[Johnny B]

Well, ya, if all your grounds were good. Or if you don't have a bunch of extra electrical load. Personally I just run a Big 1 kit. 😉
Theoretical running a ground to the engine head results in better ingnion spark. Probably would need an oscilloscope to measure the change.

 

[Johnny B]

you should notice higher numbers on the battery voltage screen

Adding the big 3 kit without a doubt helps minimize voltage drops, that said the alternator output voltage is constantly adjusted by the ECM and BCM. Putting a number on any gains would take a bit of time with a good multimeter.

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H6 battery in my cruze from cotco its a size bigger than what should be in there.

I couldn't agree more, IMHO the smallest battery anybody should be running is an H6. Especially since the H5 and H6 are the same price.