The exhaust manifold collects all the exhaust gases from different cylinders and channels them into one pipe. It is usually designed as a circular steel tube with plenty of bends and folds. The exhaust manifolds are joined at narrow angles to encourage pressure waves to flow through the outlet, and not drop back towards other cylinders. OEM (stock) exhaust manifolds are usually made of cast iron, and some drivers will switch the exhaust manifolds to an exhaust extractor that is usually fashioned from steel. The main difference between an extractor and a stock manifold is that the extractor has an individual pipe for each exhaust port per cylinder. This allows for a more efficient release of the the gases as compared to a stock manifold. Extractors can be categorised in configurations such as 4-2-1 extractors - where 4 pipes merge into 2 which merges into 1, while 4-1 extractors merge 4 pipes directly into 1. There are different combinations available in the market as to which cylinders in the firing order should be paired for best effect.