MS3Pro: Why ground to engine instead of battery?

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PelleX
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MS3Pro: Why ground to engine instead of battery?

Post by PelleX »

Hi all, new board member here!

An electrical theory question: I'm curious exactly *why* I should ground my MS3Pro to the engine and not to the battery negative post? At first it would seem equivalent?

I have a Chevrolet 350 Small block with aluminum heads. The alternator is mounted to passenger side head. It's a cheap aftermaket unit that seems to work.
I have 25mm2 (~AWG #3) welding wire cables to an Optima Yellow Top battery, cables are around 1 meter each (3 feet).

I was thinking it would be a good idea to get both +12V power and ground to the battery, insulating the somewhat from the noisy alternator. Holley and Edelbroch recommends this for their respective systems.

Apparently there is a more elaborate reason that I'm not aware of, as the MS3Pro documentation and this forum keeps emphasizing to use the engine as ground.
Is there something like high frequency impedance or something else? I mean, the head is a big block of aluminum, very large surface area and cross section, does that have and influence? The leads will be 1,5m (5 feet) shorter of course, but then the battery positive is at the other side of a very large copper wire.

Please enlighten me on this matter! (And I *am* going to ground to head, I'm finally getting old enough to take advice...)
Pelle Langlet
1979 Chevrolet Caprice, 350 SBC / Weiand Stealth Ram / MS3Pro / TKO 500 5-speed manual
MWPau
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Re: MS3Pro: Why ground to engine instead of battery?

Post by MWPau »

PelleX wrote:Apparently there is a more elaborate reason that I'm not aware of, as the MS3Pro documentation and this forum keeps emphasizing to use the engine as ground.
Is there something like high frequency impedance or something else? I mean, the head is a big block of aluminum, very large surface area and cross section, does that have and influence? The leads will be 1,5m (5 feet) shorter of course, but then the battery positive is at the other side of a very large copper wire.
Try measuring the voltage drop between the engine and -ve battery terminal while cranking the engine over ;)
It'll be more than you think, and can throw off sensor readings quite dramatically.
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Matt Cramer
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Re: MS3Pro: Why ground to engine instead of battery?

Post by Matt Cramer »

It's more of a rule of thumb than a theoretical reason. We've generally had less noise with an engine block ground than a battery ground. Many OEMs employ this same sort of grounding as well.
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Re: MS3Pro: Why ground to engine instead of battery?

Post by jsmcortina »

The alternator is also directly grounded to the engine so you are "closer" to that source of power too.

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ashford
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Re: MS3Pro: Why ground to engine instead of battery?

Post by ashford »

actually it is not so theoretical. all grounds on the ms are internally connected. so lets say your ignition modual signal ground is also physically grounded, if you have 2 volts drop across the batt cable during cranking you will also have 2 v drop across a signal ground to batt ground, and whatever current is required to make a 2v drop across the gauge of wire you are using is what current is going through your wire/ printed circuit of the ms during cranking
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Re: MS3Pro: Why ground to engine instead of battery?

Post by jsmcortina »

Yes, this is certainly the case with HEI7 where the module grounds to the engine through the distributor body. Grounding the Megasquirt to the battery or body instead of the engine block would likely cause a non-start situation or erratic timing.

James
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xrattiracer
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Re: MS3Pro: Why ground to engine instead of battery?

Post by xrattiracer »

it is important that all the grounds terminate at the same place, as has been hinted at. for engines where there are engine grounds that cannot be moved elsewhere (such as alternator or ignition module, single post sensors, etc) that means the engine block. my setup has none of those (the alternator actually has its own ground wire) so i tie everything together at a frame ground (battery is rear mounted).
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Re: MS3Pro: Why ground to engine instead of battery?

Post by elaw »

This does not directly pertain to the OPs question but for completeness: the absolute WORST thing to do would be to ground the Megasquirt both to the engine and the battery or frame.

If you did that and the engine's ground strap(s) failed even partially, cranking and alternator-output current would try to flow through the Megasquirt. And there's probably no better way to ruin a Megasquirt than to push 200 amps through the ground circuit... :(
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Re: MS3Pro: Why ground to engine instead of battery?

Post by dragbug »

jsmcortina wrote:The alternator is also directly grounded to the engine so you are "closer" to that source of power too.

James
There would be no ill effects if the MS is grounded at the same point on the engine block as the alternator correct?
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PelleX
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Re: MS3Pro: Why ground to engine instead of battery?

Post by PelleX »

Thanks all for your input!
Rules of thumb are there for a reason, and I get the point of having the MS3Pro grounded to same "source" ("sink"?) as the HEI-7 / HEI-8 module and stock 1-wire knock sensor.
Pelle Langlet
1979 Chevrolet Caprice, 350 SBC / Weiand Stealth Ram / MS3Pro / TKO 500 5-speed manual
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