Consider the source of the "theory"...

It is common to run two coils with one box.

Thanks for the positive comments.

i dont know squat about dual plug setups, never worked on one. but if i went with dual plugs, and i am considering it with the spare 3.0 i have, i would go with dual CD units. it does not matter if it is bosch, MSD or permatune, common sense just seems to tell me 2 CD units with 2 coils. i dont know why anyone would want to use just one, besides, the cool factor of the 2 units is not bad either, especially the 2 big reds.

"i dont know why anyone would want to use just one"

Very wise comment!

If you read the entire thread it tells you why. If you use two boxes and two coils and one box fails you're left with one working. One working box and coil means one working plug. If you build an engine that REQUIRES twin plugs and you loose one at 6500 rpm under load, your new engine will be toast before you can lift your right foot. That's why.

Lindy, just install a couple of small $35 fans and all your fouled plug issues, cooling issues and carb issues will be resolved. LOL

"If you build an engine that REQUIRES twin plugs and you loose one at 6500 rpm under load, your new engine will be toast before you can lift your right foot."

And with just one CDI you could most likely just have one good spark with
adequate energy and intermittently firing just one plug, or worst yet, neither sparks
being adequate. That's why Porsche on the 964/993 engines with twin plugs used
separate ignition driver controls for each coil. That way assuring adequate spark
and energy for each spark plug.

When designing any system, one designs for what's most probable to occur,
and that's that one of the two sparks or both will be inadequate and NOT that
the ignition unit will fail which is the logic for using just one CDI.

Lindy, What if you did a "right" side, "left" side with two MSD's. Would that add some additional energy but create some protection? I dont claim to know ANYTHING about a twin plug set up. Just thinking out loud.I set up my EFI with three injectors driven off each controller.

lots of talk for a $200 part (or a $2000). if you like em, run em. if you don't, run the OEM. not worth all the chatter.

I agree. Ridiculous. This is a solution in search of a problem.

I guess I'll throw away the webers, damn! Anyone wanna trade for fans?

The MSD fires two coils without issue. Loren, you're full of crap.

Not possible; one coil runs the upper plug set of six and the other runs the lower set of six. Not a bad thought though...

"The MSD fires two coils without issue."

Right, dream on!

Because of marginal sparks with one CDI, that's why Porsche/Bosch ran TWO
separate CDIs on the 935 race cars over many years.

How much will you give me in return :D:):D

I agree - it seems like foolish economics to cheap out and not run two CD boxes if you're running a twin plug system.

-Wayne

and, like aircraft, redundant systems give you a limp home mode. very handy.
Now, let's talk about wasted spark systems, what do you think or would COP (coil on plug) work more efficiently.


Wayne, saw a youtube video with you and adam corolla and a 904, classic!!

It has nothing to do with "cheap out". If there was a way of preventing the second box from firing if the first failed I would be all over it. Leaving 10% HP on the table is not in my DNA. It really doesn't matter who makes the box but the possibility to run on one plug vs two and destroy the engine, in my opinion, is too risky. Do any of you commenting have a twin plug system similar to what we're discussing? Any real world experience, or are we still talking about the hypothetical?

Then the question becomes.....does your timing light's sensor detect actual HV CURRENT flow or does it trigger simply with the presence of HV...? I haven't used a timing light for so many years I don't know, if I ever did.

Now I have to go out and find mine, buried "out there" somewhere, I remember seeing it......

I'm not sure. When I pull the plugs though, they both read almost perfect from a color visual (I know how to tune Webers). I guess one could reason that one plug firing could cause the other to look good simply because the mixture was burning completely. However, everything I read about our hemi-head engines is that anything over 9.5:1 CR with 93 octane or less will detonate without two plugs to set the fire. I'm at 10.5:1 with 28 degrees timing, pump gas and my engine shows zero detonation. I can only assume both plugs fire.

As you all point out, the theory doesn't add up and it should not work in your assessment. But, why then, has it worked for so many others? Some pretty good builders like Henry Schmidt and Mike Bruns have run this set-up with much higher CR than 10.5:1 without issue other than a loss of power. If only one plug were firing, those engines would eat themselves in short order. I'm guessing the higher output voltage of the MSD (450 volts) is the reason it works.

Loren,

The attached photo is another of my engine which uses the set-up described. It works but according to you it doesn't. Can you show me a picture of your twin plug engine with 10.5:CR that might add some credibility to you comments? What do you figure the voltage out-put of the 935 CDI were; I'm guessing 300. They had no choice but to run two units if that's the case.

http://forums.pelicanparts.com/uploa...1336833188.jpg