Singer 911 DLS, in the ring.

Singer Vehicle Design, the customization house for the classic Porsche 911, was spotted testing a version of its 911 DLS (Dynamics and Lightweighting Study) on the Nürburgring on Wednesday.

It is not clear what the company was testing, as customer examples of the 964-generation 911-based DLS have already been delivered. In all, only 75 cars will be built.

It is possible that the company was testing a new powertrain configuration, as sources say the team had multiple engines.

The standard DLS engine was a 4.0-liter flat-six producing 500 hp and was developed in partnership with Williams, with advice from Hans Metzger, the Porsche engineer who first installed the flat-six in the 911 and was responsible for several Formula 1 and 917 engines. The engine was developed with the help of Hans Metzger, who was responsible for several F1 engines and the 917 engine.

The DLS engine boasts four valves per cylinder, dual oil circuits, titanium connecting rods, aluminum throttle bodies, upper and lower injectors, carbon fiber intake trumpets, ram air induction system, and titanium and Inconel exhaust. In fact, the engine is a modified version of the 3.6-liter flat-six developed by Metzger for the 1990s 911, with lightweight materials used throughout.

The DLS is not the only custom-built Singer model based on the 964-generation 911. The company also has a wild off-roader called the 911 ACS (All-Terrain Competition Study), which is powered by a 450 hp 3.6-liter twin-turbo flat-six.