Soup up generally means "to enhance" in a slang format like so: He was going to soup up the engine in his car. Meaning he would be adding parts to improve the performance of the engine in his car. This also typically implies that the "upgrades" would be substantial. In this context it would be understood by a listener to most likely bring the car's performance up to a high performance standard. It means similar things to: "beef up", "boost", "enhance", "upgrade", etc.

soup up = to make something stronger, faster, bigger, harder. Most commonly used when creating a player in sports video games, or make cars go faster. Examples: You need to soup up your player. He is so weak right now. The Gulf of Mexico soups up the hurricanes that form there. The mechanic is going to soup up your car