor. Run what you have and be happy to have it.
Cool upgrades are cool no matter how the "internet experts" criticize them.

IMHO, Product /design criticisms should be used to evaluate future purchases not to slam what others own.

Well, I just read through this whole thread and thought I'd weigh in and offer a physics based answer on the original 'Single box for twin plug' question. I've been an electronic engineer for 30+ years and also designed our CDI+ unit.

Will it work? Short answer, depends if the CDI unit used generates enough energy for both plugs.

In the case of using the MSD boxes, you are 'getting away with it' for a couple of reasons:

1) these units do store a lot of energy. The charge voltage is around 530V, vs the Bosch unit at around 350V (RPM dependent). Voltage is the dominant number in the energy equation.

I have to make an assumption here but it is not that wild. Let's say the MSD box charge capacitor is the same value as the Bosch, which is 1.5uF. So we can calculate the energy stored in the capacitor:

Bosch: = 91 mJ
MSD: = 210 mJ

(Energy = 1/2 * C * V * V )

2) Running two coils in parallel, the current will be halved in each coil so the energy is halved (E = IxVxt). So, each coil is 'seeing' 105 mJ of energy. E.g still a little bit more than a single Bosch unit delivers to a single plug.

So, you would struggle to run two coils from a Bosch unit, but you get away with it with MSD.

Personally, I think the primary voltage used by MSD is absurdly high which is why they stipulate changing leads and using their coil. There is not that much to be gained by going much over 400V but there is lots of potential for cross-firing between leads, tons of RF noise and rotors burning out.

The old MSD 6420 has less voltage and millijoules (480v and 115mj) than the MSD 6425 (530v and 135mJ). This is from MSDs own data sheets.

So, with 2 coils, you are getting only 58mj per plug with the analog MSD units and 68mj per plug with the digital units....and half the spark duration too.

Scott

Interesting. From that data, working the maths backwards, the MSD charge capacitor in both cases is 1uF. So, the later box simply runs a higher voltage. So my maths was slightly out when I made my assumption in my previous post.

The Bosch box voltage drops off with RPM - can be as low as 275V at high RPM. So that would be only 56 mJ. So, still theoretically possible with either MSD box.

The maths proves that with one box you are getting half the energy (as you rightly said). The question is whether that is enough and based on a comparison with a Bosch unit it does appear to be.

It is true that not having enough energy will degrade performance, but at some point having more energy does not make any measurable difference to performance either. Have proven this on a dyno since MSD makes no more power than our CDI+ unit despite more voltage.

How do you conclude half the spark duration?

As this thread seems to be getting more helpful to those of interest and less of a drama show. I will give a little more helpful information for the Pelican readers.
When you split your energy between two coils, one helpful way to keep your spark stronger is by reducing the resistance of the spark plug. Silver is your best conductor therefore if you have low energy you should choose the plug with the least resistance and today you can get silver plugs again. Copper is a close second in plug choices for low resistance.

See here:

http://forums.pelicanparts.com/9158818-post1.html

Although we haven't found a need for silver or cooper plugs, that seems like a reasonable suggestion.
15+ years of using this technique taught us to use solid core wires contrary to the recommendations of MSD. That is where the analog 6ALN comes into play. The digital 6AL reacts poorly to the solid core wires.

Following the MSD wiring installation instructions to the "T" is also important to proper ignition function and longevity.

We know that beyond a certain point "extra" spark energy does not produce more horse power, JonnyH has eluded to possible detriment to excess energy. What are the possible negative effects of "too much" spark energy?

It says he DIDN’T test two coils spark duration.

“ 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. ”

Oops....wrong post:

http://forums.pelicanparts.com/9159766-post3.html

"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."

Ok, but that is just a function of the amount of energy stored in the coil. I’ll stick to physics, if you have a CDI unit with twice the energy of the Bosch units, it should ‘charge’ two parallel coils as well as two Bosch units and the resultant spark should be the same.

It is worth noting that the 8 Pin Bosch units (fitted to the race cars) have a regulated 300V supply. This means there is no drop in spark energy with RPM like on 3 and 6 Pin boxes. This suggests that Porsche considered 67mJ to be enough energy for their racing cars. But don’t underestimate the Bosch black coil in this. They have a unique spec, not found in any other coil on the market.

Porsche did not use twin plug setups that would drop spark energy at high RPM. Based on that, and using the same coil Porsche used in their twin plug setups, what was the spark energy sent to each plug?

Yes they did. Both 3 and 6 Pin boxes drop power as the RPM increases. So at 6000 RPM you would be down around 50 - 60mJ per coil.

The 8 Pin boxes are 67.5mJ constant due to regulated supply voltage of 300V.

So if a single CDI box can produce 135mJ, then it would be comparable with two 8 pin Bosch boxes in terms of energy.

Sorry, I can't be much clearer than that.

NASCAR uses a power supply that boosts 12v to 18v for their MSD.
Do you have any thoughts about what that does to energy delivery.

Porsche only put twin plug CDI setup on race cars....you said race cars used the 8-pin boxes.

Early ST, RSR etc used 3 Pin boxes. Not sure what year the 8 Pin box came in but was on 956, 962, 935 etc. 8 Pin also on early Turbo

I keep hearing about how all Porsche race cars with twin plug used two CDs because two CDs supplies a "better" spark. It may have provided a better spark with the single CDs of the time but the main reason for two CDs was ignition redundancy. The distributors had two separate mag pulse units or multiple points to produce a redundant ignition.
Very few reproduction twin plug distributors have a duel mag pulse, hence no system redundancy.

Oh Henry. Let's not modify history. The main reason for twin plug was so they could tune the compression and timing make more power. Plain and simple. Even Randy Leffingwell's book "Porsche 911: 50 Years" says that twin plugs were too they could make more power. If my Paul Frère books were not in storage I am sure I would find references to that as well.

You need two Bosch CDIs because the energy would not be enough to drive two coils. I thought I'd made that point.

Redundancy certainly isn't the 'main' reason. The twin plug was conceived for more power because you are 'lighting' the mixture from either side. Two flame fronts sees more energy released from the combustion in a shorter time, hence more power.

Twin plug engines need a lot less advance than single plug. If you 'lost' a CDI box in a race, the power drop off would be substantial unless you could advance the ignition from the cockpit. Would get you to the finish line though. I've done the single/double test on a 375HP motor, it lost 40HP when the second box was switched off.

In my opinion the two reasons for twin plug are #1) Increase volumetric efficiency of the engine, many people do not know why they benefit from twin plug... The reason is you ignite the fuel on both sides of the dome of the piston. always have to explain this on guys wanting twin plug on turbo as the pistons are pretty flat and not much gain at all other than redundancy. #2)In race cars if one failed the other could get it through the race hopefully, (2 ignitions of course) There are some side benefits like lower timing and reduced timing advance but this is due to not having to light the fuel as early since your lighting it on both sides of the dome also reducing detonation enabling more compression also.

OK, "main reason" was a misstatement but it's fun watching heads explode.
Porsche was interested in endurance races almost to the exclusion of everything else. Finishing was paramount.