Most road car engines use simple cast manifolds that are designed to get the gases out of the cylinder and away from the engine as quickly as possible. They are cheap and easy to manufacture but usually a restriction to the engine.
The problem with cast manifolds, especially on an engine that is using valve overlap to overcharge the cylinders, is that they allow interference between the cylinders and hence get in the way of that process.
Tubular manifolds solves this problem. They feature a single pipe per cylinder to make sure that each pot is effectively isolated from its neighbours so gases don’t interfere with each other. Tubular manifolds can be formed from steel, stainless steel, titanium or lnconel and the individual pipes will join further downstream, where they meet the exhaust pipe.
There are two main ways in which these individual pipes join. They can either all meet at the same point or they can become pairs that then join together to form a single pipe to the back of the car and atmosphere. Each of these has a different effect on the engine characteristics.
Whenever the outgoing exhaust gases reach a change in the system which causes an expansion, such as a join with another pipe, a negative pressure pulse is reflected back towards the exhaust valve. If the length of the pipe is correct, then that pulse will just arrive as the valve opens, creating an even greater pressure difference across the valve. This will then get the gases following out of the cylinder even quicker and hence, further improve benefits of trying to overcharge the cylinder with the incoming mixture. However, it will only do this at a narrow engine speed for each change in the exhaust section.
Therefore, a four-into-one exhaust manifold will provide one pulse at one engine speed and tends to give benefits higher up in the rev range. However, a manifold that joins pairs first, like a four-into-two-into-one will effectively provide two pulses back to the exhaust valve at different speed, and therefore the outright gains won’t be as significant as a four-into-one system, but will be spread further throughout the rev range, as they will occur at a broader range of engine speeds.