This term is used amongst petrolheads and
radio-controlled car enthusiasts to describe additional optional items added to a
car to improve its performance. It includes things like
turbos,
superchargers, big
carburettors and so on.
Typical usage: "What hop-ups are available for the T-Maxx?" or "My Mini-Z has a heap of hop-ups: front one-way, turbo circuit, foam tyres and more."