To ensure your engine works properly, you need enough engine grounds. To do this, you want electrical continuity between the transmission, engine, and chassis.
Disclaimer: Measure where you want your grounds BEFORE ordering wire. All you really need are three braided ground straps. You could use regular stranded wire but don’t forget that your engine is going to vibrate quite a lot in comparison to your chassis. Copper likes to work harden and break as it is. Braided wire is more resistant to damage from this vibration.
There’s also no such thing as too many grounds or too large of wire. For this, bigger is better.
Testing a Ground
You’ll want a cheap multimeter to make sure your locations are actually grounded. This also lets you check if you have removed enough paint to get good contact.
To perform the test, set your multimeter to the low ohms reading, which is identified by the Omega symbol. You can see it in the picture of this cheap Harbor Freight multimeter. Your chosen ground spot should have a resistance of less than one ohm. Touch one probe to where you’re planning to bolt your ground wire, and the other end to bare metal on the car’s chassis. If you see a low reading like this, you’ve found a decent ground location. If not, try sanding or cleaning the target bolt location further.
KMiata recommends grounding the engine block, timing cover, and transmission. I appear to have misread this, because I grounded the engine block, timing cover, and cylinder head. In any case, the transmission and engine are more than connected to each other, so I don’t see an issue here. I’ve had no problems with my setup. Your mileage may vary. If anything, my current setup is probably still overkill.
Engine Grounds – Timing Cover
I had a leftover bolt from some passenger side engine bay item, possibly emissions related. After some sanding, it worked great as a ground location for the timing cover.
I did have to trim off this corner in order to make the strap fit flush into the timing cover.
Engine Grounds – Cylinder Head
For the cylinder head, I ended up drilling a hole to fit a bolt.
This resulted in a pretty compact and clean grounding location.
Engine Grounds – Engine Block
My first attempt for the engine block looked like this. I don’t recommend this location on the block, as I think it didn’t quite sit flush with the flywheel adapter.
Reusing a stock Miata ground bolt location here was fine though, at least after some sanding.
The second attempt worked better. I leveraged a pre-existing tapped hole in the block, along with an extra bolt from the hardware store. I did have to watch out for the longer strap getting caught in the steering column.
Engine grounds are very important for electronics-heavy motors like the K24. You can’t really overdo it. I am happy to report that I’ve had no issues so far with this setup.