Twin Plug Approaches:

1. 2 separate CDI systems
a. Bosch CDIs - $750
b. other aftermarket CDIs - $600

2. 2 separate Inductive Discharge systems
a. 993 module driving two coils (993 setup) - $175
b. 993 module providing second coil drive (input from original coil ignition) - $125
c. Two cheap 928/964 replacement ignition modules - $175
d. Andial coil splitter system - $500

Note: All of the #2 options (except #2b & #2d) require additional circuitry when used
with a 3.2 DME ECM or additional harness mods (except #2b).

3. Inductive Discharge waste type coil (same cylinder) - inadequate spark voltage
This appoach is not equivalent to the typical "wasted spark" setup because of
the additional voltage needed for the extra cylinder charge gap versus an air
gap in the "wasted" cylinder spark plug.

Best overall (cost, performance, reliability) - #2b - Cheapest and easiest to implement
requiring no wiring harness mods and no additional "boxes" or circuitry (inverters/splitters).

Thanks Luke :)

I do know that the BEST solution is to install two MSD's or two Crane's and be done with it, but cost is a factor. A Crane HI-6 unit down here (with LX92 coil) is $500! :eek:

Have you thought about this Merv?

http://www.directignition.com/products/xdi.html

Bill

I am running 1 MSD 6AL triggering 2 Blaster II coils. 3 liter turbo with
Supertecs twin plug distributor-349 RWHP at 1 bar or 401 HP at the
flywheel with a dated K27 turbo and only 3 liters. I would have to say my single MSD is doing the job just fine. I would not even consider running 2 MSD's based on my experience with this setup.

I'm running a 993 module driving two Carrera coils in my 3.4 twin-plug and it's awesome.

Merv, your PM box is full.

There you go! Real life example of it working fine in parallel mode ;)

Patton, inbox should be free now. Cheers.

"I am running 1 MSD 6AL triggering 2 Blaster II coils."

Some never learn! Just like those who allow their stock
911 engine to revv to 7500 RPMs because it "works",
e.g. "I revv my engine there all the time and haven't
had a problem yet, & that's real world experience which
proves it's O.K.".

Loren what is your experience with the documented failures of 1 MSD driving 2 coils. What part fails in the MSD units. How short lived are the units running both coils.

A lot of odd info in this thread...
The 3.2 and later distributors don't have an ignition curve. The ignition is triggered by the DME. The only "curve like" thing in a 993 distributor is the mechanism that changes the rotor's orientation so it is lined up close enough to the distributor cap's contacts so the spark will transfer with minimal loss. It would have to be way off to miss by enough to cause a problem and it could be easily fixed by changing the springs.

A very incorrect description of what causes an ignition based miss in a high boost motor. Blowing real hard with a 14:1 air:fuel mixture is never going to put out a flame. I would be worried about taking advice from an engine builder that thought this. (Wow!)

It is very easy to induce electrons to jump a gap in a vacuum. (remember vacuum tubes?) As the pressure rises it gets harder and harder to get electrons to jump a gap. If you increase the boost you are increasing the pressure in the chamber so it gets very hard to induce a spark. It can get to the point where the coil can't drive the spark plug and the spark finds another path with less resistance.

In other words: You need a coil with a high voltage secondary output and you need the plugs, wires, cap, rotor, etc to be in tip-top shape to stop 'voltage leaks'.

The Andial splitter seemed to work for the 500+ HP stuff that Andial produced...
(I would still prefer a direct ignition system because it removes a whole host of things that can go wrong, plus each coil can have a much longer rise time to help with high RPM use.)

I think there is a misconception here about what the splitter does. The splitter doesn't split the spark. It splits the signal. The coil's signal is just voltage that goes to the coil to build a magnetic field. When the voltage is turned off the magnetic field collapses and the energy goes out the coil (spark) wire. All the splitter needs to do is to carry voltage to the coils during the time it isn't firing.

In the MSD you are bypassing this because two of them can be connected to the same trigger and it is used only as a source of signal, not as a source of power. The MSD has its own power supply to feed the coils. It will solve this nicely.

One heavily touted "advantage" of the MSD is actually a bunch of bull. (unless your engine is just on the edge of not firing) The ability to supply a bunch of sparks during a single power stroke is silly. You can't get it 'more burnt then burnt'. Once you have lit the flame the job is done. That's it, job done. If it didn't get lit the first time then you have a problem with the ignition system that you NEED to fix instead of trying to mask it by 'using a box load of matches to light the flame'.

