Wasted spark v/s direct fire?

142 guy said:

If you go to the MS Megamanual ignition section (B&G, not MSExtra), they provide a discussion and an analytical model for calculating the spark energies from a coil based upon the coil resistance, inductance, charging voltage and dwell time. I think they also link or you can find references on the web that discuss the spark energy required for reliable ignition. You already have the resistance and inductance and should have a reasonable value for your coil charging voltage. Once you find the range of spark energies required to achieve reliable ignition, you should be able to use the B&G analytical model to calculate what is a reasonable maximum dwell that you should be using for your coil. The B&G model is not perfect; but, it should provide you with a good starting point for estimating dwell if the coil vendor does not specify a maximum dwell and maximum primary current.

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I tried to find it but, couldn?t so far, can you make a link?

This is the calculation for Dwell time.

http://www.megamanual.com/ms2/configure.htm#dwell

Here is the BG link that has the spark energy calculator:

http://www.bgsoflex.com/igncoil.html

Its based upon a points type ignition system and dwell is entered in degrees rather than time. You will have to convert time to degrees which is speed sensitive. With a fixed dwell time that means that calculator can only give you a correct spark energy at the RPM that you did the conversion at. A bit clunky, but, informative.

I referred to spark energies; but, in fact what I meant was the stored energy in the ignition coil. The B&G calculator is also dealing with stored energy and not actual spark energy. In a perfect world, the stored energy in the coil would equal the energy dissipated in the spark. Reality is that because of parasitic losses and other effects, the actual spark energy will be less than the stored energy in the coil; however, it should be reasonable close. Calculating the stored energy in the ignition coil is extremely easy if you don't want to use the B&G calculator. It is given by the simple formula

Energy = 1/2(Li**2) where L is the primary winding inductance (in henries) and i is the peak primary coil current (in amps) just before triggering of the spark.

I will let you search for references about required spark energies. I recall seeing some references which suggest that 30 mJ is a good value for reliable ignition.

Thank you, I was looking in the wrong place.
It seems like slight less 0,4 would be a proper dwell.

0.4 what ??? 0.4 ms would seem a little short. How did you work that out?

142 guy said:

0.4 what ??? 0.4 ms would seem a little short. How did you work that out?

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4 ms, but I ?m not sure what values for current to put in.
The amplifier has a max output of 11,5 amp. If I put in 7 amp. dwell value raises to 5 ms and if amp is calculated to 10, dwell becomes more than 9 ms.

It?s moving forward slowly.
This is the way this direct fire coilpack would be mounted on top of engine, at first I thought I could make it more compact, but this seems to be the best solution.

I?m still confused about how to set dwell but the amplifier should have a capacity of 12 amp and the coils should stand some 10 amp each.
In that case, could I use a dwell up to 8-9 ms with no problems?, according to the calculator it would work.

per0lund said:

I?m still confused about how to set dwell...dwell up to 8-9 ms with no problems?

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General trivial

1. A given coil has a designed dwell time (charging time), and its best to not exceed this value (unless there is an electronic circuit to prevent "over-charging").

2. See: Frequently Asked Questions: Dwell

3. See: MegaSquirt-II Sequencer™ Coil Selection


4. A given coil will have a duty cycle, and its best to not exceed this value, or it will heat-up/etc.

Note - Crankshaft must rotate twice before firing a given cylinder.

Calculations for One Coil setup @ 6000 rpm in regards to coil's dwell time

6000 rpm/60 seconds per minute = 100 revolutions per second
100 revolutions times two cylinders firing each revolution = 200 sparks/second

1/200 = 5 ms total time available (subtract say 0.5 ms for spark discharge)

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Calculations for Wasted Spark setup @ 6000 rpm
100 revolutions per second
On each revolution, each coil must fire once, so 100 sparks per each coil per second

1/100 = 10 ms total time

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Calculations for Coil on Plug (Sequential Firing) @ 6000 rpm

100 revolutions per second

Each coil sparks on second revolution, so 100/2 = 50 sparks per second per each coil. (50 sparks each per second times 4 coils equals 200 sparks/second at 6000 RPM)


1/50 = 20 ms total time
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FWIW, on a V-8 engine at 6,000 RPM, with Coil on Plug setup, there is 20 ms total time available per coil cycle. Chances are, those LS2 Truck Coils duty cycle are based upon this timing parameter, which was designed for Coil on Plug (Sequential Firing) setup. Does anyone know LS2-truck-coil's duty cycle rating?