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PostPosted: April 19, 2011, 4:48 pm 
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Joined: July 15, 2010, 3:16 pm
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Location: ontario
I am in the process of figuring out MicroSquirt as ignition only and one of the figures I will have to enter in the Module is the coil dwell time. My coil is a MSD blaster Ford 4 tower Pack, a waste spark system normally used in pairs to fire V8s. I am using it to fire a regular 4 cylinder with a VR sensor. I have tried to read a bit about dwell values, I have a vague idea what it is but for the life of me I have no way to pick a number...is it 2.2 ms or is it 8 ms? I emailed MSD and they don't seem to be able to advise me. Can any of you pros offer pointers?

Thanks.

Philippe


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PostPosted: April 19, 2011, 5:41 pm 
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Joined: July 17, 2008, 9:11 am
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According to MSD's support forum, it is between 2 and 2.5mS.

see:http://www.msdignition.com/forum/showthread.php?t=10701

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PostPosted: April 19, 2011, 6:03 pm 
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Joined: December 31, 2009, 11:17 pm
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Hi Philippe,
Dwell originated with mechanical points/condenser ignition systems. It's expressed in degrees of engine rotation and it's the number of degrees that the ignition points are closed during the (720 degrees) 4 cycles.

When the points are closed (start of dwell), the ignition coil primary circuit is completed and the magnetic field is being built. When the points open, (end of dwell), the magnetic field collapses inducing into the coil secondary and creating billions of tiny sparks that look like one big one.

Modern electronics changed all of that and logic controlled transistors could vary dwell times for longer coil saturation (bigger spark) or even shortened during deceleration etc for emissions related issues.

The standard average dwell time for a 4 cylinder engine was around 52 degrees. It can only be expressed in ms (milliseconds) at a known RPM. We know how long (time) it takes an engine to rotate 600 RPM.... each revolution would take 1/10th of a second OK? That would be 100ms. At 6000RPM, one revolution would take 10ms. I would guess that electronically controlled dwell would be a constantly changing value, with that value (time/degrees) increasing with rpm.

With your waste spark system, you'd need to fire it every revolution. With a distributor it would be every 720 degrees.

This cut/paste from your Support Line

The coil charge time of the PN 8241 coil, as any DIS 'waste spark' coil is between 2-2.5 miliseconds.
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is as good as you can get to start with.

What are your options - time, degrees etc - within the program? We can likely figure out a starting point that won't fry your engine. If you have options to adjust the time, it would make sense to vary it through the RPM range with the shorter time at idle and the longer time at red line.

In case you don't understand all of this, you should know that the point at which dwell ends is the engine's ignition timing. Get it too far wrong and you'll burn a piston in a heartbeat.

Geoff


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PostPosted: April 19, 2011, 8:43 pm 
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Joined: July 15, 2010, 3:16 pm
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Thanks geoff for this remarquable explanation and Rx7 for your advice. Yes I understand this, most of it anyway. So I will try 2.4 ms and see how things go.

Philippe


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