Turbo 101

Let’s start with the basics; turbo charging is a type of forced induction that utilizes the cars exhaust system to create more power. That is to say, a turbo charged car’s exhaust system powers a turbine that forces more air into the engine — more air combined with more fuel equals more power.

The additional power that a turbo creates is measured as boost. At sea-level, normal air pressure is around 14.7 psi; this is the maximum air pressure that can be fed into a normally aspirated (non-turbo) engine. By contrast, an engine with a turbo utilizes the exhaust flow to increase the air pressure that is fed into the engine. The air pressure that is added above the normal 14.7 is known as boost. The stock ’02-’05 US WRX runs at ~ 13.5-14.5 psi boost, which means that it is supplying the engine with ~ 28 psi of air.

Boost is created as air flows though the turbine and into the downpipe (on the diagram labeled as says “From Engine” to “Exhaust Out”). This air flow spins the turbine wheel which in turn forces more air in through the compressor into the engine (on the diagram labeled as “Air in” and “To Engine”). The faster the air is allowed to exit the turbo on the exhaust side (on the diagram labeled as “From Engine” to “Exhaust Out”) the quicker the turbo spools, or reaches full boost. Less constrictive exhaust combined with some tuning can even allow the turbo to reach higher boost levels, which in turn creates more HP. One extra PSI can create up to an 8% increase in power over the stock level.

Since the turbo is powered by exhaust gases, the turbine wheel does not get up to full speed until the engine hits higher RPMs. The time in which it takes the turbo to begin to spool is known as “turbo lag.” On the WRX for instance, turbo lag is experienced until the engine rev’s to about 2800 – 3200 RPMs

As the turbo reaches maximum boost, an internal wastegate allows excess exhaust gases to bypass the turbo and exit into the exhaust system. Subaru utilizes an electronic system to open and close its wastegate.

As the car’s ECU (Engine Control Unit) detects that the car is reaching the desired boost level, it sends a signal to open the turbo’s wastegate. This is important as turbos are designed to handle certain levels of boost. Turbo and ultimately engine failure can occur if too much boost is applied.

Hopefully, this short description helped you better understand the workings of a turbo.