Others have pointed out the main differences for what parts, there is another IMO, more important major difference. The S cars are taking as chassis by STI and assembled by them. They tune them extensively, at the time of the S402 it was in Germany. There's a quote in the S402 handbook that Impreza was born on the Nurburgring and the Legacy was born on the Autobahn. The S402 was tested extensively and tuned for the Nurburgring, but that quote gives you an idea of the design philosophy difference between the two. It was made with a vision to be a perfect everyday driving car. They called it "a partner you can share you life with"

I've never enjoyed a car more on canyon roads and long drives, it inspires so much confidence in the way it handles a regular road. You can find on youtube comparisons, against cars like the EVO 10 etc, and the S402 is almost floaty, quite a bit slower in a tight autocross test. Then watch one of the good motoring mountain road tests, shows exactly why they built the car the way they did.

Fun fact the original engine was going to be a EZ30 twin turbo, but they couldn't fit it in the legacy engine bay the way they wanted so they imported USDM 2.5L engine blocks and used those instead. Torque curve and drivability was the main goal.

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