"...The Skyactiv-G[7] is a family of direct injection petrol engines. The engine compression ratio is increased to 14.0:1. To reduce the risk of engine knock at high compression, residual gas is reduced by using 4-2-1 engine exhaust systems..."
https://en.wikipedia.org/wiki/Skyactiv
Tuned length 4-2-1 exhaust system calculators are available online.
NB that the ...-1 pipe length and diameter is also important.
You can fool the exhaust into thinking it's exhausting into the atmosphere with David Vizard's Pressure Wave Termination Box, discussed in my other posts here.
Videos of the design how and why are available on his Youtube channel.
Videos of systems built by enthusiasts too.