[yangyang]

Part of it is the prevalence of "pop and bang" maps, modified ECU configuration which changes timing and allows unburnt fuel to enter the hot exhaust and explode.

There is a purpose to this in turbocharged cars in motor racing - anti-lag - as the extra gases keep the turbo spinning during gear changes. No reason for it on public roads though.

There's also a lot of "cat deletes" (removal of catalytic converters to improve exhaust gas flow and increase engine power). Illegal, but very common to see on YouTube and just like pretty much anything else illegal on the roads in the UK these days, the police don't seem to care at all.