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Old 04-07-2016, 11:58 PM   #1
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Default GM HEI

I stuck my used dizzy in with a stock GM HEI module. It's a 7 pin from an 81 Pontiac 301. I was running a 4 pin module before and it revved cleanly to 6K. Now it gets to 4500 and the tach drops down to zero if I'm still in it. I have a theory on this, I wanted to see if anyone thought it was sane and also if there were any variances in HEI modules/a decent parts store one I could get before hitting the after market.

The dwell is controlled by the HEI module. What I think is happening is the interval is too large, and by it being too large it is effectively causing the ignition system to "go static" hence the tach seeming to lose signal. With the stuff I found in my Megasquirt settings:

Dwell=3.6mS
Minimum discharge .1mS

4500/2=2250 (crank moves at 2X speed of dist)
2250/60=37.5 (revs/sec)
37.5*8=300 (8 cylinders)
1/300=0.00333333333333 or 3.3 mS.

Add in some potential time delays within the module, coil etc. I think I'm getting dangerously close. What I think I should do is set my Dwell to:

6500/2=3250
3250/60=54.1666667
54.1666667*8=433.333333
1/433.3333333=0.00230769230771 or 2.3mS

Even though there's a dwell setting in MS, I am not sure it will be functional with the module which may have a higher dwell built in. Thoughts?
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Old 04-08-2016, 09:23 PM   #2
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Solutioned it today by putting dwell to 2.0mS. I think it's too low and I have some break up, so I'm going to put it to 2.5 and see what happens. The manual says 3.5 for a 7 pin module and 2.5 for an 8 pin module. If it's a gaff in the docs it wouldn't be the first I ever found.
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Old 04-08-2016, 10:06 PM   #3
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if your using MS why not just run points and a aftermarket CD ign box

i know i have a crane cams setup somewhere on my shelf if your brave enough to find it through the cobwebs
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Old 04-08-2016, 11:35 PM   #4
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You mean a Hyfire box? Because cheap that's why, I'd run it if I had it but that's another story. The MS is a bastard child of an MS2 I burned up on the Astro so I bought an MS1 CPU for $20ish and hacked it in some years later and added an OEM 301 Pontiac HEI I got off evilbay for $30. Really debating a ghetto surge tank made from some pipe+pump+Rochester 220 mid to long term.
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Old 05-23-2016, 11:13 AM   #5
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ed:brain fart, monday blah.
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