Preheat oven to 325°.

Mix sugar, baking powder, flour and salt together in a large bowl. Add oil, egg yolks, water and vanilla and mix until smooth. Set aside.

In a separate bowl, use an electric mixer to whip egg whites with cream of tartar until stiff peaks form. Pour the cake batter over the whipped egg whites and mix with a large spoon or spatula just until combined.

Pour the batter into a well greased Bundt pan and bake for about 55 minutes. The center of the cake should spring back if you gently press it with your finger.

Let cool in the pan for about 10-15 minutes and then invert the pan and place the cake on a wire rack to cool completely.

Once the cake is cooled, slice horizontally into three layers. Spread one can of pie filling on the bottom layer and sandwich it by placing the second layer on top. Spread the second can of pie filling on top of the second layer and then place the third layer on top. (If you want a thinner filling, you can use slightly less pie filling for these two layers and just have some left over at the end.)

In a large bowl, fold the third can of lemon pie filling together with the Cool Whip until well mixed. Frost the cake and keep in the refrigerator until ready to serve.