You may have heard that BMWs are "twin turbo". However, if you look carefully on the badge of the vehicle in recent years, they are marketed as "Twin power turbo" very intentionally: there is a difference between the "Twin Power" marketing campaign and a "twin turbo" engine. Simply put, a single turbocharger would mean there is 1 turbocharger on the vehicle and twin, dual, or two turbo means there are two turbochargers on the vehicle. "Twin Power" refers specifically to a design aspect of either a single or dual turbo. That is, each turbocharger is a "Twin scroll" design. Mechanically speaking, this is the same concept as a variable length intake manifold - there is a long passage and a short passage allowing for best airflow from best of both worlds at low engine speed and high engine speed. Similarly, the turbocharger has a short "scroll" or passage and a longer passage. This allows no delay in boost pressure buildup at low engine speed yet no shortage of maximum boost potential at high engine speed.
Before this technology, a manufacturer would have to choose a smaller or bigger turbocharger accordingly. One choice was allowing no delay in boost (small turbo) and in turn sacrificing maximum boost potential at high engine speed. Or, with a big turbo, there would be lag in boost buildup at first, but maximum boost potential was available once spinning. A dual scroll turbocharger allows a smaller turbo to do both. This is a big leap in technology that allows more power more linearly, and better fuel economy.
This is also a clever marketing scheme because most people who do not know much about cars are taught a "twin" turbo is better, faster, or simply more cool. When in reality, a BMW N55 engine has a single dual scroll turbocharger with "Twin Power Turbo" embossed on the engine cover. Know your twin turbos from your Twin Power! And if you are ever uncertain about what you are getting into, call or come see us at 623-776-6126 / 8335 W Sherman St Suite #200 Tolleson AZ 85353