Loren,
Henry Schmidt from Supertec installed my system (1 MSD 6AL triggering
2 Blaster II coils). I have never been let down by anything that Henry has
built or installed for me. I consider him one of the very best when the topic
turns to anything to do with air cooled 911 engines and all of their related ancillaries.
PLEASE, answer iamchappy's question "what is your experience with the documented failures of 1 MSD driving 2 coils? What part fails in the MSD units? How short lived are the units running both coils?"
We are all ears.:D
Thanks

"Henry Schmidt from Supertec installed my system (1 MSD 6AL triggering
2 Blaster II coils)."

So, and many think MotorMeister does O.K. work and use them, and
others think it's O.K. to advance the 3.2 ignition timing beyond 40 degrees.
And then, there're those who believe the HP claims for "custom tuning" of stock
engines, etc., & etc..

"PLEASE, answer iamchappy's question "what is your experience with the documented failures of 1 MSD driving 2 coils? What part fails in the MSD units? How short lived are the units running both coils?"
We are all ears."

Re-read the posts in this thread! The all the information is there.

Make sure that the coil you choose (twin tower or equiv) is capable of supplying the number of sparks per second for the cylinders.
On a Harley...there is a coil with 2 towers (fire at the same time and waste the spark on one of the cylinders)...but you could not use this on anything more than a 2 cylinder engine.
If you were to install this type of coil on a 6 cyl. with 2 plugs per, you would be asking the coil to deliver 12 times the power that it was rated for.
The result would be very short time to failure.
Bob

I don't know anything about the MSD 6AL. Here is what I do know. If you drive coils in parallel, the coils will provide half of the energy, not good but it will appear to run quite nice to the unaware.

Coils must not be run in parallel unless you are happy with half of the spark energy that is available.

If the MSD 6AL separates the coil primaries, then it's fine. If it parallels the coils primaries, I'd look further into it.

I agree Loren. But, many DME failures are due to the 20 year old coil driving transistor in the DME. I like the idea of fresh coil drivers relieving the tired DME of it's responsibility, possibly extending DME life expectancy :)

I went for option 2a (piece of mind) and I'm very happy with the result.

So if I went with option 2a, all I'd need is an extra Carrera coil, correct? I mean, I already have the 964/993 twin dizzy, so I'm assuming all I'd need is :-

- remapped Motronic CHIP for twin plug application
- 964 ignition leads
- Patton's 993 module kit ;)
- extra Carrera coil

If the 993 module kit can reliably support ~1.2bar boosted application, I'd be very pleased ;)

Not a problem.

Every electrical component in a car is wired in parallel. (except for the switches that control each device) The positive harness is hooked to the battery on one side and feeds each PARALLEL circuit which is hooked on the other end to the battery via the ground. The only reason that wiring in parallel could limit the supply of current is if there is something in the circuit that specifically limits the current.

The MSD (or any other ignition trigger) is simply supplying a path to ground for the coils primary circuit. It would take some overly complicated circuitry to have any sort of arbitrary limiter on the quantity of current going the ground. In other words it will do fine.

...Except...

Does the power transistor in the MSD have enough capacity to keep from burning up? If the car starts it is doing fine. If it burns up then the the car is a paperweight. Easy to troubleshoot.

If you are going the MSD route I can't see a reason why you would want to stress that component when it could leave you stranded. I would wire 2 MSDs in parallel

"Does the power transistor in the MSD have enough capacity to keep from burning up?"

The MSD unit is a CDI type ignition system so it doesn't use a power semiconductor
to switch the coil current on/off as in an inductive discharge type of ignition system.
The CDI type, though, has a reliability problem the result of excessive output currents,
i.e. mutiple coils, which will damage its storage capacitor and its SCR (the device which
switches the energy stored on the capacitor to the coil).

Read here (Ignition Systems) for more info on Inductive vs Capacitive ignition systems:

www.systemsc.com/technical.htm

Quicksilver.

Yes, there is a single 12v buss running throughout the vehicle. And yes, all circuits are in parallel with this buss, one side positive and the other negative.

In the case of driving two coils off of one circuit (same 12v fused circuit and one return to ground), there is a reason why ignition manufacturers simply did not just supply larger current sinking capacity.

Loren, this is right up your alley. Please add your expertise.

OK, I've confirmed with a reputable turbo tuner that the 993 ignitor solution (with 993 coils & good plugs) WILL handle ~1.25bar of boost and can support upwards of 650HP application. This fits into my budget/performance envelope nicely :D

The other question I have is, what sort of plug gaps should I run using the 993 solution? I'm assuming 0.024" or so?

Wouldn't it be better to use the 993 dual ignition coil with the 993 Ignition Module, since it's a matched pair?