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Old 07-14-2008, 10:43 PM   #1
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Default MS dwell settings for COPs

So, I opened Megatune, went to the spark menu, opened dwell settings, changed the dwell control from "fixed duty" to "dwell control", changed to 3.5ms cranking, 2.5 ms running, and the ECU wanted to default to .5ms minimum discharge period so I left it there.

The car starts and runs fine, but idles a bit rough and AFRs are leaner, in the 17:1 range whereas before when cold it would idle around 13:1 until warm. Is there cause for concern or is this a by-product of more complete combustion?

Also, speaking of complete combustion, I'd been having some problems under boost which felt like spark blowout. I haven't driven on the COPs yet but I'd imagine those misses are gone. I'd also had problems where the plugs would come out with 2&4 looking black and 1&3 looking brown. Could this be chalked up to a failing stock coilpack? Time will tell, I suppose, but in the meantime I'm curious what the experts think.

Last edited by kotomile; 07-15-2008 at 01:25 PM.
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Old 07-15-2008, 12:47 AM   #2
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cranking 5.5ms, running 2.5ms
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Old 07-15-2008, 01:06 PM   #3
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If that's the case, it may be time to update the FAQ in the archive. Still, the only thing changed there is the cranking ms, which I doubt will address my problem.
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Old 07-15-2008, 01:18 PM   #4
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i have so much to change on that thing.....both MS and cops....
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Old 07-15-2008, 01:25 PM   #5
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So, how about the "minimum discharge period" field, is that correct? Am I right to think something is off with my AFRs, and would that be caused by the resultant more complete combustion or by the opposite; less complete combustion and fuel getting into the exhaust? (How does better or worse combustion tend to skew AFRs, anyway? Leaner or richer?)

Last edited by kotomile; 07-15-2008 at 03:01 PM.
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Old 07-15-2008, 03:03 PM   #6
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So - I've partially answered my own question, there is "quite a strong smell of petrol" [/hammond] coming from the exhaust. I think I'll pull the coils out and see if maybe they aren't seating right. There only seems to be a spring-like connection in there so the telltale "click" I've grown accustomed to with spark plug wires is absent.
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Old 07-15-2008, 03:54 PM   #7
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Well, it's not the coils themselves. I unplugged the COPs one at a time, pulling the clip from cylinders 1-3 netted a droop in idle speed and choppier idle, but pulling the clip on cylinder 4 showed no change. I switched the COPs between 3 and 4 and repeated the test with the same result, no change on cylinder 4.
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Old 07-15-2008, 03:59 PM   #8
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gotta be the wiring then....is not firing on #4 then.


betcha if you pull the #4 plug, itll be drenched.....might wanna fix that before you try to fire it again.
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Old 07-15-2008, 04:09 PM   #9
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Yup, I pulled the plugs and #4 was indeed the smelliest. Sav is sending me a new harness today and I'm sending the old one back to him, in the meanwhile I'll switch back to the stock ignition, the install was simple enough I don't mind doing it twice.

I want to stress here that Savington has been very helpful and did not hesitate to agree to send a new harness.
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Old 07-15-2008, 04:18 PM   #10
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like i said, dry out #4 if it's wet....otherwise its going to backfire so loud you'll wanna cry.
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Old 07-15-2008, 04:19 PM   #11
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I'm going to put my set of .025 gapped plugs back in until I reinstall the COPs.
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Old 07-15-2008, 04:26 PM   #12
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Quote:
Originally Posted by kotomile View Post
I'm going to put my set of .025 gapped plugs back in until I reinstall the COPs.

i understand that, im just saying, if #4 is flooded with gas, and you spark it, it blows the **** up and scares you.
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Old 07-15-2008, 06:14 PM   #13
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Scott, what the diff between 3.5 and 5.5 cranking ms?
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Old 07-15-2008, 07:23 PM   #14
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2 ms
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Old 07-15-2008, 07:25 PM   #15
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Quote:
Originally Posted by kotomile View Post
2 ms
I knew that was coming.
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Old 07-15-2008, 07:36 PM   #16
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it holds the charge 2ms longer before firing...so a tad stronger.
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Old 07-15-2008, 11:14 PM   #17
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Quote:
Originally Posted by levnubhin View Post
I knew that was coming.
You set me up

So I put the stock stuff back in and put the dwell back, just to drive around on, and waddaya know... tach needle jumping, ignitor gets boiling hot, idles on *feels like* 2 cylinders and then dies. The only things I didn't do was that I didn't put on that 12mm bitch-of-a-bolt on the back of the coilpacks, and I lost one of the 8mm nuts for the ignitor, and I put a little dielectric grease on the plug wires' connections. Wierd.
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Old 07-17-2008, 08:14 PM   #18
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Things are easier the second time 'round, no digging for tools, no bitch-*** bolt on the bottom of the coils, etc. And they work! Must've been some fluke with the harness.

I haven't driven it yet on them but already just moving it in the driveway idle seems smoother and throttle response seems quicker. Rain, rain, go away...
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Old 07-17-2008, 08:58 PM   #19
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Here is a scope capture which shows my stock 1.6 ignition coil running with a dwell time of around 3.7ms. The yellow line is voltage on the trigger line between ECU and igniter, and the blue line is current going through the coil primary. Ignore the fact that the yellow line is sloped- it turns out that the stock 1.6 ignitier is an amazingly capacitive load and I wasn't driving it with a very strong trigger.




So when the igniter first turns on, the amount of current initially flowing through the coil is quite low. This is because the coil is an inductive load, resistant to change in current flow. As time passes, the amount of current flowing through the primary steadily rises. As the current is rising, the electromagnetic field inside the coil primary is also growing stronger. It is this field that will ultimately produce the spark. When the coil primary snaps off at the end of the dwell period, the field suddenly collapses, inducing a high voltage in the secondary winding and causing the spark to occur across the plug gap. The stronger this field, the higher the voltage and the hotter the spark.

You can see that about midway up, the rate of increase begins to taper off slightly- it's still increasing, but not as fast. Had I left the primary on for long enough, the current would tapered to a steady state, neither increasing nor decreasing. At that steady state point, the coil is said to be fully saturated- leaving it on longer will not cause the strength of the field to increase any more.

The trick in setting dwell is to determine how long it takes the coil primary to reach this saturation point. Different coil designs require different dwell times. Those with a higher impedance (all else being equal, those which are physically larger) require a longer dwell.

In the above example, I had the dwell time set too short. The coil turned off before reaching its maximum potential current, so I sacrificed some potential spark energy. On the other hand, if I'd have left it on after the current leveled off, I'd simply be creating excessive heat in the coil, and possibly damaging it.

So, levnubhin, that's the difference between 3.5 and 5.5 msec.
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