i remember when i put 98 octane fuel (in america i think you call it 92??) i took it out on the highway and put it in 4th gear pulling 4.5k rpm (only has a 4 speed gearbox atm) i remember a big puff of thick black smoke coming out the back, after that it idled so much better and got better fuel economy!!, i felt sorry for the person behind me though
