VDASH Guide
How the turbocharger works
Basic explanation of turbocharger regulation, wastegate, bypass valve, blow-off valve, and the most common faults.
Basic principle
How the turbocharger creates boost pressure
The turbocharger draws in fresh air, compresses it in the compressor section, and sends it through the intercooler, throttle valve, intake valve, and into the engine combustion chamber.
After ignition, exhaust gases flow back into the exhaust side of the turbocharger, where they spin the turbine. The exhaust turbine is connected by a shaft to the compressor side, creating boost pressure.
The explanation below is simplified and based on a basic turbocharger control schematic, for example on a 2005 Volvo V70 T5.
Bypass / blow-off valve
Protection of the compressor section
On the intake or compressor side, there is a bypass or blow-off valve. This valve releases excess pressure created when the accelerator pedal is released and the throttle valve closes.
When the throttle valve closes, the turbocharger is still spinning and generating airflow. This air stops before the throttle valve and has nowhere to escape. Without intervention, it could damage the throttle valve, pressure hoses, or turbocharger.
The bypass / blow-off valve is controlled by vacuum behind the throttle valve. When it opens, pressure before the throttle valve drops quickly, protecting the system from damage.
The difference is where the excess pressure goes. A bypass valve sends it back before the turbocharger, while a blow-off valve releases it into the atmosphere, creating the typical “racing” sound during gear changes.
Wastegate and TCV
Turbocharger performance regulation
Before the exhaust turbine, there is a valve called a wastegate. It protects the turbine from excessive exhaust gas pressure and from exceeding critical RPM.
The wastegate also regulates turbocharger performance. If the turbocharger overspins, the turbine blades, bearing, shaft, or other components may be damaged.
Wastegate regulation is handled by the TCV valve (Turbo Control Valve). It is a three-way valve that uses pressure and vacuum through hoses to control wastegate opening. Its operation is controlled electronically by the engine control unit based on data from engine sensors and the accelerator pedal.
Most common faults
Where turbo regulation most often fails
- Leaking bypass / blow-off valve or damaged closing spring. Some air may escape back before the turbocharger or into the atmosphere, reducing engine performance.
- Incorrect wastegate preload. If the preload is too low, the wastegate may open earlier than requested by the engine control unit, allowing exhaust gases to bypass the turbine.
- Faulty TCV valve. A faulty or leaking TCV valve may cause the wastegate to open when it should not, reduce performance, or cause excessive boost pressure.
- Deteriorated, leaking, or worn vacuum / pressure hoses. If the hoses connected to the TCV or bypass / blow-off valve leak, regulation will not work correctly.
- Leaks in intake hoses, intercooler, or other system parts. Air or exhaust pressure leaks will prevent the system from delivering the expected performance.
This quick explanation may help you understand why your Volvo does not perform as expected, or why you did not feel a major difference after a performance optimization loaded with VDASH from D5T5.com.
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