Wiktionary
vb. To modify the engine of a motor vehicle to give a higher performance than the specifications.
WordNet
Usage examples of "soup up".
After all, the rules made it so that the final limiting factor was physical ability, not tinkering knowledge to soup up an airors.
And soon afterward found ourselves in monkeysuits wandering around in thick pea soup up to our navels (if we could have located our navels without a map, surrounded as they were with lifesaving devices) waiting for somebody to open a door.
I used to steal cars and soup up the engines just for the fun of it.
Yossarian put another can of soup up to cook and ate that too when it was hot.
Michael took soup up to his grandmother and returned to report that she was ready to sleep.
Painting this stuff on was easier than customizing the hot rods he used to soup up as a kid.