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Let’s Talk Grounds….

Technique Tim

Some people are willing to go to great lengths to avoid doing anything electrical. When complex projects arise people are more than willing to drop ten thousand dollars on a new engine and drivetrain and are surprised  when they need to touch a wiring harness which was never designed for their custom setup. Go to any car show and have a look under the hood of almost any car. Once you’re done looking at all the pristine valve covers and intake plenums, shift your eyes over to the ratsnest of wires tucked into the fender. With its countless cut wire stubs and crimp connections going nowhere, it’s like a mechanical trypophobia.

Few things can ruin someone’s day faster than a car fire and few things that can cause a car fire faster than poor wiring. Routing the fuel-return hose into the distributor would be faster, but requires significantly more effort than letting vibration and time work their magic.

Modern automotive wire insulation (Not PVC) has a maximum working temperature rating of 221℉ (105℃). For safety, wire insulation is rated at minimum ⅔ its true effective working temperature of 347℉ (175℃). In the event your insulation is burned, charred or missing due to heat. You have exceeded 650℉(343℃)(MSDS). Consistent temperatures of at least 1000℉ (537℃)* are possible from starter motors grounding through the wrong wire. 1000° is more than enough to ignite motor oil and the rest of the engine bay in addition to destroying the ground wire or rest of the harness. All this can happen in less than 3 seconds while you were cranking your car. 

*≈250 amperes 18awg 1.02mm2 free air model @ 300℉/sec normalizing @ ≈1400℉

Why didn’t my car catch fire? There are a lot of reasons but the easiest thing to say is: You got lucky!

Installing and maintaining an effective ground system is essential for any car enthusiast and not difficult. Any car enthusiast must be willing to inspect and maintain their pride and joy. At Rat2 Motorsports we can and do make custom order ground cables but learning to do it yourself is an invaluable skill.

For almost all cars a 4 awg wire is the ideal mix between current capacity, strength and space limitations. 4 awg (≈5.2mm2) can easily handle 475 amps continuous current while remaining at less than ½ its rated operating temperature. For most cars, this size cable doesn’t increase in temperature at all during use. Correctly assembling a copper cable requires both heat-shrink and compression lugs and are more difficult to hand assemble due to the crimp connections but can provide the longest lasting and most reliable connections through the lifetime of the vehicle.

Braided ground straps are a good alternative to copper cabling, they have more flexibility to turn corners due to their construction but care must be taken to avoid a poor selection. Crimped stainless steel ground straps are the ideal type of braided strap. Avoid tack welded ends and avoid aluminum or copper ground straps as these will all corrode surprisingly quickly inside the strap ends. While stainless steel does have a higher resistivity than copper, unless you plan to run more than 10 feet of strapping it doesn’t matter. If you are still worried, upgrade your stainless strap by one gauge (for 4 awg copper go to a 3 awg stainless strap) otherwise, enjoy your drive.

It’s a misconception bolt threads can’t be used to make an effective electrical connection. While it’s true rusty metal is not a good conductor and threads can be notoriously hard to clean, bolt size current ratings can be found in almost any technical encyclopedia. M8 bolts and up are rated for greater than 100 amps continuous current through their threads and are sufficient for carrying the required electrical loads of both the starter motor and rest of the car.

If you cannot guarantee the cleanliness of the threads, you will need to remove paint in the area of the surface connection. Rust prevention can be achieved through the application of a good silicone dielectric grease to the surface. In this case “A little dab'll’ do ya” Note: Threadlocker is non-conductive and will prevent an effective electrical connection. If you need conductive thread locker and no, you really don’t, Loctite 3888 can take whatever you can throw at it. If you have a paint job you particularly care for, its often worth your time to make sure you have extremely clean threads to protect against needing to scrape away paint. Chasing a thread with a tap is a good way to clean large debris out of the threads. Going up a size by tapping a new hole is also effective. Rotary brass wheels (the type for use in dremels) are decent at thread cleaning. Never attempt to use a sheet metal screw to mount a ground cable as this provides an exceedingly limited contact area. Nothing in a car should ever be affixed with a sheetmetal screw. If a new connection needs to be made in sheet metal, weld a nut to the backside whenever possible. For non-welding operations involving sheet metal you must scrape paint from either the front or rear of the metal. It also needs have a vibration resistant nut or lock washer on the rear and a correctly sized bolt head (flanged) on the front or appropriate washer.

Because ground connections are often forgotten when removing or installing an engine. Having a cable which is physically strong enough to resist damage or breaking is extra insurance if the worst happens and you drop your engine with the cables still attached.

Tips for grounding the vehicle back to the battery.

  • If the starter bolts to the block, ground the block to the battery/frame. If it bolts to the transmission, ground the transmission to the battery/frame.

    • The ground cable needs to be at least as big as the power cable to the starter motor. Current in equals currents out!

  • Connect the cylinder head to the battery/frame.

  • Connect the chassis to the battery. (If your car has a separate frame and body, make sure both the frame and body are both connected to the battery)

  • Avoid daisy chaining ground straps

If you would like some custom ground kits made for your car, please visit the Rat2 Motorsports website


If you are interested in learning about this or other related topics don’t hesitate to request an article topic for the future.

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