Tom wrote: Tue Oct 07, 2025 9:41 am
Thanks for all that! Can I interpret that to mean you wouldn't be surprised or concerned to see the amps keep climbing up past 9, 10, 11, etc. whenever there is enough dwell time to get there? I'm still just a bit stumped why the first time I tested it, it seemed to be limiting current to something around 9 or 10amps -- as seen in the video I posted in #232 above...
Yes, I would expect the amps to climb throughout typical dwell times.
In your video, the VPP was exactly 10.0V. Likely, it was just the limit of the probe/scope input with however you had it configured.
homer.webp (11.22 KiB) Viewed 588 times
Thank you! You must be right. And now realizing it was flattening out at 10 due to the instrumentation (and not 9 due to a constant current from the DME), it brings the time to 9 in line with john's tests and mine when using the stock DME.
Re: Looking for DME BIN files
Posted: Tue Oct 07, 2025 1:31 pm
by johnb
I can confirm there are definitely different coils...whether they behave differently or not, I'll post later when I've had a chance to test the new one.
Here's the new one compared to an old one. The old one here looks exactly like the one in my car. The new one is narrower and has a smaller + post so I can't install it without a trip to the hardware store to look for an M5 nut. It came out of a Bosch box, from Pelican parts so I have to assume it's legit.
new_coil.jpg (168.42 KiB) Viewed 584 times
20251007_162018.jpg (185.07 KiB) Viewed 584 times
I'll post an update when I've had a chance to test it. I'll try measuring the inductance too. I have a cheap component tester that I've tried using but it keeps coming up with 0.4-0.5mH which seems too far off to be realistic. I'll try setting up something more accurate with the scope and see what I come up with.
Re: Looking for DME BIN files
Posted: Tue Oct 07, 2025 2:30 pm
by Tom
Joe replied and confirmed his ignition drivers do not hold current constant at 9 amps like the factory DME. It was a design choice allowed by the more modern IGBT drivers now available, intended to avoid the heat build-up (and potential failures) inherent in the factory design. He does have a failsafe in place in case of run-away current, but that would mostly likely take a bad coil or harness to trigger. Otherwise, the dwell tables themselves and the design of the modern drivers eliminate the need to hold constant currents under ordinary operating conditions.
As for different coils, I plugged the PET part number into the Porsche parts database (via Sunset) and see that the coil was officially superseded to that smaller diameter version. Not sure how I feel about using foam tape to make it fit in the bracket. Looking forward to seeing if the new ones charge any differently...
coil superseded.jpg (348.17 KiB) Viewed 583 times
Re: Looking for DME BIN files
Posted: Tue Oct 07, 2025 7:16 pm
by johnb
OK, here's my result with the new coil:
new_bosch_coil_1.png (92.15 KiB) Viewed 569 times
Exactly the same, ~3.5ms or so.
Later in the week I'll make a serious attempt at measuring the inductance of the one I removed, maybe both, using something like this technique:
Re: Looking for DME BIN files
Posted: Tue Oct 07, 2025 7:31 pm
by Tom
So, that's 4 different coils that all measured in that same range (ignoring my errant readings when running the fTech dme) when tested in that 13.8- 14.2 volt range. At some point, if it walks like a duck, and quacks like a duck....
Re: Looking for DME BIN files
Posted: Mon Oct 13, 2025 5:36 pm
by johnb
I tried to measure the inductance of the coil using a few approaches with pulse generators and signal generators but it was hard to get values I could trust. I was able to reproduce known measurements of small PCB mount inductors with these techniques but it seems that the physical size of ignition coils and their capacitance makes it trickier.
So I decided to try a different approach - why not just try to measure all the voltage drops and see what the formula I used originally says? It should be ok once I know all the voltage drops right?
Here's what I came up with:
ramp_up_to_8a.png (29.07 KiB) Viewed 516 times
The red trace is obviously the coil current, and the blue trace is DME pin 1 (with the probe ground lead on pin 16 or 17, one of the DME grounds). So blue measures the voltage drop across all the coil driving circuitry.
The initial ramp up of the blue trace is (I presume) the voltage drop caused by the coil current. The sudden jump just before peak current is obviously the DME's current limiting kicking in. So I limited my measurement to the linear part of the voltage drop, and just took the middle value, which is a drop of around 1.2v.
I also separately measured the drop between the battery and the positive side of the coil during charging and it's much less, just a couple of hundred mv over the same interval. The drop between the DME ground and battery ground is even less still.
So ultimately using the formula I used originally this suggests the coil inductance is in the order of 3.2 - 3.3 mH.
@Rogue_Ant how did you measure the 5.85mH?
Re: Looking for DME BIN files
Posted: Tue Oct 14, 2025 2:38 pm
by Tom
johnb wrote: Mon Oct 13, 2025 5:36 pm
I tried to measure the inductance of the coil using a few approaches with pulse generators and signal generators but it was hard to get values I could trust. I was able to reproduce known measurements of small PCB mount inductors with these techniques but it seems that the physical size of ignition coils and their capacitance makes it trickier.
