The flow of exhaust gases mixed with oil from the engine to the exhaust form deposits on the exhaust manifold. If the width of the exhaust hole decreases because it gets dirty, this will worsen performance. This is why the exhaust manifold must be cleaned every two/three days on a track. Sandpaper should not be used to do this because there is a risk of changing the internal dimensions of the manifold, which in some classes are defined by the technical regulations. An abrasive sponge must be used, which will lengthen cleaning time but will avoid removing material from the exhaust manifold.