Preheat oven to 350 degrees F (180 degrees C). Grease and flour one 9x13 inch baking pan.
Sift together the flour, sugar, cocoa, baking soda, baking powder, and salt. Add the oil and eggs; mix well. Add the water and mix well again. Stir in the walnuts. Pour batter into prepared pan.
Bake at 350 degrees F (180 degrees C) for 30 minutes. Cool cake before frosting.