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Power Ground I'm Confused

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So I'm wiring up my new Atom2 and the last thing I'm doing is the Power Ground. In the manual in section 17.3 is says to attach the 2 Black wires to the back of the head. Now isn't that about the noisiest place you can put a ground as the ignition current is travelling thru the cylinder head. Now I've only wired up 2 other brand of ECU and they both said to "not use the engine" and I always assumed that was because there was no current path at the engine other than the ignition. Please explain if you would. Thanks

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Most cars will run a rather large cable from the battery negative direct to the engine, and a smaller one to the body. This means your best "source" of ground is usually the engine block because its got the biggest path to battery negative. 

By the same logic, the engine is a big solid chunk of metal that conducts current pretty well so you get very low resistance from one side of it to the other. The sheet metal in the body is very thin, has numerous stich or spot welds, and is a relatively high resistance path back to battery negative.

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Its a tube chassis race car with a solid mounted engine and I have a ECU specific ground strap from the battery, which is in the rear. Can't get more clean and direct. Still doesn't jive well with a "not noisy" ground path using the head. Link must have a reason for wanting 2 separate lines to the engine. Before I connect anything, as a engineer, I need to know the why's. I have found most installers just do what there told without wanting to know why. Especially when counter intuitive. And the big reason to ground to the block is usually for starter motor current draw.

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Second bit of logic then - your most important sensors are those in the engine. Fuel level sensor a bit off, who cares, crank position a bit off = badness. 

If the ECU is using the same reference point for ground as the sensor, then any ground offset is minimized. There is always going to be some electical noise in the engine bay and any engine sensors + wiring are going to see it. If the ECU sees the same level of noise (on its ground, not directly on the chips etc inside it), from a relative standpoint, the signal is still "correct". You can shield individual wires but you cant shield the whole engine on the ground side.

The assumption here is always that a cable designed to flow current is going to be lower resistance and more consistent than a path through the body - even in your tube frame there are welds, possible different types or thicknesses of metal etc, and certainly no shielding. By the same logic, putting the ECU near the engine not only helps packaging (usually), but means any signal wires including ground have a shorter run, reducing losses in the cable and essentially the length of the antenna(wire) that can pick up EMI.

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I will add a little bit too:

In an automotive environment, the best grounding strategy to avoid ground offsets etc is generally star point grounding, this is where all grounds are tied to a single point.  If that was our only consideration then that star point could equally be the engine or chassis (provided there were no sensors that ground through the engine).  However a big part of the consideration of that star point location is that we also need to minimse magnetic loop noise from the ignition system.  To minimise this you need to make the magnetic loop as small as possible.  For most common ignition systems this means the ignition coil secondary ground needs the shortest path possible to the spark plug electrode.  Since we are using a star point ground and our coils need the shortest ground path possible then that means our star point needs to be on the engine.

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On 3/29/2018 at 4:59 AM, Adamw said:

I will add a little bit too:

In an automotive environment, the best grounding strategy to avoid ground offsets etc is generally star point grounding, this is where all grounds are tied to a single point.  If that was our only consideration then that star point could equally be the engine or chassis (provided there were no sensors that ground through the engine).  However a big part of the consideration of that star point location is that we also need to minimse magnetic loop noise from the ignition system.  To minimise this you need to make the magnetic loop as small as possible.  For most common ignition systems this means the ignition coil secondary ground needs the shortest path possible to the spark plug electrode.  Since we are using a star point ground and our coils need the shortest ground path possible then that means our star point needs to be on the engine.

Does this include 'Gnd Out' (D5 Thunder)? or is this treated differently and does it have a different purpose?

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