How Does a Variable Geometry Turbocharger Work in Diesel Engines? 


Variable geometry is a fancy term for extra moving parts. Unless you fly aircrafts, variable geometry will always refer to certain turbochargers (VGTs, or Variable Geometry Turbochargers). VGTs have turbines with vanes that move depending on the needs of the engine theyre attached to.

How the vanes move depends on the VGT design; some manufacturers design them to pivot, and some to slide. Early VGTs regulated the parts positions using pressure or vacuum actuators, but most current designs use electronic control units to determine the positions of the parts.


vgt turbine | Highway & Heavy Parts


When vane positions are altered, the geometry of the turbine housing changes. These changes affect the speed of the spinning turbine, allowing it to be optimized for engine performance.

When the engine speed is low, the space in the turbo is expanding, decreasing the speed of the air flowing through the turbine. When the engine speed is high, the space in the turbo is restricted, increasing the speed of the air flowing through the turbine. 

It is important to remember that VGTs change the speed of the turbine, not the amount of exhaust air. The amount of exhaust air never changes.

VGTs were created to work with EGR systems to control emissions and are essential for Diesel Particulate Filter (DPF) regeneration. During DPF regeneration, the air speed needs to be completely controlled to get the temperature of the exhaust air high enough to burn off the accumulated matter on the filter.


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What are the advantages of a diesel engine VGT? 

VGTs are much more versatile than fixed-geometry turbochargers. In general, diesel engine VGTs help deliver more power to the engine by increasing torque and creating the optimum geometry for an engines current speed. Especially at low speeds, VGTs can impel EGR flow and greatly reduce turbo lag.

Optimum geometry also fosters increased efficiency. And doesn't everyone wants to get more for less? Turbo technology is always improving, meaning that as advances in aerodynamics occur, diesel VGTs will only get better.

Want to keep your turbocharger in great shape? Check our blog on how to keep your turbo healthy!


What are the disadvantages of a variable geometry turbo?

As far as function goes, VGT is the ideal way to go, right?

Well in theory.

VGTs are much better for lower speeds, but some have been known to run out of air at higher speeds. In addition, as with every piece of new technology that comes out, there are going to be the typical drawbacks. VGTs are generally much more expensive than similar turbos without the variability. And while theyre better than standard turbos when working, theyre more likely to fail. As everyone whos ever worked with engines knows, more moving parts mean there are more failure points and opportunities for things to break.

Something up with your diesel engine turbo, but not sure what? Our Failure Diagnostic Checklist can help!

Is a VGT right for your diesel engine? Talk to our ASE Certified Technicians to find out!

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Purchasing a Variable Geometry Turbocharger

The decision on whether or not to get an engine with VGT depends on what you will use your vehicle for and how much you can spend. Ultimately, we just present the facts and the decision to go VGT or not is up to you and your vehicle.

There may not even be options for the engine you need. Once you have an engine, you cannot convert a VGT engine to a non-VGT engine. Some conversion kits do exist for particular engines, but it is illegal to actually convert them and authorized repair shops will refuse to work on your engine after the conversion.

For more turbo questions, see our blog on common turbo problems and failures here.


If you have any additional questions about Variable Geometry, or any other general questions you need answered, please call our ASE Certified Techs at (844) 215-3406. Or, you can request a quote online

Originally Posted May 23, 2017; Edited October 1, 2019