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Practical Motorsport Wiring - Professional Level

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Looking at building my first ground up concentrically twisted harness (I have built many "normal" harnesses and also repaired some motorsport stuff... but this is a step up for me).

I am trying to choose my core. I have 5x 10awg wires (1x injector +12, 4X for coil ground and power (even/odd V8) along with 6 shielded cables (knock x2, DBW (uses two), cam/crank).

Should I choose the 1x injector 10awg as the core, and then start around that with the rest of the big stuff? or can/should I twist all the 10awg stuff and start with that as my core? is it a big deal to have the source of the highest noise (probably) directly next to the cables I am trying to shield?

Depending on the final layup, I could split that injector +12 into 8x 18 gauges if I needed to (vs fill wire). My original intention was to keep that single 10awg and splice behind a transition boot at the harness area.



Hello Jordon,

I would try running one shielded cable for the core and seeing how the other 5 lay around that. As for the 10AWG conductors, I would use more, but a much smaller AWG. What injectors are you using? Most late model injectors are high impedance and can be run (individually) with 22AWG conductors. Also, what coils are you running & how high will the engine rev? Because if you are below 7000rpm running 4 coils per 12V supply & ground, you will likely only have one coil dwelling at a time (depending on dwell time also), so you should be able to go a fair bit smaller than 10AWG for those, making it easier for you to construct. You are using M22759/32 to construct this harness?

Sorry for the delay, for some reason I wasn't notified of the response.

Thanks Callum. Yes I am using M22759/32. Good point on the duty cycle and timing.

For injectors I actually did what you suggested in my planning and went to all 22awg as it helps with the layer design

I left the 10awg (4x, power/ground for each side of the engine). I was going off the recommendation from holley in this case who recommend a 40amp relay PER side, but obviously I don't know the real current draw.

Using holley 'smart' coils (common), V8, M150 ECU

Even if you wanted to run separate 20-18 AWG 12V & grounds to each coil individually from the relay, I imagine it would make you concentric layer design & construction much easier.

Are you using Holley EFI part number- 556-112? The documentation for these says to use 20-18 AWG conductor for each coil & splice them into one 12-10 AWG conductor. So according to Holley's recommendation the 20-18 AWG conductor is able to support one coil. If you are running sequential ignition, keeping RPM below 7000 RPM & dwelling the coils for 4 ms, while supplying & grounding the coils groups of 4 (ensuring they are grouped correctly), only one coil will be dwelling at a time on each 12V supply & ground, which means you could run 20-18 AWG for the 12V supply & ground, per group of 4 coils. Also, the 556-112 appear to be ING1A coils (which draw between 5-7A average on a 4-4.5ms dwell), which 20-18 AWG M22759/32 will comfortably support (even Zac uses 20 AWG in the RX7 worked example).

Thanks for the analysis, makes sense. Yes that is the correct coil. I presume since the firing order is 1-8-7-2-6-5-4-3 I would tie 1-7-6-4, and 8-2-5-3

Wasted Spark with that firing order will be done using the following pairings.

1 - 6

8 - 5

7 - 4

2 - 3

Attached Files

Yeah that's right Jordon! If you have one of the 12v & grounds for coils 1-7-6-4 & the other 12v & ground for 8-2-5-3, only one coil will be dwelling at a time (provided you keep below 7000rpm and maximum dwell at 4ms). If you were running 4.5ms of dwell you would need to stay below 6600rpm (or use either an 18AWG for each 12v & ground, or two 20AWG for each power & ground if you wanted to still hit 7000rpm.) Hopefully that makes sense.

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