The Volvo XC90 T8 "Twin Engine" plug-in hybrid SUV is now the most powerful production Volvo ever, thanks to tweaks from in-house tuner Polestar.

Polestar started out as an independent race team and Volvo tuner, and was bought outright by the Swedish carmaker last year.

Over the years, it's offered everything from parts to dedicated performance models like the current S60 Polestar and V60 Polestar.

DON'T MISS: 2016 Volvo XC90 T8 Plug-In Hybrid: An Owner's First 21 Days

In addition, it now offers "Polestar Performance Optimization" tuning packages, which increase performance while maintaining the factory warranty.

It was one of these packages that turned the XC90 T8 plug-in hybrid into Volvo's most powerful production model.

With the Polestar upgrades, the T8 produces a combined 421 horsepower and 501 pound-feet of torque in hybrid mode.

2017 Volvo XC90 T8 Twin Engine with Polestar Performance Optimization

2017 Volvo XC90 T8 Twin Engine with Polestar Performance Optimization

That's compared to 400 hp and 472 lb-ft for the standard XC90 T8.

The extra power will get the XC90 from 0 to 62 mph in 5.5 seconds, Volvo says.

The powertrain remains mechanically unchanged, with a 2.0-liter turbocharged and supercharged four-cylinder gasoline engine and eight-speed automatic transmission working with an electric motor to drive all four wheels.

ALSO SEE: Volvo Leads Plug-In Hybrid Sales In Europe, Higher-Power Polestar Plug-Ins Coming (Jul 2015)

Besides unlocking more power, Volvo says the Polestar tweaks—which may be primarily software related—also improve throttle response.

The XC90 T8 version of the package also introduces a new gearshift-hold feature.

This prevents the transmission from shifting while cornering, which can upset a car's balance.

2017 Volvo XC90 T8 Twin Engine with Polestar Performance Optimization

2017 Volvo XC90 T8 Twin Engine with Polestar Performance Optimization

Expect the same tuning package to be offered on plug-in hybrid versions of the S90 sedan and V90 wagon in the near future.

Those two models share the XC90's Scalable Product Architecture (SPA) platform, and the T8 "Twin Engine" powertrain.

MORE: 2017 Volvo S90 Plug-In Hybrid Luxury Sedan To Join XC90 T8 'Twin Engine' (Dec 2015)

Polestar has already confirmed performance upgrades for some versions of the S90 and V90, but not the plug-in hybrids.

The Polestar Performance Optimization for the XC90 T8 was announced only after packages for other XC90 models were released, so Polestar may follow a similar schedule with the S90 and V90.

2017 Volvo XC90 T8 Twin Engine with Polestar Performance Optimization

2017 Volvo XC90 T8 Twin Engine with Polestar Performance Optimization

When it announced its acquisition of the tuner last year, Volvo also said that it planned to develop dedicated plug-in hybrid performance models.

That may include 600-hp versions of the S90 and V90, with powertrains based around an upgraded 2.0-liter four-cylinder engine, Volvo R&D chief Peter Mertens told Motor Trend last week.

In the meantime, the Polestar Performance Optimization package for the XC90 T8 "Twin Engine" will be available for order in October.

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