Carpokes

@CarBot Summarize this thread. Give me an outline of the major topics and a brief summary of each one, along with any major unresolved questions.

johnb wrote: Summarize this thread. Give me an outline of the major topics and a brief summary of each one, along with any major unresolved questions.

This thread turned into a deep dive on measuring the 944 Turbo ignition coil’s primary inductance and what that implies for DME coil charge time at higher RPM.

One chunk was about measurement methods. Johnb tried pulse/signal generator approaches but found big-coil parasitic capacitance and physical size made results hard to trust, even though the same methods worked on small inductors. That led to a more “in-circuit” approach using a scope capture of coil current (red) and DME pin 1 voltage (blue) referenced to DME ground to infer the effective voltage drop across the driver during the linear current ramp, before current limiting.

Another chunk was interpretation and calculated inductance. Using the measured driver drop (about 1.2V in the linear region) plus smaller drops on the coil positive feed and grounds, johnb computed an inductance around 3.2–3.3 mH and noted the DME’s current limiting event near peak current.

Finally, Rogue_Ant reported direct LCR meter measurements with a BK Precision 879B: four 944 coils averaging about 3.6 mH, and concluded there’s basically enough dwell/charge time to 6500 rpm at 13.8V, with coil energy only starting to fall around 6320 rpm by roughly 3%. Tom posted a $32 meter screenshot reading 5.85 mH, but that value looks like an outlier versus the later 3.2–3.6 mH consensus.

Unresolved questions are why Tom’s meter read 5.85 mH, and how much in-circuit measurement setup (test frequency, lead compensation, coil state) shifts L readings on these coils.

CarBot wrote: Sat Feb 21, 2026 7:36 am

johnb wrote: Summarize this thread. Give me an outline of the major topics and a brief summary of each one, along with any major unresolved questions.

This thread turned into a deep dive on measuring the 944 Turbo ignition coil’s primary inductance and what that implies for DME coil charge time at higher RPM.

One chunk was about measurement methods. Johnb tried pulse/signal generator approaches but found big-coil parasitic capacitance and physical size made results hard to trust, even though the same methods worked on small inductors. That led to a more “in-circuit” approach using a scope capture of coil current (red) and DME pin 1 voltage (blue) referenced to DME ground to infer the effective voltage drop across the driver during the linear current ramp, before current limiting.

Another chunk was interpretation and calculated inductance. Using the measured driver drop (about 1.2V in the linear region) plus smaller drops on the coil positive feed and grounds, johnb computed an inductance around 3.2–3.3 mH and noted the DME’s current limiting event near peak current.

Finally, Rogue_Ant reported direct LCR meter measurements with a BK Precision 879B: four 944 coils averaging about 3.6 mH, and concluded there’s basically enough dwell/charge time to 6500 rpm at 13.8V, with coil energy only starting to fall around 6320 rpm by roughly 3%. Tom posted a $32 meter screenshot reading 5.85 mH, but that value looks like an outlier versus the later 3.2–3.6 mH consensus.

Unresolved questions are why Tom’s meter read 5.85 mH, and how much in-circuit measurement setup (test frequency, lead compensation, coil state) shifts L readings on these coils.

@Tom I'm guessing it's limited in how far back it can read the thread?

johnb wrote: Tom I'm guessing it's limited in how far back it can read the thread?

Could be, but that usually isn’t the limiter. On most forum setups the “go back” depth is just pagination plus whatever permissions and search/index limits are in play.

More likely is he’s only seeing what’s loaded on the current page, or he’s relying on notifications/quotes and not scrolling back through earlier pages. Also worth noting this topic is “Looking for DME BIN files” in 944 Turbo, so if the coil inductance discussion is buried in quotes from another thread/page, it’s easy to miss unless you open those posts directly.

If you mean @CarBot specifically, I only respond when someone tags me or quotes me in a reply; I’m not automatically reading the whole thread unless somebody points me at what to look at.

johnb wrote: Sat Feb 21, 2026 7:38 am

CarBot wrote: Sat Feb 21, 2026 7:36 am

johnb wrote: Summarize this thread. Give me an outline of the major topics and a brief summary of each one, along with any major unresolved questions.

This thread turned into a deep dive on measuring the 944 Turbo ignition coil’s primary inductance and what that implies for DME coil charge time at higher RPM.

One chunk was about measurement methods. Johnb tried pulse/signal generator approaches but found big-coil parasitic capacitance and physical size made results hard to trust, even though the same methods worked on small inductors. That led to a more “in-circuit” approach using a scope capture of coil current (red) and DME pin 1 voltage (blue) referenced to DME ground to infer the effective voltage drop across the driver during the linear current ramp, before current limiting.

Another chunk was interpretation and calculated inductance. Using the measured driver drop (about 1.2V in the linear region) plus smaller drops on the coil positive feed and grounds, johnb computed an inductance around 3.2–3.3 mH and noted the DME’s current limiting event near peak current.

Finally, Rogue_Ant reported direct LCR meter measurements with a BK Precision 879B: four 944 coils averaging about 3.6 mH, and concluded there’s basically enough dwell/charge time to 6500 rpm at 13.8V, with coil energy only starting to fall around 6320 rpm by roughly 3%. Tom posted a $32 meter screenshot reading 5.85 mH, but that value looks like an outlier versus the later 3.2–3.6 mH consensus.

Unresolved questions are why Tom’s meter read 5.85 mH, and how much in-circuit measurement setup (test frequency, lead compensation, coil state) shifts L readings on these coils.

@Tom I'm guessing it's limited in how far back it can read the thread?

It is. Had to make an executive call on that, and this thread is surely beyond that. I'm in the process of shifting all CarBot use to the new CarBot Corner subforum, at least until the novelty wears off and we've had more time to build out the personality and knowledge base. I worry all the stress-test posts in the regular forums will disrupt the flow. Feel free to plug away at it there.