What size wire should I use

2 ga. or 4 ga.

To the engine? I would go 2 ga to the engine. You can get away with 4 but 2 will get a LOT more current to the starter.

Jon

depends on how close the batts are to the engine, the futher away they are the thicker the cable should be to handle the amperage. if the cables are longer than 4 ft use 2ga.

Use the 2.
I actually have a set of 1 gauge to my most distant batts. (out of 4)

Use 4 gauge for your power amps!

listen to gusman