Answer: soupup

AnswerCrossword Clue
SOUPUPImprove, slangily
SOUPUPImprove, as a motor
SOUPUPEnliven
SOUPUPIncrease the power of, as an engine
SOUPUPAdd horsepower
SOUPUPAdd juice to, as an engine: 2 wds.
SOUPUPMake more awesome, as a car
SOUPUPImprove a muscle car engine
SOUPUPGoose the auto engine
SOUPUPMake more powerful