So I decided to try a different approach - why not just try to measure all the voltage drops and see what the formula I used originally says? It should be ok once I know all the voltage drops right?
Here's what I came up with:
ramp_up_to_8a.png
The red trace is obviously the coil current, and the blue trace is DME pin 1 (with the probe ground lead on pin 16 or 17, one of the DME grounds). So blue measures the voltage drop across all the coil driving circuitry.
The initial ramp up of the blue trace is (I presume) the voltage drop caused by the coil current. The sudden jump just before peak current is obviously the DME's current limiting kicking in. So I limited my measurement to the linear part of the voltage drop, and just took the middle value, which is a drop of around 1.2v.
I also separately measured the drop between the battery and the positive side of the coil during charging and it's much less, just a couple of hundred mv over the same interval. The drop between the DME ground and battery ground is even less still.
So ultimately using the formula I used originally this suggests the coil inductance is in the order of 3.2 - 3.3 mH.
johnb wrote: Mon Oct 13, 2025 5:36 pm
I tried to measure the inductance of the coil using a few approaches with pulse generators and signal generators but it was hard to get values I could trust. I was able to reproduce known measurements of small PCB mount inductors with these techniques but it seems that the physical size of ignition coils and their capacitance makes it trickier.
So I decided to try a different approach - why not just try to measure all the voltage drops and see what the formula I used originally says? It should be ok once I know all the voltage drops right?
Here's what I came up with:
ramp_up_to_8a.png
The red trace is obviously the coil current, and the blue trace is DME pin 1 (with the probe ground lead on pin 16 or 17, one of the DME grounds). So blue measures the voltage drop across all the coil driving circuitry.
The initial ramp up of the blue trace is (I presume) the voltage drop caused by the coil current. The sudden jump just before peak current is obviously the DME's current limiting kicking in. So I limited my measurement to the linear part of the voltage drop, and just took the middle value, which is a drop of around 1.2v.
I also separately measured the drop between the battery and the positive side of the coil during charging and it's much less, just a couple of hundred mv over the same interval. The drop between the DME ground and battery ground is even less still.
So ultimately using the formula I used originally this suggests the coil inductance is in the order of 3.2 - 3.3 mH.
@Rogue_Ant how did you measure the 5.85mH?
@johnb -- you are one scary-smart dude...
Lol well in fairness Rogue called it as 3 - 3.5 a page or so back without the benefit of a scope trace to see the voltage drop!
Although I am impressed with that meter after seeing how tricky they are to measure.
johnb wrote: Mon Oct 13, 2025 5:36 pm
I tried to measure the inductance of the coil using a few approaches with pulse generators and signal generators but it was hard to get values I could trust. I was able to reproduce known measurements of small PCB mount inductors with these techniques but it seems that the physical size of ignition coils and their capacitance makes it trickier.
So I decided to try a different approach - why not just try to measure all the voltage drops and see what the formula I used originally says? It should be ok once I know all the voltage drops right?
Here's what I came up with:
ramp_up_to_8a.png
The red trace is obviously the coil current, and the blue trace is DME pin 1 (with the probe ground lead on pin 16 or 17, one of the DME grounds). So blue measures the voltage drop across all the coil driving circuitry.
The initial ramp up of the blue trace is (I presume) the voltage drop caused by the coil current. The sudden jump just before peak current is obviously the DME's current limiting kicking in. So I limited my measurement to the linear part of the voltage drop, and just took the middle value, which is a drop of around 1.2v.
I also separately measured the drop between the battery and the positive side of the coil during charging and it's much less, just a couple of hundred mv over the same interval. The drop between the DME ground and battery ground is even less still.
So ultimately using the formula I used originally this suggests the coil inductance is in the order of 3.2 - 3.3 mH.
@Rogue_Ant how did you measure the 5.85mH?
@johnb -- you are one scary-smart dude...
Lol well in fairness Rogue called it as 3 - 3.5 a page or so back without the benefit of a scope trace to see the voltage drop!
Although I am impressed with that meter after seeing how tricky they are to measure.
Josh is the theoretical physicist; you are the experimental physicist, and I'm a good Amazon shopper. $32 for the meter! And now I can finally measure all those capacitors I have in a box.
Re: Looking for DME BIN files
Posted: Mon Oct 20, 2025 10:57 pm
by Rogue_Ant
johnb wrote: Mon Oct 13, 2025 5:36 pm
@Rogue_Ant how did you measure the 5.85mH?
Using my BK Precision 879B LCR Meter...
I just measured four 944 coils, they were all right around the average of 3.6mH.
There is close to enough time to charge the coil at 6500 rpm & 13.8 volts (coil energy starts dropping around 6320rpm, but it doesn't drop by much, about 3%).
Looking forward to seeing if the new ones charge any differently... 
$32 for the meter! And now I can finally measure all those capacitors I have in a box. 