Fresh Grapefruit Cake

• 5 eggs
• 250 gr sifted cake flour
• 250 gr sugar
• 80 ml water
• 80 ml salad oil
• 60 gr squeezed grapefruit (fresh)
• 9 gr baking powder
• 6 gr grapefruit peel (fresh grated)
• 1 gr salt
• 1 ml cream of tartar