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The PFC and dwell

Old 04-29-08, 12:34 PM
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The PFC and dwell

Did some work with this today and figured I'd post my initial findings. I want to go back and play with the settings to confirm their operation but what I have already may interest some. I hooked up my Innovate LMA-3 to the trailing coil trigger feed. I typically use this to log RPM. This time I also setup the LMA-3 to log dwell on this channel. You end up with a percentage value that corresponds to the percentage of on-time versus available cycle time. Think of it as duty cycle if you will. The log is shown below. This was done with no load. Since the PFC shows no adjustment for dwell v. load I have to assume at this point the dwell will remain constant under load. It will be easy to verify this later on.







Doing a calculation to find actual charge time is simple enough. Below I have arranged the PFC settings to what I have actually logged. Unfortunately I don't see a correlation between the PFC numbers and actual dwell or charge time. I did not rev the car out to 8000 rpm to get a reading there. You can get an idea of what the PFC would interpolate at 7000 and compare it to the actual numbers.







Thoughts?

The charge time looks about right for the coils I'm using until the revs pass 5500-6000. I'm going to play with raising the numbers in the dwell settings on the PFC to see if that really will raise the charge time at the coil.
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Old 04-29-08, 01:47 PM
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Thanks for the interesting data. I think the PFC dwell settings have always been somewhat of a mystery, even to the experts (Chuck). I'd be interested to see what happens when you increase the PFC values.

I'd also be interested to see someone do the same test with an HKS twinpower installed because there have been claims that all it does is increase the dwell time.
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Old 05-01-08, 08:35 AM
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Posed the question "what do the numbers in the datalogit represent on the dwell settings" on EFI101 and someone responded that they represent degrees of crankshaft rotation. Makes perfect sense if this is the case and sounds entirely plausible to me. The highest number the datalogit will accept is 255.

Here's where things got funky for me. Obviously, with a wastespark leading configuration you can not charge the coil(s) for 255* of crank rotation on a rotary application. You need to charge it, discharge it, and allow a slight amount of recovery time in 180* of crank rotation. At 7500 RPM (the highest revs this particular engine should see) we have 8.57 mS available during one 360* revolution for the crankshaft. So we would have about 4.2 mS to complete each 180* spark event on the leading coil(s). Every system I have worked with until now would allow you to program in whatever charge time you wanted and would automatically discharge the coil at the appropriate time even if the charge time was greater than the available cycle time at high RPM. It's look like the PFC might not do this.

When setting the dwell to 255 at 8000 RPM (obviously a greater charge time than is available on the leading coil(s) on a rotary) the engine hits what can best be described as a very smooth rev limiter around 7000 RPM. In this case the actual rev limiter was set at 8400 so we were not on that. I can only assume that the PFC was holding the true charge time and not discharging the leading coils for every event. Lowering the charge time back below 200 allowed revs to 7500 RPM (the highest I wanted to rev the engine anyway).

So, it seems that there is a practical limit to the upper end of the dwell setting, at least in a rotary application. I'll go over my data some more and make another post on my final settings and how they relate to logged charge time.
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Old 05-01-08, 10:44 AM
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I don't know much about dwell. How could adjusting dwell times improve performance or driveability?
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Old 05-01-08, 10:55 AM
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Dwell (in points, distributor speak) or charge time (in distributorless speak) is the amount of time the primary ignition coil is charged before the spark event. Any given coil will have it's need for peak charge time. Some quite short, some quite long. Anywhere from 3 to 10 mS usually. Charge times shorter than this leaves the coil less than saturated and produces a weaker spark than a fully saturated coil. Any charge time over the amount needed to saturate the coil simply produces heat and will eventually ruin the coil.
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Old 05-01-08, 01:59 PM
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Originally Posted by C. Ludwig View Post
Posed the question "what do the numbers in the datalogit represent on the dwell settings" on EFI101 and someone responded that they represent degrees of crankshaft rotation. Makes perfect sense if this is the case and sounds entirely plausible to me. The highest number the datalogit will accept is 255.
That makes sense. Using the RPM & PFC dwell values you posted you can calculate a charge time and it looks to be pretty constant over the RPM range. However, it doesn't seem to line up with the actual charge times you recorded in your log.

Interesting stuff none the less. I wonder what the ideal saturation time is for an FD coil.
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Old 05-01-08, 04:04 PM
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Ok this is weird. So I'm looking through my Banzai Racing PFC basemap (for a series 4 Turbo II) and all the dwell settings are set to 30, both leading and trailing. I wonder why that is. I know that two trailing coils are essentially spliced together due to the differences in FC and FD ignitors, but really the two generations aren't too different in the way they do ignition from what I understand. I also wonder if playing with those settings could improve spark performance. I have been getting intermittent ignition blowout despite having an MSD 6AL box on the leading (FC) coil and B10EIX plugs gapped to .28 .

I wonder what the relationship is between optimum dwell time and plug gap, if any.

