Twin plug DME splitter?
 

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........:confused:

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.

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

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.

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.

http://forums.pelicanparts.com/uploa...1074562236.jpg
-Chris

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

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.

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.

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

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?

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.

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

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

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

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

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).

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?

I am not totally following what you all are discussing (undergraduate EE was a long time ago), but bottom-line it sounds like Andial's $600 splitter could be replaced by $50 of standard electronic components? Thereby reducing the cost of a twin-plug conversion by 25% or so? Is that correct?

Yupp.
Andial splitter is nothing else but 25$ of electronic components.


rick:
I don't have schematics of DME but woulnd't it be a good idea to rather intercept signal before it goes into single Darlington in the box itself and just connect it to two Darlingtons on a cooler in separate box - connected to it's own coils.

Anyway, it's a simple hack. HW cwould cost something like 30 bucks...

I think the Andial splitter has about $150 of components in it. Two Bosch ignition modules "ignitors" (~$50 each), connectors for them ($10 each?), two 65 amp airplane pull switches ($10 ea) and about $15 worth of Leds, connector strips, regulator, and some descrete components.

If you replaced the $100 worth of Bosch ignition modules with power transistors you could reduce the cost significantly.

My own thoughts are that I want the splitter to reside in the engine compartment to avoid hacking the harness up and running another wire. I intend to spring for the Bosch ignitors because of their proven robustness in the engine environment.
-Chris

Is the signal into the darlington a 5v square wave?

Darlingtons are just two cascade-connected transistors. You cannot feed them with TTL-logic levels, but i believe there are components that can be switched directly with TTL (maybe those Bosch "ignitors" Bennet is talking about). That way, you could make your own splitter. 7205 for 12-to-5 volt downpull for TTL logic Vcc, 7400 as inverter, couple of condensors to get rid of EFI and two "ignitors" to hit the coils with.

Dunno what "ignitors" cost but other components total cost is aound 12 bucks, not counting the box and heat-sink.

Loren: yeah, 7400 has four gates but it's dirt-cheap...something like $0.95. Electronic compontent pricing is weird and has often no correspondence with device's complexity.

You can pay 2 bucks for 10 000 gates VHL circuit and 10 bucks for one single stupid transistor, just beacuse IC is being made in bigger series.

PC motherboards are real-life examples of it....they are book-typical proof of how availability and competition cuts prices....we are talking about 5-million gates of IC, hundreds of components, multi-layered PCB and whatnot...for $90.

i was just on the phone with Nology and they have a 2 channel ignitor that will fire 2 coils but it uses a 5v square wave signal. Warner (the owner) ws very helpful he drives a 930 daily (woo hoo) and had a lot to say about twin vs. single plug.

Great! Tell us what he said then!! http://www.pelicanparts.com/support/smileys/wat3.gif

I was afraid you'd say that. :rolleyes: Didn't want to highjack my own thread but here goes. (I still hope to complete this discussion)
Warner doesn't think that twin plugging is still required given the progress in ignition systems recently. He cited examples including CMW racing's 4 and 4.5 (yeah they have a 4.5 in development) and several Baja racers who use 911 motors converting BACK to single plug using high power CDi, with nology coil, hotwires and silverstone plugs. They also have BBC (big block chevy for the uncultured;) guys running 7 to 8 liters with bores that are huge and no problems.

Given what I know about 911 combustion chamber shape etc. I'm not sure it overcomes the problem of having a big honking hi-comp piston dome dividing half the mixture from the flame front. I'd like to see some dyno sheets and get some real world exp.

I believe you will still need the Bosch modules for Dwell control.

The entire splitter could be built with $15 worth of components, a $5 Radio Shack breadboard and $7 box/enclosure ... for a total of under $30! Selling it for $100 would be ... [fill in the blank] ...

That depends, Bosch makes both smart and dumb ignition modules, some ECU's control dwell, others dont. I dont know whether motronic controls dwell.

I have aftermarket EFI that can be set for either module

Ok, so do any of the EEs in the audience know if at any point the DME system uses the 5v square wave or it is not required in the design? It seems that if there is the equivalent of an ignitor in the DME box then it's triggered by X?? kind of signal?

I found the schematic and, well maybe after a few years of EE classes this will make sense ;) Seriously though, I can trace 1 and 21 back and find the components. Where does the crank signals come in?

..an excellent way for you to make a little money on the side Warren. ;)
-Chris

Has anyone replicated a splitter box as yet?

Why can't one just run two ignition systems (MSD or Crane) off the single OEM trigger signal to drive two high powered coils?

Bump! Chris, have you engineered a cheap splitter box yet, to drive two ignition coils and a 964/993 Distributor?

I'm not to sure I'm willing to spend $600 on an Andial unit, or ~$1k on two Crane/MSD ignition systems :eek:

Won't a Electromotive XDi crank-fired ignition easily solve what you're trying to do without reinventing the wheel?

Yes, but how much is the Electromotive setup going to cost, inc. installation?

How about something like this TWIN FIRE Ignition Coil (typically used for Harley's etc)?

http://www.mpsracing.com/products/Dyna/ig01cd.asp

40,000 Volts per output.

I came up with something that worked - on the bench at least. Between losng my "test car" back in 2003 and the new business I haven't had a chance to do anything further with it.
-Chris

Would this work?

http://www.mwignitions.com/pdf/Pro12_S3.pdf