[Noel]

SSI's are not ALL stainless steel heat exchangers. This is a common miss-belief. They are heat exchangers (Porsche Header with the integrated air to air heating system) manufactured by Stainless Systems, Inc. They are a pre-1974 design, which was the way Porsche designed the exhaust before catalytic converters were required. The reason that many upgrade to them is that the post 1974 design was very much a compromise in order to meet the changing environmental regulations and thus, they didn't flow as well as the pre-1974 exchangers. The installation of the SSIs or any pre-1974 heat exchanger really opens up the flow and increases horsepower across the entire RPM range. They have the most dramatic effect on SCs and a smaller effect on the 1984-1989 cars. This is due to the pipes in heat exchangers on the Carrera’s already being bigger and thus flowing more air already. Many state that for an SC, the increase is about 17-22HP. The increase on the Carrera's is closer to 13hp.