dwell control

[zguy360]

I am currently having some problems with my ignition setup on my megasquirt II. I am trying the direct coil control without having much luck. To get the ignition to fire correctly I had to change the predictor algorithm to 'last interval'. I don't know if this is a problem. Any other setting would cause the tach to jump around and the engine to miss badly. Changing those settings, I did get it to run well enough to drive around. While trying to tune with boost (4000 rpm and 10psi), the engine also starts to miss badly and backfires out the exhuast. The a/f mixtures are 13:1 and timing is 20 degrees advanced, neither of which should cause a backfire. My question is about the dwell control and direct coil driver.

The direct coil driver limits coil current to 7 amps. Is this sufficient to get a strong spark? Pulling an plug wire and seeing how far it will draw a spark seems to indicate a weak spark. (it barely jumps out of the plug boot) compared to the stock system which would jump 1/4 inch beyond the plug boot. I hooked up my coil directly to the batter through an amp meter and it pulled 24 amps. Seven seems a bit less than 24. Would this cause a weak spark and which is causing the misfires under boost?

I was thinking that to get a stronger spark, you can increase dwell. I am currently running a max of 3ms dwell. I would think that if I increased this, the misfire should go away since I would get a stonger spark. Doing a little bit of math though, the maximum milliseconds at 7000rmp (redline) would be 2.8ms. Increasing dwell beyond this would not allow the plugs to fire at this rpm. So just increasing the dwell would not fix the problem if I want to be able to run the engine at the speeds I would like, just at the lower rpm would it be better.

Any help would be greatly appriciated. The car had no problems running these boost levels on the stock ecu a week ago, so there shouldn't be any problems with the plugs or wires. Thanks a ton for any info.
-jeremy-

[krisr]

I had a similar problem with my MSD Blaster SS (e-core style) coil while trying to get the VB921 to work, would get a very weak spark to the point the car would struggle to get out of my driveway.

I have gone back to HEI7-pin - also just installed MSD6AL on top - and you should see the sparks now! AFR went from 13.5 --> 14 and RPM increased 150rpm.

[Bernard Fife]

jeremy,

In addition to increasing the dwell (try 3.5), you can decrease the 'maximum spark duration' from it's default of 2.0.

When the time gets to short to fit the dwell plus maximum spark duration between the spark events, MegaSquirt-II starts to reduce both the dwell and spark duration proportionally to fit in the available time.

So reducing the maximum spark duration allows for more time for dwell when the rpms get high. I'd reduce it to 1.3 to 1.4 to see if that helps. You can even try as low as 1.0.

Lance.