[shorod]

It is "just for ground" but the ground is critical. The ground connection completes the circuit for every electrical thing running in you car. If you use a ground that is not clean, you will have electrical issues and probably noise which can screw up the PCM, sensors, and the radio. If the ground is not capable of sinking enough current, your starter motor will not be able to get full current, nor will your alternator be able to supply full current to the system.

Hmmm, where would the two circuits be that require/provide the most current? The starter and the alternator, and both are mounted to the engine block. Therefore your ground cable should also be connected to the engine block. It is also critical that you make sure the ground straps/cables between the engine and the unibody of your car are properly connected, clean and tight.

-Rod