I’m pleased to know it’s tight, but I’ve never heard it called a budget before. Is it true for all of them?
It varies. You'll find self financing racers with their mates/family in support rubbing shoulders with the top flight TT racers.
The Supertwins are 700cc Kawasaki twins making up to 90hp.
You need to be pin point accurate and have absolute confidence to produce consistently fast laps. It's a bit like two stroke racing, you need to keep it pinned everywhere because once you lose ground it's hard as hell to make it up again.
EDIT: it started out as just Kawasaki ER6 twins, but other twin engines are allowed now.