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Wire size for wasted spark coils?

EFI Wiring Fundamentals

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I'm trying to work out the wire size for wasted spark coils +12v power wire?

The bit I'm struggling with is trying to understand the current draw on 2 wasted spark coils running a V8 engine.

The coils are Bosch 0221503407. (see attachement)

Firing order 1-5-3-7-4-8-2-6

The ecu is omex 710 with built in Ignition amplifiers.

The ecu universal wiring is set with 4 ign wires going to the 2 coils. So that part is taken care of, but what wire size should the +12v coil power wire be?

Also, how would I connect the rev counter to these coils?



Your 12V supply wire to each coil will want to be able to handle about 7A. 18AWG would do.

The tacho would generally be driven from an aux output on the ecu rather than from the coils in a multicoil setup.

Thankyou Adam.

I've now found the documentation for the ecu to tacho in the manual also. Thanks.

Hi Adam,

how did you get the 7amp current figure for the coils? From the above Bosch coil attachment?

And would 18AWG handle that current ok?


Wasted spark @ 8000RPM would give a cycle time of 7.5ms. To achieve the max current on the data sheet at 14V would need a dwell of 4.0ms, this is about 53% duty cycle. So 10A x 53% = 5.3A. This would be worst case - you will unlikely need to run these at the max current and I guess you are probably not running to 8000RPM either (or if you are it wont be for very long), so in real life your current will probably be quite a bit less.

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