I only use engine when I 'm going to win the car. when I don't have enough engine, I upgrade the car to max 5555 pro 0000 and save all box(es) unopen, then leave it.
I do this, because I think will better to use for one car then win it and leave another cars. well... many cars are useless, so don't need to win all of them. just win the best car or the car that you like.