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Old 04-03-2009, 02:22 AM   #1
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Default Dwell setting for my MSM?

Short version, per the headline: what numbers should I use for cranking dwell and running dwell on my MSM (parallel MS, stock coils)...?

Long version: my settings were 3.5 and 2.5 since the beginning of recorded history (last summer when I first 'squirted). Also I've had misfire problems at idle for almost as long. From last summer until now, I've tried myriad things to address the misfires .... new plugs, new wires, new coils, DIY spark mod (which worked for a short time and then stopped working), several fuel injectors, a whole new injector harness (which it turned out was needed anyhow) .... plus a bunch of headaches, a bunch more money, and massive bunches of my time and Paul's.

As a last resort I decided recently to attempt COPS. The concept of "dwell" came up during this process thanks to y8s, Jason, and Sam. I have to admit that until today I had NO CLUE what dwell is all about. Sam explained it to me in an IM tonight. Then he said my settings were too low, so we both checked with AbeFM who also had an NB. He suggested that I use 5.0 and 4.0.

So with very little charge left on my laptop battery, I rushed outside and uploaded the changes in a new msq (didn't have enough juice to do any datalogging)....

Initial impression: car started MUCH stronger .... it no longer lingered on lean AFR for a few seconds as it's been doing the past few months, but instead jumped right to properly rich settings .... good stuff.

Then I warmed up the car and went for a short highway drive, maybe 15 minutes at 70mph .... exited, stopped for a red light .... no misfires!!!!!!!

One negative thing: this change somehow mangled the idle itself. Before, despite the misfires, idles was stable around 850. Now it's not misfiring but the idle kept jumping up and down between nearly stalling (600) and way too high (1300) ... what's going on here???? (Normally that only happens for a short time after I turn the car off and start it again soon after, because of IAT heatsoaking, but that wasn't the case tonight.)

I repeated this test a couple of times. Highway driving, exited, no more misfires but retarded idle. LOL so I traded one problem for a new problem.

Meanwhile, I also cleared the misfire CEL before I left my driveway, and it stayed off the whole time tonight. So that's a good indication.

If I can fix the idle, then I might sell the COPS that I just bought .....

PS - I read the thread at https://www.miataturbo.net/forum/t11744/ and only partially understood.

Last edited by evank; 04-03-2009 at 02:42 AM.
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Old 04-03-2009, 08:21 AM   #2
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Dwell is easy.

Understand that the plugs fire when the coils turns off, not when it turns on.

each individual coil is actually two coils. I don't mean 1/4 and 2/3, I mean an ignition coil consists of a primary winding (12v) and a secondary winding (high >10 kV). The primary is what we control with the ECU, and the fields we generate with it wind up in the secondary.

When the primary side of the coil is conducting, it's building up a big electromagnetic charge inside. The longer it stays on, the stronger this field gets, until it reaches a maximum point, called saturation. After that, more on time is just wasted heat. When the coil primary turns off, that big field collapses, but it can't conduct out the primary, so it conducts out the secondary, causing a spark.

Ok, so more dwell = more "on" time before firing = stronger EM field = hotter spark.


So yeah, your previous dwell times were a bit short, so the coil didn't have an opportunity to build a huge field, ergo, a weak spark.
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Old 04-03-2009, 09:15 AM   #3
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Quote:
Originally Posted by Joe Perez View Post
your previous dwell times were a bit short, so the coil didn't have an opportunity to build a huge field, ergo, a weak spark.
Thanks for the additional explanation. I was just telling Sam last night that it's my nature to ask "why" instead of just accepting "do this".

And it's ******* amazing. Paul and I, sitting around since last summer, cursing the supposedly "weak" MSM coils.

Looking forward to going out to my car later and making sure last night's results were not a fluke!

So ummm ... why did this change also butcher my car's idle and how should I fix that?

Last edited by evank; 04-03-2009 at 11:30 AM.
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Old 04-03-2009, 11:11 AM   #4
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What timing are you idling at?

How are the AFR's at idle?
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Old 04-03-2009, 11:29 AM   #5
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I think idle timing is 17, not 100% sure.

AFR is good, 12-13, when in the 850-950 rpm range. It's just not staying in that range, bouncing all over the place. Today I will do a bunch of tuning. Open to ideas of course.
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Old 04-03-2009, 12:38 PM   #6
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Bump idle timing to 20 and try playing with the idle set screw to stabilize the RPM.
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Old 04-03-2009, 03:12 PM   #7
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Quote:
Originally Posted by Saml01 View Post
Bump idle timing to 20 and try playing with the idle set screw to stabilize the RPM.
That helped slightly.

I'm going to table any additional changes until Jason can test my stock coil (I think I have an extra somewhere) or until someone else chimes in with definitive numbers to use.
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Old 04-03-2009, 03:47 PM   #8
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Stock? I've tested '01 coils and posted the proper dwell numbers.
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Old 04-03-2009, 07:07 PM   #9
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Big public thanks to Jason .... got my numbers from https://www.miataturbo.net/forum/t11744-3/#post390845.
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