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Turbocharger Basics

While a turbo charger system can be explained in very simple terms, a lot of people tend to confuse a lot of the basics. There are many similar terms, and even more little nuances that are easy to get mixed up. I’d like to take a moment to help simplify and expand your knowledge.

A turbocharged internal combustion engine is quite literally a hybrid engine. A turbocharger is essentially a centrifugal jet engine, minus a combustor. An internal combustion engine acts as the combustion source, supplying the turbine with exhaust gases to drive the compressor, as well as allowing for the extraction of power through the crank.

How a Turbocharger Works

As a baseline, we all need to know how a turbo works. I’ll refer to the diagram below (source: Turbo Technics)

The process starts in your engine’s combustion chamber. After combustion has completed, the exhaust flows through the head, through the exhaust manifold and at some point, to your turbo’s turbine. The turbine is a simple fan wheel located in the exhaust housing of your turbo. When there is enough exhaust gas flowing through the turbine, it will spin. The turbine wheel is then connected by a shaft to the compressor wheel?. The compressor is a part of the intake system, and the spinning of its wheel? blows air in to your intake manifold.

In a normally aspirated engine, there is a vacuum in the intake manifold, as the movement of the pistons inside the combustion chamber tries to draw air in. As the throttle is opened, allowing more air to be drawn in, the vacuum approaches atmospheric pressure. But we want more. As the compressor blows more air in to the intake manifold, rather than reaching atmospheric pressure and stopping, the pressure will continue to rise. This pressure is called boost. With a pressurized intake manifold, your engine is now able to get more air to the combustion chamber, which means more power.

The Blow-Off Valve


A compressor bypass valve (CBV? or BPV) also known as a compressor relief valve is a vacuum-actuated valve designed to release pressure in the intake system of a turbocharged or centrifugally supercharged car when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurized end of the intake (before the turbo) but after the mass airflow sensor.

A blowoff valve, (BOV, sometimes hooter valve, dump valve) does basically the same thing, but releases the air to the atmosphere. This creates a very distinctive sound desired by many who own turbocharged sports cars. Some blowoff valves are sold with trumpet shaped exits that amplify the “Psshhhh” sound, these designs are normally marketed towards the keen boy racer. For some owners this is the only reason to fit a BOV. Motor sports governed by the FIA have made it illegal to vent unmuffled blowoff valves to the atmosphere. In the United States, Australia and Europe cars featuring unmuffled blowoff valves are illegal for street use.

Downsides of releasing air to atmosphere

This unique sound sometimes comes at a price. On a car with a mass airflow sensor (MAF), doing this confuses the engine control unit (ECU) of the car. The ECU is told it has a specific amount of air in the intake system, and injects fuel accordingly. The amount of air released by the blowoff valve is not taken into consideration and the engine runs rich for a period of time.*

Typically this isn’t a major issue, but sometimes it can lead to hesitation or stalling of the engine when the throttle is closed. This situation worsens with higher boost pressures. Eventually this can foul spark plugs and destroy the catalytic converter (when running rich, not all the fuel is burned which can heat up on and melt the converter).

   * Note that engines using a MAP? (manifold absolute pressure) system are not affected.

   * BOV’s are illegal in most countries now, your car will get defected for it

Purpose of Relief and Blow Off Valves

Blowoff valves are used to prevent compressor surge?. Compressor surge? is a phenomenon that occurs when lifting off the throttle of a turbocharged car (with a non-existent or faulty bypass valve). When the throttle plate on a turbocharged engine running boost closes, high pressure in the intake system has nowhere to go. It is forced to travel back to the turbocharger in the form of a pressure wave. This results in the wheel? rapidly decreasing speed and stalling. The driver will notice a fluttering air sound. In extreme cases the compressor wheel? will stop completely or even go backwards. Compressor surge is very hard on the bearings in the turbocharger and can significantly decrease its lifespan. In addition, the now slower moving compressor wheel takes longer to spool (speed up) when throttle is applied. This is known as turbo lag.

With the implementation of either a bypass valve or a blowoff valve the pressurized air escapes, allowing the turbo to continue spinning. This allows the turbocharger to have less turbo lag when power is demanded next.

A blow-off-valve is connected by a vacuum hose to the intake manifold after the throttle plate. When the throttle is closed, underpressure develops in the intake manifold after the throttle plate and “sucks” the blowoff valve open. The excess pressure from the turbocharger is vented into the atmosphere or recirculated into the intake upstream of the compressor inlet.

Tuning adjustable valves

Most aftermarket valves are adjustable leaving customers curious on how to set them properly for their vehicle. Typically the adjustment lies in the spring preload. Here is how to set it.

You want the spring as soft as possible without leaking boost at peak pressure. If the spring is set too soft then the valve will not close fully resulting in a boost leak and idle problems. If you set it too hard then the valve will not fully open, close too early, and have compressor surge.

Trial and error with an accurate boost gauge is the perfect way to find the right setting for your vehicle….

Allard, Alan. Turbocharging and Supercharging. Cambridge, England: Patrick Stevens Limited, 1982.

Gorla, Rama, and Khan, Aijaz. Turbomachinery Design and Theory. New York, New York: Marcel Dekker, 2003.

Society of Automotive Engineers. Turbochargers and Turbocharged Engines. Warrendale, PA, 1979.

Watson, N, and Janota, N. Turbocharging the Internal Combustion Engine. London, England: Macmillian Press Ltd, 1982.


What is Duty Cycle Control?

This method is used for controlling the Wastegate and the Turbo Pre-Control. Both of these actuators are required to operate between simple fully open or fully closed. The actuators need to be moved/controlled to adjust for various operating conditions, i.e. Wastegate only needs to be opened 10% at lower RPMs and 75% at higher RPMs to regulate the boost pressure.

So what is this duty-cycle control?

The Wastegate and Turbo Pre-Control actuators are supplied air pressure directly from the Primary Turbo Compressor. Between the Primary Turbo and the actuators there are air restrictors or pills, these restrict the amount of air that can flow into the actuator. With a small opening it will take longer to pressurize the actuator than with a larger opening. Now add onto this a solenoid that can vent some of the pressurized air from the actuator. The ECU can only turn the solenoid On or Off, (no in-between analog stuff here, just digital). However, if the solenoid is turned On/Off quick enough that only some air is vented for intervals of time that are shorter than the time it take the pressurized air to leak in through the pill you can adjust the pressure in the actuator. Thus, a change in the On time versus the Off time, (duty cycle control) will give a different pressure and move the Wastegate or Pre-Control valve some amount.

Kind of like a tank of water with an adjustable valve on top feeding water into the tank and making it fuller, and a valve at the bottom that can be only fully open or fully closed. Someone, (the ECU) is turning the bottom valve open/closed and you don’t know how long it is open or closed, but you can adjust the water level by increasing or decreasing the water flow from the adjustable valve, (pill hole diameter).

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Page last modified on April 26, 2007, at 12:47 AM
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