Dwelling upon the ignition (pun intended) - Miata Turbo Forum - Boost cars, acquire cats.

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Old 12-13-2007, 12:37 PM   #1
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Default Dwelling upon the ignition (pun intended)

I've noticed in many different places that people tend to write things like:
IF you are using VB921s for direct coil control, THEN you must configure dwell. (emphasis is mine)
This seems to imply that if you are NOT doing direct coil control (retaining the igniter) THEN you do not have to configure dwell. This is technically a false negation, but it still strongly suggests that the normal course of action would not involve configuring dwell.

WTF?

I've observed (by comparing IGN trigger to coil primary current) that on a 1.6 anyway, the igniter is just a simple switch, and that dwell must be controlled at the ECU.
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Old 12-13-2007, 12:40 PM   #2
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Yep you are rigth the Dwell needs to be managed via the settings on the DIYautotune site but I think they, are talking about a mod to the baord when you tap into the vb921's, to control the dwell through them. At least that is my take on it. Maybe Lazer or Mat could clear this up better though. I agree that it is abit confusingly if not contrarily worded though.
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Old 12-13-2007, 12:55 PM   #3
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I don't see why they feel it necessary to point out. Whether you're using a VB921 or not, the pulse duration of PTC0 and PTC1 needs to be controlled at the CPU. The only thing that changes when you go to direct coil control is that you install a beefier transistor and invert the polarity of the signal. It doesn't affect the timing / duration of the signal one whit.

Case in point: Ignition capture of my car running at 2,000 RPM with no load. Upper trace is IGN-A trigger voltage (from the ECU to the igniter) and lower trace is Coil-A primary current.



The coil's current profile is following the ECU's trigger signal exactly.
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Old 12-13-2007, 01:51 PM   #4
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There are only a few setups where you do not need to control dwell, and Miata ignitors aren't one of them. It's usually things like the Ford EDIS system or various "smart coils" with built in dwell control.
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Old 12-13-2007, 06:01 PM   #5
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MSD ignition boxes also don't need dwell configured. Basically just hook up the trigger and go.
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