### Instruction: What is the most powerful Porsche flat six engine?
### Input: The Porsche flat-six engine series is a line of mechanically similar, naturally aspirated and turbocharged, flat-six boxer engines, produced by Porsche for almost 60 consecutive years, since 1963. The engine is an evolution of the flat-four boxer used in the original Volkswagen Beetle.


1967 Porsche 911R
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2006 Porsche Cayman S
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The flat-six engine is most often associated with their 911 model, Porsche's flagship rear-engined sports car which has used flat-six engines exclusively since 1963. The engines were air-cooled until 1999, when Porsche started using water-cooled engines.

In April 2011, Porsche announced the third generation of the 997 GT3 RS with an enlarged 4.0-litre engine having a power output of 500 PS (368 kW; 493 hp). The naturally-aspirated 4.0-litre flat-six engine (the largest engine offered in a street-legal 911) was introduced with their 911 (997) GT3 RS 4.0, in 2011. The engine itself uses the crankshaft from the RSR with increased stroke dimensions (from 76.4 mm to 80.4 mm). This change increased the power output to 500 PS (368 kW; 493 hp) at 8,250 rpm and 460 N⋅m (339 lbf⋅ft) of torque at 5,750 rpm. giving it a power-to-weight ratio of 365 hp per ton. Only 600 cars were built. At 493 hp (368 kW), the engine is one of the most powerful six-cylinder naturally aspirated engines in any production car with a 123.25 hp (92 kW) per litre output.

Other Porsche models that use flat-six engines are the 1970–1972 Porsche 914/6 (mid-engine), the 1986–1993 Porsche 959 (rear-engine), and the 1996–2021 Porsche Boxster/Cayman (mid-engine).

The Porsche 962 sports prototype also used a twin-turbocharged flat-six engine.

These engines have also been used by German tuning company RUF in various replica Porsche sports cars.
### Response: The most powerful flat six engine made by Porsche is the 997 GT3 RS 4.0. It had nearly 493hp on just 4.0L of displacement. This is a very rare car that hardly ever come up for sale. It is one of the most powerful six-cylinder naturally aspirated engines ever made.