Why do you put a ground wire from the engine block to the chasis?

I seen a few people put as much as 2 or 3.

 

I think it's supposed to cut down on radio interference, but I don't know why someone would put 2 or 3

 

I can tell you from personal experience many [years ago ] when my ground strap rusted into.The transmission cable welded itself in the Drive possission one rainy day.

 

You want all the metal parts of the car to be at the same voltage for the ground. Since a car is made up of parts that are bolted together, corrosion, paint, and other non-conductors can disrupt these ground paths, leading to bad grounds and electrical problems. The negative battery cable bolts directly to the engine, since the starter is the highest current draw. Some negative cables have a pigtail that bolts to the fender, some to not. Even if it's there, the pigtail may become corroded. A ground strap from the block to the body provides a redundant path to ensure a good electrical ground for the body. In addition, since the fenders bolt to the body shell, there may be paint of rust in the path from the pigtail to the body shell anyway. To be honest, you can't have too many ground straps. Gaskets, rubber motor mounts, paint, corrosion, can all disrupt the ground path.

 

I guess that's why my Chevy truck has 5 of them. More steel, more straps? Anyway, Joe , where are the straps located on a '72 Cutlass? Chumley

 

A lot of mystery electrical problems are caused by missing or rotted ground straps. I install a 9/16 bolt thru the fender adjacent to the battery and run the ground from the battery to the bolt and from the bolt to the starter. I then run a strap from the intake manifold to the body of the car. Completes the circuit. Never have any electrical mystery problems with this setup

 

You can get ground strap material from the hardware store.

 

All of that info helped alot.
I have the pigtail of my negative cable going to the body and one from the motor to the body.