Twin plug DME splitter?

[anh911]

I've gone through all the old threads on twin-plugging and have a question for the electrical engineers out there.
What's really involved with designing a signal splitter so the DME can trigger 2 coils? And the wasted spark systems use coil packs which use coils that have single input and dual spark output. I'm guessing that you can't use one of these in conjunction with a distributor cuz they won't cycle fast enough? But if they build a single in dual out (sounds like muffler talk) coil for wasted spark, why not one for use with distributor?

Sorry long........

[69911e]

The splitter may not be needed. I see no inherent reason you couldn't hook up two coils on one DME output, other than specific driver limitations of the DME. The coils would need to be identical in specs, otherwise very odd currents would flow.

[Oldporsche]

I think I understand your question.

1. The distributor is designed to distribute secondary voltage to the spark plugs. If you triggered a coil with two outputs you would still have to distribute the secondary voltage you have created somehow.

I do think a distributorless fully electronic ignition system could be built however using the output of the dme for its trigger.

I hope this at least partially answers your question.

Good luck,
David Duffield

[anh911]

Thanks Ed, that's what I thought too.

"I do think a distributorless fully electronic ignition system could be built however using the output of the dme for its trigger."

That's an option that is also interesting, that way you wouldn't lose the DME control over the ignition. I guess I wasn't clear though, let's say you have the 3.6 dist., 3.2 DME and 2 coils. But you still need the splitter to fire 2 coils from one DME.

[ChrisBennet]

All the splitter does is invert the coil signal before feeding it to two "ignitors" (Bosch power transistors I assume). I'm planning on making one to replace my Andial one. In the diagram, T1 and T2 are the Bosch ignitors.


-Chris

[anh911]

Thanks Chris, I noticed the 2 bosch ignitors in a pic of the Andial piece. But - forgive my ignorance- can you elaborate on the inverter?

a~~

[ChrisBennet]

Originally posted by anh911
Thanks Chris, I noticed the 2 bosch ignitors in a pic of the Andial piece. But - forgive my ignorance- can you elaborate on the inverter?

a~~

I'm not an electrical engineer so hopefully someone will correct or augment my discription.

The inverter converts the "signal" to the opposite "state". You can think of the coil signal as having 2 "states - say 10v and 0 volts. (Numbers picked out of the air.) For example, say the DME raises the coil line to 10 volts and drops it momentarily to 0 volts to "fire" the coil. The ignitors change the state of that signal i.e. reverse it so if 10volts was fed to the ignitor, the ignitor's output would be 0 volts. To get around this, an inverter is used to "invert" the signal before the ignitors invert it (again). The inverter's output is the opposite state of the it's input. E.g. 0 volts in produces 10 volts out. Does that make any sense?

I'm curious why a coil with secondary windings isn't used instead of a splitter box. It would seem to be much simpler.

-Chris

[anh911]

Ok, I'm with you now. I didn't realize the ignitors had that effect. Couldn't you wire 3 ignitors then? Use one as the inverter to fire the second two for the coils?
I also think the simplest solution is the secondary coil. I'm gonna do some searches.

[anh911]

hmm, seems that dual output coils are relatively common in motorcycle apps. Nology has a couple of products that I am going to call about tomorrow. Dual output coils and they have multi-channel ignitors. The dual coils are smaller than the singles so I don't know if they will work.

[Oldporsche]

Since I work on relatively "old Porsches" I haven't been under the hood on any late model stuff. I take it that the 3.2 DME has only one output to trigger the coil. To use it with the twin distributor you would need two outputs that the later DME would have?

The Andial splitter may be actually an amplifier switching a couple of large power transistors?

Thanks,
David Duffield

[anh911]

Yes, David that's it. In Chris' diagram above T1 and T2 are the power transistors. I'd love to hack into a late DME and see how Bosch does it.

Anyone have access to a 3.6 DME schematic?

[Lorenfb]

You really can't drive two coils with a DME output. The DME has a current
limit function which is set at about 8 amps. Based on the coil inductance,
you'll not have enough energy (a weak spark) driving two coils. This
should answer Chris' question about an additional secondary. Checkout
this web site (www.systemsc.com) on the Technical page about Ignition
Systems. It may provide some additional insights about spark energy.

You can, though, add another transistor inside the DME unit and pickoff
the right internal signal and have a second output to drive another coil.
I've done this before and it works great. You need to also have a current
limiting function on the second output. Obvious, the second transistor
must have a heatsink.

[anh911]

Thanks Loren, that's good info. Right now I'm leaning toward an add-on.
Chris, do you have a source for the inverter?

[ChrisBennet]

Originally posted by anh911
Thanks Loren, that's good info. Right now I'm leaning toward an add-on.
Chris, do you have a source for the inverter?

The Andial splitter is available from Andial or Rennsport (Steve Weiner). I'd recommend Steve. Price is around $600 I think.
I'm pretty confident I can build a better mousetrap so if you want to buy my Andial spltter box let me know.
The Andial splitter box requires modifying the harness and running a 2nd wire all the way from the DME to the 2nd coil. I intend to make a splitter that sits in the engine bay so the harness can remain uncut. I betcha Loren could design something like this (to sell) in his sleep.
-Chris

[ChrisBennet]

Originally posted by anh911
Thanks Loren, that's good info. Right now I'm leaning toward an add-on.
Chris, do you have a source for the inverter?

Sorry, I didn't answer the question before. I don't know of a source for an 0-12v "inverter" though one probably exists. Andial made their invert using TTL logic which required a regulator (TTL needs 5v) and a few resistors. I was just going to make one from a transistor or two.
-Chris

[Lorenfb]

If you use the tach output (pin 21) of the DME, you have the inverted signal of
pin 1 (coil). This could drive a buffer transistor which then drives both coil
transistors or ignition modules like on the 964 3.6.

[ChrisBennet]

Originally posted by Lorenfb
If you use the tach output (pin 21) of the DME, you have the inverted signal of
pin 1 (coil). This could drive a buffer transistor which then drives both coil
transistors or ignition modules like on the 964 3.6.

Loren,
Do you think you could drive the ignitors directly? The Andial box feeds them both from a wimpy 7400 NAND gate. Wouldn't a buffer transistor invert the signal? Sorry if this a stupid question, it's been over 20 years since I studied this stuff.
-Chris

[rick-l]

Just for info the 3.2 DME is a current source that sinks (if I remeber right) about 0.5 amps when it is not firing the plugs. It is controlled by that long tailed pair and the darlinton on Wilkes DME schematic. To fire the plug the dme interupts that half amp.

In an ideal world if you parrelleled coils each would have half the energy but....


edit

on second thought don't even try to put a coil in parrellel

[Lorenfb]

Chris, you're right. A simple transistor inverter should work. Andial's use
of a TTL device (7400) was a bad application, since it requires a +5v supply,
is not a good current sink, and wastes three other nand gates. Andial would
fail a EE design class.

You need to select what ignition module you'll use and then determine
the sink/source current required. Once that's determined you can design
a simple single transistor inverter using a 2N4401 (NPN) or a 2N4403 (PNP).

[anh911]

Ok, so if I'm following right the trick way to do this is to tap into the tach feed, use a buffer and trigger the ignitors. Can you use the existing coil feed to power one and the new tach feed>buffer>ignitor to trigger the second?