The main electrical ground for the engine is the main ground cable from the block to the chassis to the battery.

The small ground straps at the rear of the head help to reduce RF* interference (noise) created by the ignition and the pulsing injectors. (Keep in mind that Volvo employed the "radio suppression relay" up near the engine to reduce the radiated interference from the injectors that can reach inside the car to the radio.)

RF noise will have wavelengths on the order of a meter or two, which is about the distance from the main ground cable to the distributor. Thus, the distrubutor area of the head is NOT grounded at those wavelengths (even though it IS grounded for DC).

That means that electrical RF noise can be radiated into the harness, and this can possibly reach the radio -- or maybe even interfer with other systems, such as the injection. Those ground straps help to suppress RF noise that would othereise be radiated under the hood.

* RF = Radio Frequency

--
Don Foster (near Cape Cod, MA)