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"Spurious Signal"-Ignition on separate circuit?

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"Spurious Signal"-Ignition on separate circuit?

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Old 06-14-18, 03:18 PM
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"Spurious Signal"-Ignition on separate circuit?

I recently built an MS2 V3.0 with 4 individual ignition outputs to run LS2 coils using the "resistor method" in the megamanual.

The manual states that with this method the ignition coils will receive a "spurious signal" from the MS when powered on. I take this to mean that the ignition circuits will send out a short 5v signal when powered on, thus causing the coils to dwell and subsequently release a charge. I am guessing this isn't a major issue otherwise the internet would be filled with horror stories, which it doesn't seem to be. However, it does concern me so here is my question:

The manual recommends wiring the relay for the ignition coils to the same signal that turns on the fuel pump(s). I get why that is the safe way to do it. However, my car is a race car and so I have a simple switch panel for necessary items, and of course a master kill switch.

Why not then wire the megasquirt with it's own relay and have a separate relay for the megasquirt. Then I just turn on the squirt first and then the ignition second so the initial bad signal won't cause a spark.

Any specific negatives I'm not thinking about? Just curious any thoughts.
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Old 06-15-18, 09:48 AM
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152 views and no responses huh.

Well, I'm going to go ahead and wire the coils to their own separate relay and I'll let everyone know if my car blows up.
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Old 06-18-18, 10:40 AM
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Since the engine was sitting & off, there will be no combustible mixture in the cylinders, so a quick discharge when the ECU powers on won't be a problem for 99.9% of installations. Its only if the engine was forced to stop while still being fueled, and attempted to be restarted quickly enough that there was still some compression left, and that there was not a ton of internal exhaust residual at the low speeds the engine spins through as it stops that it could possibly be a problem. Basically, if you were to very abruptly stall a carburated engine with an MS for ignition, that has very low-overlap cams, and almost immediately cycle power to the MS then it could maybe possibly cause a single cylinder to fire once if the engine stopped in the right spot. So, a very low chance overall, and basically 0 chance if the MS is controlling fuel too.

There shouldn't be a problem with wiring the coils to their own switch, it won't harm anything at the very least.
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Old 06-18-18, 11:51 AM
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Originally Posted by toplessFC3Sman View Post
Since the engine was sitting & off, there will be no combustible mixture in the cylinders, so a quick discharge when the ECU powers on won't be a problem for 99.9% of installations. Its only if the engine was forced to stop while still being fueled, and attempted to be restarted quickly enough that there was still some compression left, and that there was not a ton of internal exhaust residual at the low speeds the engine spins through as it stops that it could possibly be a problem. Basically, if you were to very abruptly stall a carburated engine with an MS for ignition, that has very low-overlap cams, and almost immediately cycle power to the MS then it could maybe possibly cause a single cylinder to fire once if the engine stopped in the right spot. So, a very low chance overall, and basically 0 chance if the MS is controlling fuel too.

There shouldn't be a problem with wiring the coils to their own switch, it won't harm anything at the very least.
Gotcha. Thanks toplessFC. My main thought would be that this condition might affect rotary more than piston motors since they generally run richer and don't have the 4 separate cycles like a piston motor.

I'm crossing it off my list as a non-issue though. Too many real issues to deal with
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