Twin Plugging 3.2 Carrera question

[911TT33]

Hi all,

My blown 3.2 motor has been machined for twin plugs and I now want to start looking at hooking up the secondary lower plugs.

I'd like to know whether the Motronic DME ignition output signal can drive the two MSD / Crane CDI ignition boxes directly, or do I need some sort of amplifier inbetween the DME and the two CDI units?

Also, is it better to just install a 964 twin distributor in or go with one of the aftermarket JB Racing 12 plug distributors?

How are you guys hooking up your twin plugs?

Pics would be appreciated

[schnellmann]

The DME can't drive two coils directly. You'll need a splitter which takes the DME signal and amplifies the voltage for two "channels," one for each coil. CDI boxes are used on CIS injection cars, not the Motronic cars.

I used a rebuilt 964 distributor because it fits perfectly and I know I'll never have trouble finding caps and rotors, the belt that drives the second shaft, other parts.

For pics, have a look-see at my 3.4 build thread:

How to buy your 911 twice, one box at a time...

[911TT33]

Originally Posted by schnellmann

The DME can't drive two coils directly. You'll need a splitter which takes the DME signal and amplifies the voltage for two "channels," one for each coil. CDI boxes are used on CIS injection cars, not the Motronic cars.

I used a rebuilt 964 distributor because it fits perfectly and I know I'll never have trouble finding caps and rotors, the belt that drives the second shaft, other parts.

For pics, have a look-see at my 3.4 build thread:

How to buy your 911 twice, one box at a time...

Thanks for tuning in schnellmann.

Just to clarify - I don't want to drive two coils directly from the Motronic DME.

I want the Motronic DME to send the ignition trigger signal to two MSD 6AL CDI units, and each MSD 6AL CDI box will fire a separate MSD Coil (one for top plugs and one for bottom plugs).

I need the extra ignition power because my motor is turbocharged and runs ~1.2bar of boost.

I currently run a single CDI unit for the top plugs only. I need to fire the second plugs and will be looking at wiring in a secondary CDI unit.

So, are you saying I need FROM the Motronic:

Motronic -> Signal Splitter -> two MSD 6AL CDI units-> dual MSD coils?

Who sells the Signal Splitter?

[Elombard]

what about one of these distributor-less systems?

F/S TEC1 EFI Twin Plug

[911TT33]

Originally Posted by Elombard

what about one of these distributor-less systems?

F/S TEC1 EFI Twin Plug

Looks like you need trigger wheels and crank sensor etc... seems complex.

Seems I can use a M&W Pro-14B CDI unit which will take a single ignition trigger signal from the Motronic DME and fire two coils simultaneously to the twin distributor. Costs $1k for the CDI unit

[911TT33]

Wiring for M&W Pro14B CDU Unit:

[schnellmann]

Close but not exactly. You'd need either the coils or the capacative discharge units. They serve the same purpose. And all you need is enough voltage to jump the plug gap and properly ignite the charge, boosted or not.

You should be able to get that with the right coil for your plugs or an aftermarket CDI. But they work differently (electrically speaking) in how they develop their output and need different kinds of input signals from an ignition controller. Though aftermarket ones are made to use with a range of input pulses usually.

But if your existing CD boxes can accept the DME ignition signal as is, I'd think you still might need a splitter. Or ditch it all and use a coil-on/near-plug distributorless type system. But that is a pretty complex change from your current EFI setup.

Keep in mind the DME was designed to push out a very specific voltage with a specific range of resistance to a single coil in the Motronic system. Motronic DMEs not very forgiving in terms of electrical tolerances. If you asked the DME to drive two CDIs it might damage the DME. It is a small computer after all. It uses the same Intel CPU used in early 1980s arcade games and is on a board with etched circuits which move very small currents between components. Or send a higher current to a remote component as in the case of the ignition signal output. If you know an electrical engineer they could do some quick math based on the DME ignition output specs and the input specs of your CD boxes and tell you for sure whether it would be OK. If OK, they could advise how to wire the CD boxes (in series or parallel, etc).

In any case, Andial makes the splitter I used. It basically takes in the single DME input and creates 2 output signals. It's not cheap, but then good used DMEs are not either if you toast one.

[schnellmann]

By the way, I think I paid ~$600 for the Andial splitter in 2007.

Speed costs... how fast do you want to go?

[mysocal911]

Originally Posted by schnellmann

Close but not exactly. You'd need either the coils or the capacative discharge units. They serve the same purpose. And all you need is enough voltage to jump the plug gap and properly ignite the charge, boosted or not.

You should be able to get that with the right coil for your plugs or an aftermarket CDI. But they work differently (electrically speaking) in how they develop their output and need different kinds of input signals from an ignition controller. Though aftermarket ones are made to use with a range of input pulses usually.

But if your existing CD boxes can accept the DME ignition signal as is, I'd think you still might need a splitter. Or ditch it all and use a coil-on/near-plug distributorless type system. But that is a pretty complex change from your current EFI setup.

Keep in mind the DME was designed to push out a very specific voltage with a specific range of resistance to a single coil in the Motronic system. Motronic DMEs not very forgiving in terms of electrical tolerances. If you asked the DME to drive two CDIs it might damage the DME. It is a small computer after all. It uses the same Intel CPU used in early 1980s arcade games and is on a board with etched circuits which move very small currents between components. Or send a higher current to a remote component as in the case of the ignition signal output. If you know an electrical engineer they could do some quick math based on the DME ignition output specs and the input specs of your CD boxes and tell you for sure whether it would be OK. If OK, they could advise how to wire the CD boxes (in series or parallel, etc).

In any case, Andial makes the splitter I used. It basically takes in the single DME input and creates 2 output signals. It's not cheap, but then good used DMEs are not either if you toast one.

Overly confusing!

The DME ECM can easily drive two CDI units assuming that they require a zero
to 12 volt square wave input. Pin 1 of the DME can supply that signal via a load
resistor (replaces the coil) to 12 volts and will trigger the CDIs on a positive going
signal, which is the correct phase for generating the correct spark timing. Pin 21
of the DME ECM (tach output) can be used, but it may not have enough drive for
the CDI inputs and cause the tach not to work.

[Elombard]

XDi Dual Plug Ignition Kits for 911 Porsche : Clewett Engineering, The complete solution for ignition & fuel injection

Clewett sells a plug and play kit - if you have not seen it. Not cheap but not too bad especially if you consider you might be able to sell your old stuff and off set some cost. I had one on a car for a while and it was fault less.

[schnellmann]

Originally Posted by mysocal911

Overly confusing!

The DME ECM can easily drive two CDI units assuming that they require a zero
to 12 volt square wave input. Pin 1 of the DME can supply that signal via a load
resistor (replaces the coil) to 12 volts and will trigger the CDIs on a positive going
signal, which is the correct phase for generating the correct spark timing. Pin 21
of the DME ECM (tach output) can be used, but it may not have enough drive for
the CDI inputs and cause the tach not to work.

To the OP - take free forum advice for what it is. I'd say you need to do your own research/homework on what will "easily" work and what won't. We are all just well-meaning yahoos and it's your car/money/time.


Bottom line is that nothing about twin plugging is simple or cheap. It's the most expensive 10HP you can buy for a 3.2L.

[yayo_ayala]

Hi Elombar, I am in the process of fixing my engine. I gave up the twin plug, and I can tell you at want point my mechanic was suggesting the Clewett, but there is one thing you wont be able to do and for me the main one - change the chip and adjust the time. The clewett will not use the DME for timing. Therefore I stop considering the Clewett and cancel the twin plug project.