3.2 Motronic ignition query?

[911TT33]

Can anyone confirm whether the 3.2 Motronic motor uses rising edge or falling edge to trigger ignition?

Just wondering which setting to use on an aftermarket CDI unit?

The Motronic will trigger the CDI unit and the CDI will fire two coils simultaneously to the twin plugs.

[JJ 911SC]

Free bump as this may or may not help...

Check the 1st five post of Z1, flywheel pin and the Reference Sensor Relationship...

[304065]

I haven't scoped the output but here's my answer:

On a motronic car (let's say 964) the coil primary positive (terminal 15) is connected to switched +12V and the negative (terminal 1) goes to the coil driver. The coil driver in turn is driven by the DME.

When the coil driver grounds terminal 1, the field builds up in the coil. When the ground is removed, the field breaks down and the coil fires. So the coil fires on the RISING edge of the coil driver output.

In a 3,2 there is no separate coil driver, the coil is driven directly from terminal 1 on the DME control unit. The positive goes to 15 as in the 964 example. The nature of an inductive ignition system relies on energy being stored in the coil itself, so that suggests that it always triggers on the rising edge. Good if you are planning on using an MSD box, they trigger on the rising edge. I have one in my '71 which triggers with points which are certainly a rising edge trigger.

All that said, why would you want to downgrade from the fast-switching inductive trigger of the Motronic system to the very short spark duration of the CDI? There's a good reason Porsche stopped using CDI when the Carrera was introduced-- the old worry of not being able to charge the coil fast enough between ignition events was solved by lower resistance coils and high-amp switches.

The Bosch coil drivers can handle something like 7 or 8 amps in normal operation. Points are only good for around 3-4 amps and the old coils would burn up if you tried to charge them that fast.

[Lorenfb]

"Just wondering which setting to use on an aftermarket CDI unit"

The 6 pin Bosch CDI unit requires a negative pulse to trigger for proper timing,
i.e. the distributor signal (SC/Turbo) must be oriented (connected) correctly
or the timing advance will be incorrect.

So, if one were using the DME ECM to trigger a Bosch CDI unit, one would to need
to invert the pin1 signal of the DME ECM and have it go negative. A better signal
to use for that is the pin 21 tach signal with the correct polarity.

A better solution is to use a 993 coil driver module for the two coils and use the pin 21
signal (with proper polarity) to trigger that module.

[911TT33]

Thanks all. This is for a 3.2 Motronic motor running 18psi of boost, so I need an aftermarket CDI to provide a stronger spark. The OEM ignition couldn't handle high boost levels... It would misfire under load, thus the reason for an aftermarket ignition to be triggered by the Motronic dme.

[Tippy]

Have you tried a plug gap of 0.025"?

[rsscotty]

Check with Todd @ Protomotive. I am sure he has done this.

[Lorenfb]

Spark is all about; energy, voltage, rise time, duration/burn time

- CDI - Capacitive Discharge Ignition

energy - 80 mjoules (typically), more than adequate
voltage - 400 volts (initailly) @ primary, 40KV @ secondary, more than adequate
rise time - very fast (< 100 usec), more than adequate
duration/burn time - short (few hundred microseconds),
problem under some loads and with emissions

- TCI - Transistor Coil Ignition, IDI - Inductive Discharge Ignition

energy - 40 - 50 mjoules (typically), more than adequate
voltage - 350 volts @ primary, 35KV @ secondary, more than adequate
rise time - moderately fast (< 200 - 300 usec), adequate
duration/burn time - longer than CDI, better for heavier loads and emissions

Bottom line; Using a CDI system to replace a TCI system may not always
solve a potential ignition problem and may result in other problems.

[304065]

I agree, I would get an external coil driver and a low resistance, high-amperage coil.

[Tippy]

I run 0.020" - 0.025" plug gap which is standard with high boost.

Interestingly, the stock Motronic ignition can support 15 PSI fine but once I get to 18 PSI, it is hit or miss on "flaming out" the spark.

More times than not though, it handles 18 PSI fine on my setup @ 515hp.