Terminating engine ground strap

[jimgreen]

I'm not clear why it is standard to run the crankcase ground to the firewall grounding point. Seems better to run it to battery negative terminal.
That gives one fewer bolted joint and less wire for the starter current.
Since everyone does it I assume there's a good reason.

 

[elippse]

I'm not clear why it is standard to run the crankcase ground to the firewall grounding point. Seems better to run it to battery negative terminal.
That gives one fewer bolted joint and less wire for the starter current.
Since everyone does it I assume there's a good reason.

I agree! I run my 14V return directly from the battery (-) post to the engine, THEN run braid from the engine to the firewall to supply a battery return for whatever is connected to the firewall. I also supply a braid from the alternator case to the engine rather than depending on the bolted joints to supply the alternator return.

 

[WenEng]

Does it have anything to do with airframe grounding?

With the rubber engine mounts, isn't the engine somewhat isolated from the rest of the airframe for overall grounding purposes? Running the negative battery to the engine block seems to me to bypass using the airframe as the primary ground and relies on the strap connection from the block to the firewall for continuing the ground from everything using the airframe for a ground. Just a guess on my part. Hope electrical gurus can better explain....

 

[pierre smith]

Either way, Wendell....

....is fine. You absolutely need a ground from the battery to the firewall, plus a ground to the engine...it doesn't matter whether it comes directly from the battery or the firewall to engine. I have only one ground strap to the battery from the firewall, so I only have one cable if/when I have to remove the battery. The flex ground strap to the engine is attached to the same bolt on the firewall as the battery ground.

Best,

 

[WenEng]

Pierre, Ok. I think I got it...... but

I thought the earlier poster said he routed the ground from battery to engine, then engine to firewall without going battery to firewall. That's what got me wondering if that was trying to use the engine as the main ground rather than the firewall (airframe). I think I got it now.

 

[jimgreen]

re: Terminating engine ground

I guess I got started on this line after thinking about the least resistance for starter ground. After reading Bob Nuckolls' article on ground paths I counted up the number of bolted joints in my starter circuit and got a surprise. Using his math for cct resistance it seems more important to reduce the number of joints than whether you use #2 or #4 wires.
In my case, using crankcase to battery ground means I don't need a #2 ground strap, battery to firewall, since it doesn't carry starter loads. #6 should be more than adequate since the biggest current won't exceed alternator output.

Just a small point, I guess.

 

[Rick_A]

Both

I have 2 braided ground straps, one direct to the battery and the other to the firewall side of the ground block. They both go to the same ground location on the engine block, so that bolt is a potential single point of failure.

 

[Walt]

Ground straps...

I use 3 grd straps: bat/eng, bat/firewall, eng/firewall

 

[rv8ch]

Failure modes

Some electronic ignitions may require a good ground in order to run, so it's important to think about failure modes in case you lose ground to the engine. Also, if you lose your normal ground but don't realize it, when you try to start the engine, the ground may become something unexpected, like a fuel line.