Made the engine sound good (5)
I believe the answer is:
tuned
This clue might be a double definition.
'made the engine' is the first definition.
The answer and definition can be both related to change as well as being past participle verbs.
Perhaps there's a link between them I don't understand?
'sound good' is the second definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I don't see?
Can you help me to learn more?
(Other definitions for tuned that I've seen before include "Adjusted (eg, a piano)" , "Got engine right" , "arranged for reception" , "(Of radio) set to a frequency" , "Given the correct pitch" .)