Preheat oven to 350 degrees. Butter and flour Bundt pan.
1st bowl- sift flour, (1 cup of) sugar, baking powder and salt.
2nd bowl- mix egg yolks, oil, grapefruit juice, zest and vanilla. Blend bowl 2 into bowl 1. Stir until smooth.
3rd bowl-use your mixing bowl and whip the remaining 6 egg whites along with cream of tartar until stiff peak appear. Add the remaining 1/2 cup sugar very slowly. slowly pour batter over whites, cutting with frosting knife until large foamy batter is evident. Do not beat batter. Spoon into prepared Bundt pan and bake 40-45 minutes or until tester comes out clean. Set aside to cool. May prepare glaze w/ 1 Tbsp. grapefruit juice and powdered sugar.