Bi-turbocharged and twin-turbocharged are often used interchangeably during vehicle specification discussions – but there is a difference between these two terms.
While both configurations work with two turbos connected to one engine block, their layouts and benefits are what set them apart.
Below, we find out what this means for the average motorist.
How a turbo works
Turbos are turbine-driven parts that accelerate compressed air into an engine to increase power without sacrificing efficiency. This process is known as forced induction.
Forced induction pushes more air into the cylinder’s combustion chambers, thus allowing these to fill up and ignite fuel faster than a non-turbocharged engine.
“The more quickly and efficiently the chamber is filled, the more fuel can be used – and therefore more power can be produced,” states Mercedes-Benz.
A downside of this method is “turbo lag”, which can be experienced when a turbo takes a long time to spool up before sending compressed air into the engine.
This causes a hanging feeling when stepping down hard on the accelerator, before being thrown back into your seat when the rush of air finally hits the motor.
Fortunately, turbo technology has advanced drastically over the years and this propensity to hang has been cut down using various methods of turbocharging.
Two popular technologies are bi-turbo and twin-turbo set-ups.
Bi-turbo motors are also classified as parallel twin-turbo engines, as this layout features two turbochargers that are the same size working in unison.
The configuration is mostly found in vehicles with V-cylinder motors, with one turbo assigned to one bank of the engine while the other turbo takes care of sending compressed air to the opposite bank.
According to Mercedes-Benz, this configuration creates boost faster than the other layouts while being more efficient, as each turbo has less work to do.
However, the main drawback of this engine type is a greater loss of top-end power when compared to a sequential twin-turbo setup.
The traditional twin-turbo moniker is often used to refer to a sequential twin-turbo engine, in which two differently sized turbos work independently to provide boost throughout the rev range.
A smaller turbocharger is used for low engine speeds when the paired, larger turbo has not started to register any airflow. This provides a linear boost sensation while accelerating.
As you speed up the smaller turbo will spool until enough pressure has built up for the larger turbo to kick in right before it is needed, thus causing minimal turbo lag without compromising on boost output or engine power.
In addition, sequential turbocharging provides a lower boost threshold and increased power output at lower engine speeds.
The main drawback of this setup is the costs, as it is expensive to build – and equally expensive to repair.