Sift together flour, 1 3/4 cups sugar, cocoa, soda, and salt in a large mixing bowl. Make a well in center of dry ingredients; add oil, egg yolks, water, and vanilla. Beat at high speed of electric mixer 5 minutes or until mixture is smooth.
Beat egg whites (at room temperature) in a large mixing bowl at medium speed of electric mixer until soft peaks form. Add remaining 1/2 cup sugar, 2 tablespoons at a time, and beat about 4 minutes at medium speed or until stiff peaks form.
Pour egg yolk mixture in a thin, steady stream over entire surface of egg whites. Gently fold the chocolate-yolk mixture into whites.
Pour batter into an ungreased 10-inch tube pan, spreading evenly with a spatula. Bake at 350° for 1 hour or until cake springs back when lightly touched. Remove from oven; invert pan, and let cool completely before removing cake from pan.
Beat whipping cream until foamy; gradually add powdered sugar, beating until soft peaks form. Frost top of cake with whipped cream, and garnish with walnuts, if desired.