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Engine Ground

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paul330

Well Known Member
I have used a buss bar with numerous #8 screws as my firewall earth. I have run a #12 wire from each end (for redundancy) to bolts very close together on the firewall.

I cannot remember taking off the powder coat at the back of the engine mount and am concerned that I may not have a solid ground between the engine and the airframe. I am therefore considering running a #2 cable between the engine strap and one of the firewall bolts mentioned above.

Would that be OK or is the current from the starter going to be too much for the thin firewall and fry it?
 
Here is a picture of my strap. It goes between an engine case bolt annd a bolt on my forest of tabs. The engine mount does not have a ground on it.

My oil and fuel lines function as a secondary ground. Look up "Fairiday (sp?) Cage", if that makes you nervous.
 
Paul, I see you are building a 10------

Bill's photo shows a firewall mounted battery, and his wiring is great for that setup. Note that with the firewall battery, he is also able to use the same bolt for the battery ground as the forest of tabs, and engine ground strap.

In the 10, the design uses the airframe as the ground between the battery in the rear, and the firewall area-------you need to get the engine load path back to the firewall, including the heavy load of the starter.

What I did is to install a #2 wire from one of the unused vacuum pump studs to the bolt holding the master cylinder reservoir to the firewall.

Been working fine for the last almost 4 years--------yes the thin stainless sheet of the firewall is more than enough to carry the electrical load.

Pretty much any thing to get the load path from the engine case back to the airframe should do just fine as long as it is capable of handling the starter current.
 
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Thanks for the replies.

Yes, Mike, I understand the system for the -10. I was simply worried that the thin SS firewall might not stand the 200A or so when the starter cranks. The engine mount bolts, of course, attach to something a lot heavier but, as I said, I can't remember taking the powder coat off the back of the mounts.

I want to avoid the situation where the ground is poor and the current gets to earth via the cables and piping - despite Bill's assurances.

I still fear the thin SS is likely to burn around the bolt with all that current. How about a 1/8" aluminium doubler plate behind the bolts..........?
 
paul330 said:
I have used a buss bar with numerous #8 screws as my firewall earth. I have run a #12 wire from each end (for redundancy) to bolts very close together on the firewall.

I cannot remember taking off the powder coat at the back of the engine mount and am concerned that I may not have a solid ground between the engine and the airframe. I am therefore considering running a #2 cable between the engine strap and one of the firewall bolts mentioned above.

Would that be OK or is the current from the starter going to be too much for the thin firewall and fry it?
Click to expand...

Paul,

You should ground the engine case to the firewall with a thick wire - don't rely on the engine mount. The firewall has sufficient area to take the current - but no harm in providing a doubler. I think its rather poor practice to use flexible hoses as an electrical conductor, even as a back-up.

Pete
 
penguin said:
Paul,

You should ground the engine case to the firewall with a thick wire - don't rely on the engine mount. ....Pete
Click to expand...

Don't use a "wire". The connection between the vibrating / moving engine should be made with a grounding strap. The reason is, the grounding strap is much more flexible and less prone to breaking.
 
Understood.....

I am using the straps to ground from the engine to the mount. I then intend to run a #2 cable from one of those points on the mount to one of the firewall ground bolts.

From all the info here, that should be good.
 
You can use a wire if it is the correct type and size, locomotive cable or welding lead works as well as straps. Reason is that they have a great many strands as a braid does. With a lug properly crimped on they will provide a good ground and last as long as any.

Bird
 
Paul,

What I did in my RV-10.......

#2 from the rear attaches to the bolt holding the forest of tabs to the firewall on the cabin side.

#2 from the same bolt on the FWF side going to an engine bolt

#2 from the same bolt on the FFW side going to the engine mount

no circuit will depend on the stainless to conduct current

I do have a B&C forest of tabs mounted on both sides of the firewall installed with a common set of bolts.


bob
 
I am partially rebuilding the electrical system in my RV-8 during its avionics MLU. The original builder installed what looks like a #2 or #4 cable between the firewall mounted battery's negative terminal and a bolt in the lower crankcase. There is another #6 or #8 wire that leads from the negative terminal aft to what looks like a negative buss attached to an aluminum frame that supports a small gauge sub panel in the lower right footwell just aft of the forward luggage compartment. Lots of spaghetti in there...

Anyway, I'm skeptical of this system design. I've been thinking that I should install a brass post in the firewall and run the negative terminal to that, and then run either a strap or the original cable from crankcase to the brass firewall post. A third lead would run from the brass post back to a FOT mounted behind the panel.

Questions:

A. Am I overthinking this?
B. Should I reuse the original crankcase cable, or replace it? I am concerned about engine vibration being transmitted through a stiff cable - I discovered the neg terminal bolt loose enough to be turned by hand...
C. If I replace the cable, should it be welding cable or a braided strap?

Comments / suggestions?
 
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