There are a few ways to make homemade foam car wash. One way is to mix 1 cup of baking soda with 2 cups of water and pour it into a small bowl. Add a few drops of dish soap to the mixture and stir until the foam is formed. Pour the mixture into a large bowl and add more baking soda and water until the mixture is thick. Use a hand-held foam car wash cleaner to clean the car.
There is no definitive answer to this question as the foam cannon requires a different PSI for each type of ammo it fires. For example, a standard foam cannon fires small rounds that require around 30 PSI while a bigger foam cannon fires large rounds that require around 100 PSI.