Video: Porsche’s Latest Variable Geometry Turbocharger Explained

Video: Porsche’s Latest Variable Geometry Turbocharger Explained

As an enthusiast, you’ve likely heard something about variable geometry turbocharger technology. While they are more prevalent in diesel applications for a variety of reasons, Porsche has been leading the charge in their use on performance gasoline engines. Jason Fenske of Engineering Explained dives into the subject in this video about the technology, and how it’s being applied to Porsche’s latest powerplants.

To truly understand what the Porsche variable geometry turbocharger is accomplishing, you need to understand how a turbocharger works. Fenske does a good job explaining the relationship of the turbine wheel and housing sizing to overall power production in the video. The smaller the exhaust side of the turbo, the faster the compressor side “spools up.” However, the tradeoff there is that at a certain point, the small exhaust side acts as a restriction, limiting the total power potential of the turbocharger.

Defining Variables

Getting into the variable side of things, we need to make it clear, that the actual turbine wheel is fixed, both in size and geometry. What is variable is essentially the effective size of the exhaust housing. In the world of aftermarket turbos, this is referred to as the A/R ratio – or Area to Radius ratio. A/R ratio can be an article in and of itself, but putting it simply, a for a given compressor wheel size, a small A/R ratio prioritizes exhaust gas velocity for quick spool times and responsiveness, while a larger A/R ratio prioritizes exhaust volume capacity, allowing for more ultimate power at the cost of “lag.”

On the left, you can see the vanes in the "closed" position, allowing the exhaust housing to act much smaller. This promotes exhaust gas velocity and a quicker spool time. On the right, you can see the vanes in the "open" position, where the exhaust housing has more effective volume, allowing for more efficient flow of a larger volume of exhaust gasses.

By using adjustable “vanes” inside of the compressor housing, the variable geometry turbocharger is effectively changing the internal volume and design of the exhaust housing. At one setting, the turbocharger acts as a small A/R ratio version of itself, while at the other extreme of the vane travel, it behaves like a much larger A/R ratio variant.

Fenske also points out, that it’s not just a binary system. By being adjustable on the fly, the turbocharger can be the best version of itself based on the specific conditions it’s in. At low RPM, it can maintain exhaust velocity and get the compressor spinning quickly. At higher RPM, the vanes can “get out of the way” and allow for a larger volume of gas to move efficiently, creating more top-end power.

The latest 911 Turbo engine features a pair of 61mm Borg-Warner variable geometry turbos. The oversquare engine (102mm x 76.4mm) will spin to 7,200 rpm, and makes 640 horsepower at 6,750rpm. Thanks to the variable geometry of the turbos, it makes peak torque (590 lb-ft) all the way down at 2,500 rpm, and carries it out to 4,000 rpm.

Putting Theory Into Practice

Porsche has been utilizing this technology for almost two decades now. However, for a host of reasons outlined in Fenske’s videos, the technology really hasn’t been widely adopted on the gasoline engine side. Instead, being more commonly used on the diesel engine side of things, thanks to some beneficial conditions present in those types of engines.

The current Porsche 911 Turbo S utilizes a pair of 61mm Borg-Warner Variable Geometry Turbochargers with 55mm turbine wheels (that’s a 3mm increase on the compressor side and a 5mm increase on the exhaust). The enhanced turbochargers have been paired with electronic wastegates. This allows for incredibly precise boost control, along with opening the door for a host of interesting OEM tuning strategies for both emissions and performance.

Another cool feature is the electronic internal wastegates incorporated into the turbochargers. This allows for a much wider array of tuning strategies to be employed for both performance and emissions.

The turbochargers used are also mirror-image turbos, instead of true twins, in order to simplify routing and packaging. With all of the turbocharger improvements, coupled with larger intercoolers, the 3.7-liter engine is rated at 640 horsepower at 6,750, while making peak torque of 590 lb-ft down at 2,500 rpm, and carrying it out to 4,000 rpm (making it more of a plateau than a peak).

While variable geometry turbochargers are far from novel concepts, they are relative novelties on gasoline engines. As Fenske points out, the costs to manufacture gasoline turbos with variable geometry are still on the more exorbitant side due to the cost of the advanced alloys used in their construction, relegating them to use in vehicles with equally exorbitant pricetags. Hopefully, we see the technology continue to mature and bring the price down, as it is quite interesting and has some interesting potential for mass-produced turbocharged engines.

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About the author

Greg Acosta

Greg has spent seventeen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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