The Civic Si sedan concept makes enough sense that it makes one wonder why they haven't done this before. The new Civic Si has unique performance hardware that differentiates it from the standard coupe offering. The Si sedan concept shows what would happen if those same changes were applied to the four door model. Specifically that means a 2.0L, 200hp four-cylinder motor, 6-speed manual transmission and limited slip differential. Dubbed a concept car, the Si concept is really a prototype for a 2007 production model.