Battery and ground connections

[cmonref]

I am planning to MS a Porsche 911 and am concerned about the battery and grounding connections available in the engine compartment. There is sufficient switched 12v power in the engine compartment, but the chassis-return ground path to the battery in the front worries me -- too much chance for noise. To insure good power and grounding, I am considering running battery and ground wires to the battery requiring about 15' of cable. In this planning, I would ask folks two questions.

1. For those with rear engine/front battery configurations, how did you solve the keep noise in your MS system to an acceptable level?

2. For dedicated battery and ground wires from front to rear, how many of what size conductors would be advisable in order to achieve an acceptable resistance, minimum inductance, and other electrical factors that would be introduced? (I'm assuming that both the battery and the ground wires need to be multiple conductors, say three 12ga wires in each bundle.)

Brian Haggard
Chester, VA

[Matt Cramer]

Our Miata is the opposite, front engine with a trunk mounted battery. On these cars it seems to work best to ground the Megasquirt to the engine block and the engine block to the battery. We tried grounding the MS to the battery directly and it turned out to be noisier.

[cmonref]

Interesting that there is more noise if you ground MS directly to the battery. Does the Miata have a chassis ground that you supplement with a dedicated ground from the engine to the battery -- if so, what size wire do use for that ground line? Likewise, do you run a dedicated line from the battery positive to MS, or do you take power from the normal engine system?

[john.p.clegg]

Well fancy that,I've just been investigating why I've problems starting my Lotus Elan Plus2,battery at the rear,and it turns out that during cranking the Ms voltage can drop to 6v,(just been looking at the trace in MLViewer)this is fed from the starter solenoid in the engine bay,am looking at a direct feed from the battery....

John

[Matt Cramer]

The Megasquirt is using the factory ground wires; IIRC they're something like three 14 or 16 gauge wires. There's a sort of frame that runs from the engine to differential, and the battery negative cable is connected to the engine / frame / differential assembly. I can't recall exactly where it connects off the top of my head.

[cmonref]

Matt,

Thanks -- the info I was seeking was the number and size of the ground wires between engine and battery. Multiple wires (like Bruce suggests) and big enough to keep the resistance down over the 16' run.

Brian

[FixItAgainTony]

Brian,
My Fiat Spider has a set up is similar to the Miata, front engine / rear battery. I just pulled a positive power lead from a junk car and routed that from the battery to the engine block. That worked out easy in terms of gauge (if good enough for +, good enough for -) and for connecting to the battery / engine (connectors already on the cable). I can't say that I saw a lot of before / after difference. Biggest effect is probably during cranking. MegaSquirt is grounded to the engine block through the old Bosch harness which is probably my biggest noise contributor. The original ground was 1 foot of battery cable welded to body of the car.
I ran another Fiat (X1/9 - rear engine and front battery, the car frame was used to conduct ground up to the battery). MS was grounded to the engine block; I had no electrical problems on the X1/9. The X1/9 is a smaller but heavier car than the Spider - it also had much better electrical system in general.

-Charles.