Porsche-O-Phile

There have been a few good posts recently with regards to the ignition system on the 944, but I'm wondering if anyone has a source of a good FAQ or file on this system, as there are still a few aspects of it that confuse me. TIA.

I'm wondering how the DME can control spark timing through coil pulses if the ultimate completion of the circuit from the coil to the spark plug is determined by mechanical means - the position of the rotor inside the cap and whether or not it's in contact with (or close enough to) the point for that particular plug wire. I suppose there is a certain "range" that is set, determined by how far the spark can jump from the rotor tip to the point on the inside of the cap. Put another way, for a given coil voltage, a spark can only be so big / long. Based on this, the DME could only vary ignition timing within the limits length of this spark within the cap. If the DME sends the signal to "FIRE NOW" to the coil, the coil energizes, but the rotor is positioned outside the distance it can "jump" from the plug contact given the coil voltage, nothing should happen. Is this a correct assessment? It seems to make sense given my knowlege of the speed & reference sensor setup, DME function, etc.

Someone also said recently that it doesn't matter which plug wire goes to which position on the cap (i.e. there is no requirement for the #1 wire to be at the 12-o-clock position, the #3 wire to be at the 3-o-clock position, etc.) Supposedly the DME can adapt to this, but I am still lacking understanding as to how the DME could, simply by sending a signal to the coil to fire, control which plug gets the juice. This is ultimately determined by rotor position, is it not? As such, the plug wire positions WOULD matter. Can someone clarify this?

Matt H

To the best of my knowledge this is incorrect.
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Porsche-O-Phile

Everything else makes sense then. Thanks, I'm pretty sure I've got it now.

Danno

The width of the rotor-tip gives a large range of ignition-timing values that the distributor can pass on to the plug-wires. In fact, due to the dielectric insulating properties of the cap, you can actually have the rotor mis-aligned such that the rotor isn't even near the post for the plug-wires and it will still jump across. Albeit, it'll be a weak spark, which may not be sufficient to ignite the mixture.

I was fighting a problem like this for weeks. ALL of the spark-tests were positive with spark jumping out to a plug inserted into the center coil-wire. Even got spark at the ends of the plug-wires with plugs in the wires laid on top of the intake-manifold. But they wouldn't fire the car! Turns out I had left off the 2nd bolt on the rotor and it spun about 30-degrees off. Just enough of the spark jumped across the generate a spark at the plugs in open air, but it wasn't strong enough when faced with the compressed air-fuel mixture.

FWIW, I had dynoed a car @ 262rwhp with stock K26/6 turbo and found out later that the owner had not kept up with regular maintenance, he'd only repair things when it broke and left him stranded. The distributor rotor looked like this:



And the car run just fine before and after replacing it with a new one! :eeK:

Matt H

Big difference between the car "might" start and you could put the wires on any post and the car would run fine.
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Porsche-O-Phile

Wow. That just miiiiiight have been a contributing factor. . . yep.

Danno

"Big difference between the car "might" start and you could put the wires on any post and the car would run fine."

Yup, I think you might be able to swap the #1 & #4 plug wires and the #2 &#3 and the car would still run correctly.