How low can I safely set "Max spark duration"?

[BottleFed70]

I've found that with my current coil, I need a fairly long dwell time. Approx 4.0ms

I've ran into a problem with missfires at above 4000rpm. Playing with the dwell time only made things worse or had no change, but dropping the max spark duration to 1.5ms made it so that I didn't experience the problem until 5000rpm.

Can I safely lower the max spark duration to 1.0ms so that the engine will rev to 6000rpm? My old points system used the same coil and I was able to rev the engine up to 6000rpm no problem, so I believe the points had the spark duration pretty low at high RPM.

What are the risks of running too low of a max spark duration?

[64Vair]

Too short a duration spark will fail to light the mixture, or at least do a poor job of it. You did not say, but I am going to guess that you have a mostly stock motor, and do not have a lot of compression. High compression will cause problems, as it is hard to get the spark to jump the gap with a lot of compression. I also guess you are running MS2 as MS1 gives "Minimum" spark duration, not max.
Go ahead and make your changes, you can always go back. If the points were good enough, then the electronic should be way more than enough. Don't know what system you are using, but you should be able to remove the resistor in line to the coil, so the coil gets full 12 volt. Points systems used a resistor, (GM used a resistor wire, Chrysler used a ceramic resistor, can't remember what Ford did), so the points would not burn out. The coil would get a full 12 volts, but only for starting, then it would go resisted volts. If I were you, I would be sure to be on full 12 volts all the time.
Tom

[woh]

If you do short out the ballast resistor (assuming it is not inside the coil) then be sure to connect the + coil lead to the fuel pump relay. Not doing so will risk blowing the coil driver.

[PSIG]

On the Extra site, there are running examples (not just theory) such as a Rover V8 with 5ms cranking, 4ms running and a 0.1ms min discharge. Sparks fly pretty fast...

David

[Bruce Bowling]

I've found that with my current coil, I need a fairly long dwell time. Approx 4.0ms

I assume this is a stock points/resistor coil. These have a somewhat high primary resistance and inductance, both of which drive the long dwell time.

If you can get ahold of a lower-inductance coil then this would be of benefit. Of course this requires a inductance meter and access to ignition coils. In general, wasted spark and some of the aftermarket coils have a lower primary resistance and inductance. I measured a pretty low inductance on the MSD HEI super coil. However, these have a lower turns ratio so more charging current is required.

If you measure inductance, measure the primary circuit with the secondary both shorted and unshorted. The unshorted represents the coil charging time and the shorted value will help determine the spark burn time. Also measure the DC resistance of both primary and secondary.

To determine turns ratio, feed a low-voltage AC signal, like a 6.3 volt signal from a AC transformer plugged into the wall, into the secondary coil, and measure the primary coil induced voltage. Note that the voltage will be lower by the ratio of turns. The turns ratio tells you the secondary voltage from the flyback.

- Bruce

[PSIG]

... If you can get ahold of a lower-inductance coil then this would be of benefit. ...

Hi Bruce -

Since he stated that the points would fire the engine to 6000 RPM, shouldn't MS be able to take the same coil to the same RPMs easily and without issues?

David

[BottleFed70]

Thanks guys,

Most of my info is in my sig. MSII, V3 PCB, 2.36firmware, MT2.25

I've already bypassed the ballast resistor.

64vair: I don't think the MS controlls the actual spark burn time. I always thought that burn time would be determined by the amount of energy stored in the coil and the rate in which that energy is disapated across the distributor and plug gaps. So long as the MS fully charges the coil, I can't see how it could control the spark duration.

Bruce: Thanks for the information. Yes it is the stock style coil. I have another coil with a lower primary resistance but it's a much larger coil and I have a fealing the inductance is higher. I guess I'll need to get my hands on a LCR meter for some testing.

PSIG: Thanks for the info. Definately sounds like I can turn down that maximum spark duration setting a little more.

[BottleFed70]

Can anyone tell me what the symptoms/consequences of setting the max spark duration too low would be?

[Bruce Bowling]

Hi Bruce -

Since he stated that the points would fire the engine to 6000 RPM, shouldn't MS be able to take the same coil to the same RPMs easily and without issues?

David

Yes, you would think. Addressing the earlier question, there is nothing wrong with dropping the spark duration number to 1 ms. Or even lower.

- Bruce

[Mike_Robert]

what are the ramifications of starting a coil charge before the burn is done? The magnetic field is still collapsing and would cease to do so, stopping spark burn, if a primary coil charge is initiated before it's done collapsing, right? I'm guessing (bigtime!) it depends if your application needs the entire spark burn time to reliably ignite the mixture charge.

-Mike, definitely not an EE!