Porsche is set to release a family of three heritage versions of the iconic 911, with the first one already unveiled as the 911 Sport Classic. However, the 911 ST is the next in line, and it is expected to be revealed later this year.

The upcoming 911 ST was spotted earlier this year while testing in the cold. The spy photos gave us a glimpse of what to expect from this upcoming model. This time, however, the second heritage model was seen while testing at the Nurburgring, courtesy of YouTube's Carspotter Jeroen.

Gallery: Porsche 911 ST spy photos

One of the key features that distinguish the 911 ST from a regular 911 prototype is the double-bubble roof, which is also found in the 911 Sport Classic. Additionally, the 911 ST will be based on the Porsche 911 GT3 Touring, and the spy photos reveal center lock wheels on all four corners of the car.

Sources suggest that the 911 ST will feature the same 4.0-liter flat-six engine as the GT3 Touring, without any tweaks, delivering an output of 502 horsepower and 346 pound-feet of torque. Hearing the prototype setting the Nurburgring ablaze, the 911 ST sure did sound like the GT3 Touring. There is also speculation that an automatic transmission will be the only option available, though it's not clear if Porsche will offer a manual version.

The 911 ST is inspired by a small-run racing model from the early 1970s, and its design is expected to reflect this. There may not be significant changes compared to a standard 911 GT3 Touring, but there will likely be some retro-inspired touches here and there. For instance, the Porsche Heritage emblem is probably hidden under the black circle on the left side of the engine mesh cover at the back.

If reports were to be believed, the 911 ST will not be the most exciting 911 heritage version. Rumors suggest that a revived 911 Carrera RS is coming after the 911 ST. No timeline has been set just yet for the third heritage model.

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