Preheat the oven to 350 degrees F (175 degrees C). Lightly grease two 9x5-inch loaf pans.

Beat sugar and oil together in a bowl with an electric mixer; beat in eggs, one at a time, beating well after each addition. Stir in vanilla extract.

Sift flour, baking soda, salt, and baking powder into a separate bowl; stir in sugar mixture, alternating with sour cream, stirring until just combined after each addition. Fold in zucchini and walnuts until just combined. Pour batter into the prepared pans.

Bake in the preheated oven until a toothpick inserted into centers comes out clean, about 1 hour 20 minutes.