In a large bowl, stir together the dry cake and the entire can of crushed pineapple with its juice. Stir well until all the dry mix is incorporated. (The mixture will get really foamy.)
Pour the batter into a 9x13 pan which has been lightly greased with nonstick cooking spray.
Bake at 350F degrees for time specified on the box for size pan. When the sides pull away from pan and toothpick inserted in the center comes out clean, the cake is done. This should take somewhere between 30 and 40 minutes.
Remove from the oven and place on a wire rack to cool.