When in the pursuit of a more potent and efficient engine, turbochargers are one of the first directions many people look. Through the use of forced induction, turbochargers increase the amount of air that is sucked into the engine to be compressed and ignited in the cylinders, leading to a more powerful engine.
For vehicles with 4-cylinder engines, single turbochargers are sufficient but when you get into V6, V8, and V12 vehicles, dual turbochargers are preferred. When talking about dual turbochargers you may hear the words “twin turbo” and “biturbo” thrown around. The following goes over what these words mean and what the different types of dual turbochargers are:
Against popular belief, the terms biturbo and twin turbo are synonyms. They are interchangeable and have the same meaning. The “bi” in “biturbo” is a prefix meaning “twice” or “two” in Latin. Twin turbo also carries a similar meaning. When used in this context, there is no difference.
Sometimes a vehicle manufacturer will use “twin turbo” or “biturbo” when describing their vehicle’s dual turbocharger system, though there is no agreed upon criteria regarding when to use one term over the other. Sometimes parallel dual turbochargers are called “twin turbos” and sequential dual turbochargers are called “biturbo”, but this is not the rule, and you will often find that they are called the opposite term.
Parallel Dual Turbochargers
Parallel dual turbocharger systems have two turbochargers that are the same size, both being assigned to their own engine bank. For example, if the engine is a V6, the first turbocharger may be assigned to cylinders 1 through 3 and the second turbocharger may be assigned to cylinders 4 through 6. If instead you have a V8 engine, one turbo of the parallel dual turbocharger system would handle cylinders 1 through 4, and the other would handle 5 through 8.
The fact that the two turbochargers are the same size is likely why this type of system is often called twin turbos. They generally have less turbo lag than some other turbocharger systems and can be found on the Nissan 300ZX and the Mitsubishi 3000GT.
Sequential Dual Turbochargers
Unlike parallel dual turbochargers, sequential dual turbochargers utilize two turbos of different sizes. Regardless of which you choose or your vehicle has, you will have to determine if your turbo is legal in the State of California. If it is part of your vehicle’s original manufacturer equipment (OEM), you should be fine.
With two differently sized turbos, sequential dual turbocharger systems can provide enhanced efficiency even at lower speeds. At lower revolutions per minute, the smaller of the two turbochargers functions. When you accelerate, the larger turbo will kick in through the use of a compression valve. Generally, the term “biturbo” is used for this type of setup.
Sequential dual turbocharger systems are often found with inline engines such as those in the BMW 5 Series. For example, the 500i is equipped with a sequential dual turbocharged V8 engine that allows it to have an impressive 445 horsepower. Another example is Subaru’s Legacy B4s. Some vehicles that are currently banned from the United States by the National Highway Traffic Safety Administration also utilize sequential dual turbocharger systems. These include Toyota’s MK4 Supra and Mazda’s FD Rx-7.
Even though biturbo and twin turbo is usually interchangeable with some trends towards either parallel or sequential dual turbocharger systems, twin-scroll turbochargers are something entirely different. In fact, twin-scroll turbochargers consist of only one turbo.
Twin-scroll turbochargers are a type of pulse-turbocharged system where the exhaust ports of the engine’s cylinders are routed to the turbine through spiral shaped channels, also called scrolls, with both scrolls being a different diameter. Doing this allows there to be no pressure interference between cylinders.
Twin-scroll turbochargers are great for improving performance during low and medium engine speed through the economical use of pulse energy. Performance during acceleration is also improved through twin-scroll turbochargers. Unlike dual turbocharger systems which are good for V6 and V8 engines, twin-scroll turbochargers are best for 4-cylinder engines. For example, the twin-scroll turbocharger found in the 2012 BMW 3 Series, not only consumes 15% less fuel than it would have without a twin-scroll type turbocharger, but it also gives this V4 engine the power of a V6.
We can not forget about the disadvantages of twin-scroll turbochargers despite all of the advantages. One of the downsides of twin-scroll turbochargers is that they are not very efficient at high revolutions per minute, nor with high loads. Additionally, they require more complicated and expensive exhaust manifolds and turbine casing than other systems.
Lastly, depending on where you live you will still have to investigate local regulations. For example, if you live in Los Angeles, you will need to know if tuning is illegal in California. If your vehicle comes with a twin-scroll turbocharger, it will be accepted by the state of California. Otherwise, you must ensure that you know how to pass the emissions test with a modified vehicle.
Dual turbochargers often come by different names, some of which include biturbo and twin turbos. In the vast majority of cases, these terms are used interchangeably. Although manufacturers choose which term they are going to use to describe their dual turbochargers, it seems that parallel dual turbochargers are called twin turbo chargers (likely because the two turbochargers are identical in size) more often.
On the other hand, biturbo is often the term used to describe sequential dual turbochargers. Sequential dual turbos are good for maintaining performance even at low revolutions per minute. Parallel dual turbos are better at reducing turbo lag. Both sequential and parallel dual turbochargers may be called either biturbo or twin turbos depending on the preference of the vehicle manufacturer.
One term you need to look out for is “twin-scroll turbocharger” because twin-scroll turbochargers are not dual turbochargers at all. Instead, it is one turbocharger with paired spiraled shaped channels or scrolls between the cylinders and the turbine. Eliminating pressure interference between cylinders makes for a much more efficient system.