This BGM BigBox was designed as replacement for the awesome exhaust of the special Vespa PX T5. The T5 engine by Morini has an adapted 125cc motorbike cylinder, which is high in performance and has special characteristics. The T5 engine reacts extremely to an exchange of the exhaust for instance. In order to achieve a similarly high torque in the low and medium revs area, the BGM BigBox for T5 was designed to fit the standard exhaust. Depending on the setup you can then reach higher revs.
This is why the BigBox fits the original engine but also Polini 152 or Malossi 172 tuning cylinders. The larger volume of the BigBox Touring in combination with tuned engines leads to significantly improved torque and performance compared to the original exhaust.