Last edited by arghx; 05-01-08 at 04:27 PM.
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Old 05-01-08, 04:25 PM
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Originally Posted by afterburn27 View Post
That makes sense. Using the RPM & PFC dwell values you posted you can calculate a charge time and it looks to be pretty constant over the RPM range. However, it doesn't seem to line up with the actual charge times you recorded in your log.

Interesting stuff none the less. I wonder what the ideal saturation time is for an FD coil.
True. The higher RPM values do match up relatively close. I need to put a scope on it instead of relying on the LMA-3. I think there is certainly some loss of accuracy there. It was just the handiest way at the time to take a quick look at what was going on.
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Old 06-02-08, 02:15 PM
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Revisited this with the scope today. Below are the results. The scope measures actual charge time in mS instead of trying to convert a measurement from the Innovate logger. I'm confident these measurements are pretty accurate. The scope would fluctuate slightly by about .25mS up or down at each measurement but no wild swings at all. The "Theoretical Charge Time" is calculated by assuming that the Datalogit dwell number is indeed a measurement of degrees of crankshaft rotation. Such that for 1k RPM for example 12* would represent 360*/12* equals 30. At 1k RPM one revolution takes 60mS. Dividing 60mS by 30 nets us 2.0mS which is exactly what the measured charge time is. The numbers are very close. I can't explain why they begin to separate at 7k. Part of this may be that there is not an input for 7k and I extrapolated the 174* measurement assuming the calculation is linear between 4k and 8k. Also, I didn't want to hold the engine at 7k for an extended period of time so the measurement may not be the most accurate.

In the end I feel safe in assuming that the Datalogit numbers do indeed represent degrees of crankshaft rotation and will program accordingly from now on.

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Old 06-02-08, 02:28 PM
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This is pretty interesting... people reading this should remember that adjusting the dwell time has no effect on charge time if using "intelligent" igniter's to include any brand or type of CDI, such as the twin power, MSD etc.
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Old 06-02-08, 02:34 PM
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True. FWIW I have been measuring this on a customers car equipped with the 034 inductive coils.
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Old 06-02-08, 05:32 PM
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^^ that explains why changing dwell did jack **** then.
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Old 06-18-12, 06:06 AM
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I just found this thread out of my own interest in dwell times. Ludwig, I've usually found that when the powerfc has a 0-255 range, it usually represents 0-100%. Would that make more sense in your crank calculations?

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Old 06-18-12, 01:11 PM
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what PFC number in the 0-255 range represent 3.0, 3.5, 4.0 and 4.5ms dwell???
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Old 06-18-12, 07:22 PM
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Originally Posted by thewird View Post
I just found this thread out of my own interest in dwell times. Ludwig, I've usually found that when the powerfc has a 0-255 range, it usually represents 0-100%. Would that make more sense in your crank calculations?

thewird

Not based on what I measured with the scope. When I used the numbers as degrees of crank rotation, the charge time jived with the available time in each rev range.
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Old 06-18-12, 07:24 PM
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Originally Posted by neit_jnf View Post
what PFC number in the 0-255 range represent 3.0, 3.5, 4.0 and 4.5ms dwell???

That depends on the RPM range.
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Old 06-18-12, 07:24 PM
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Originally Posted by arghx View Post
^^ that explains why changing dwell did jack **** then.

Old post, but what could you of meant by this?
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Old 06-19-12, 06:54 AM
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Originally Posted by slo View Post
This is pretty interesting... people reading this should remember that adjusting the dwell time has no effect on charge time if using "intelligent" igniter's to include any brand or type of CDI, such as the twin power, MSD etc.
Originally Posted by arghx View Post
^^ that explains why changing dwell did jack **** then.
Originally Posted by C. Ludwig View Post
Old post, but what could you of meant by this?

I am assuming arghx is using an inteligent igniter and that is why changing the dwell times in the pfc had no effect on his ignition system.
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Old 06-05-15, 03:55 AM
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Originally Posted by arghx View Post
Ok this is weird. So I'm looking through my Banzai Racing PFC basemap (for a series 4 Turbo II) and all the dwell settings are set to 30, both leading and trailing. I wonder why that is. I know that two trailing coils are essentially spliced together due to the differences in FC and FD ignitors, but really the two generations aren't too different in the way they do ignition from what I understand. I also wonder if playing with those settings could improve spark performance. I have been getting intermittent ignition blowout despite having an MSD 6AL box on the leading (FC) coil and B10EIX plugs gapped to .28 .

I wonder what the relationship is between optimum dwell time and plug gap, if any.
Old topic, I know. I have a customer with an S5 with AP Engineering PFC and like you say both leading and trailing dwell is set to 30 everywhere. for leading this doesnt interest me as the car is running a Dynatek ARC-2 with stock leading coil, but for the trailing side its worrying me a bit since I have no clue if this number is meant to be there for some reason because of the funny trailing coils setup on the FC or if its just running way to less dwell at high rpm.
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