Beat sugar and oil at medium speed with an electric mixer until blended. Add eggs, 1 at a time, beating after each addition. Add sweet potatoes, beating well.
Combine flour and next 5 ingredients. Gradually add to sweet potato mixture; beat 2 minutes. Stir in pecans and vanilla. Pour into 2 greased and floured 9- x 5-inch loafpans.
Bake at 350° for 55 minutes or until a long wooden pick inserted in center comes out clean. Cool cakes in pans on wire racks 10 minutes. Remove from pans, and cool on wire racks. Sprinkle cakes with powdered sugar.