CyCloNe!

That's why he's married, at high speeds she pulls out the spray bottle and he turns on the wipers

CyCloNe!

The deed is done

TurboTommy

I know this is a little late, but was just curious.
Does the current not have to flow thru one of the relays in the fuse box, and if it does, would that not be the point of highest resistance anyway.
Not sure.
What exactly is the path of electrical power pre-coil and how much voltage drop should we expect?

TurboTommy

So, when Porsche developed the software, originally, did they come up with a fixed dwell time that reflects the average parameters of voltage and RPM?
Or, is the dwell time "moving" with varying conditions? (This would imply that the DME is aware of voltage, then)
Sorry for my slow learning curve.

Rogue_Ant

The DME has a table for looking-up / calculating dwell time. The table has two axis; one rpm, the other voltage. So, yes, the DME is able to dynamically change coil dwell depending on operating conditions. And yes, the DME is aware of system voltage (also necessary for injector lag-time adjustment).
No worries about the questions, that is why I made the thread - to promote understanding.

s14kev

I admit I have not bothered to learn how the ignition system works in the 944. The last ignition system I routinely worked on was back in the day when cars came with mechanical ignition systems. My old mitsubishi had a distributor which was easy to understand. Ignition rotor makes contact with the terminals inside the distributor cap to allow current to make its way through the leads to fire the spark plugs. There was a mechanical advance based on centrifugal weights to advance timing as rpm increased together with an equally as simple vacuum advance system to do the same with increasing manifold vacuum (ie higher load).

What I don't understand is the 944 also has a distributor. Since the ignition rotor turns at camshaft speed (1/2 crank speed), and the rotor has to make contact with the distributor terminals, how does the DME control timing? There has to be mechanical contact (well not actually contact but very close for the arc to jump from rotor to distributor terminal) to allow the spark to fire and since this contact is controlled by the speed of the ignition rotor which is directly linked to the camshaft, I can't see how the DME varies timing. I know I would probably find the answer if I just read the appropriate section in the Bosch automotive handbook however can anyone in brief explain how this works?

blown 944

Simplest answer is that it is using the S/R sensors to trigger the ignition event. Rather than the old school "phasing" of the cap and rotor.

Just an FYI, the timing wasnt controlled by the "speed" of the rotor, but by the relationship to the position of the rotor and the cap. A vacuum advance allowed the rotor to be phased towards a more advanced position.

Also, vacuum drops under higher load. Vacuum advance is for better em missions and better low end throttle response

s14kev

Yup. Thanks. I think I misworded the vacuum advance paragraph.

I understand the speed/ref sensors tell the DME crank position and when to trigger the ignition event but an ignition event can only occur when the rotor is in phase with one of the terminals in the cap right? Since the rotor is only in phase with one of the four terminals inside the distributor cap for a very brief period of time how does this allow for complete control of ignition timing by the DME? Overall control is still seems to be somewhat mechanical? That's what I don't understand (not that I've done any self learning to try to find an answer!).

Rogue_Ant

Sure.

On the 944 the distributor & rotor are only used to correctly direct the ignition event to the correct cylinder.
The rotor in the 944 is fairly wide. This allows a "range" of possible ignition timing, because the DME can fire the coil to the correct cylinder when it wants, and as long as the rotor is near the distribution terminal, it will be directed to the proper cylinder.
In-fact because the camshaft is spinning at half the speed of the crankshaft, this effectively doubles the range of possible timing that the DME can fire the coil. A quick measurement of the rotor (width and rotor disc diameter), shows that the rotor provides over 80° of possible ignition timing range.

s14kev

Since the 944 does not run 4 independant coils, I understand the distributor does exactly what is name implies (ie. distributes the spark to the appropriate spark plug). It's just getting my head around how it does this.

s14kev

Excellent! Thank you.

Reading this thread has inspired me to finally learn about electronic ignition systems. This is something I have never needed to do since they have always been so trouble free compared to the points based ignition systems I had to play with so long ago. I think I will have to pull out the Bosch handbook and do some light reading this week.

URG8RB8

An excellent question followed by an excellent answer! I looked at the width of the rotor electrode the last time I changed it and thought to myself this is really wide! Now, I fully understand why.

bebbetufs

I take it reducing the Dwell time will eliminate the need for the 0,8K resistor that is recommended to protect the DME when running the MDS blaster coil. Is this correct?

refresh951

Tom - Thx for the tip. I did order a coil wire from Magnecor as mine was not long enough. Cost was $12 plus shipping.

User 52121

+1 thanks Joshua!

I had my own theory over how the DME controlled ignition timing with a cap and rotor - which was what you described, that the possible connection points were pretty wide so you had a range with which to fire. Glad to see my guess was correct.

Thanks for explaining and clearing it up!