It is usually understood that the less restrictive the exhaust system is, the better performance it will provide. Unfortunately there are many factors that come into play.
The design of an exhaust system not only influences the engine performance and noise volume, depending in the design choice it will also provide very different exhaust tones.
I would like to split this topic into two parts. In the first one we will discuss how an exhaust system may improve engine performance. In the second one we will explain why engines with similar architectures produce different noises.
In today’s post we will focus on performance improvements.
How does the exhaust system improve the volumetric efficiency of the cylinder?
The improvements in the volumetric efficiency of a cylinder are obtained by using the exhaust pulse of the previous cylinder so that a low pressure wave is generated behind it, helping with the extraction of gasses from the combustion chamber and the filling thereof while both the exhaust valve and the intake valve are open.
Not only the extraction of exhaust gasses is improved. When the inlet valve opens, still during exhaust phase, the gas-air mixture can enter the cylinder even though the piston has not yet reached the TDC (that is, while it is still travelling upwards) favoured by the vacuum generated by the exhaust pulse.
This effect is called exhaust scavenge.
The main goal in the design of an exhaust system is to adjust it so that the low pressure trail of the exhaust pulse from one cylinder reaches the exhaust pipe of the following one just when its exhaust phase begins.
There are two types of exhaust scavenge:
- Inertial scavenge: This process begins as soon as the exhaust valve is opened. The gases run through the valve, the exhaust pipe and the primary pipe of the collector. During this journey the gases leave behind them a low pressure area, the greater the further away from the cylinder. If the low pressure area affects the exhaust ducts of the cylinder that follows in the ignition order at the right moment (when the exhaust valve is starting to open), this low pressure zone will help to extract the exhaust gases. The size of the low pressure zone can be tuned by manipulating the diameter and length of the primary pipes in the exhaust manifold.
- Wave scavenge: This process takes place due to the sound waves generated in the engine. As soon as the exhaust valve opens, a pressure wave travels through the exhaust system at the speed of sound, dragging part of the gases with it, helping in the emptying of the cylinder. On the other hand, when this wave reaches the end of the primary pipe it is reflected. The reflected wave travels in the opposite direction towards the cylinder, dragging gases back with her. The system has to be optimized so that the reflected wave reaches the cylinder immediately after the exhaust valve has closed.
Length and diameter of primary pipes
By modifying the length of the primary pipes, we will be able to tune the time needed for the low pressure zone to travel through the exhaust system until reaching the manifold.
The need for a long primary tube is reduced proportionally to the engine rotational speed, since the time between cylinder explosions is shorter and the low pressure wave has less time to travel through the exhaust system. On the other hand, the smaller the diameter of the tubes, the faster the exhaust gases will move.
At lower engine speeds, bigger performance gains are achieved with long primary pipes. As the engine speed increases, time between exhaust pulses will shorten, therefore better performance is achieved with shorter tubes. Because of this reason the exhaust system must always be tuned to an optimal rpm range. It is also very important that all the primary tubes have the same length, so that there is no difference in the scavenging effect between them.
When the main pipes meet in the manifold, they will be placed consecutively with respect to the ignition order, so that the effect of the exhaust wave from a cylinder is improved by the previous one.
Soon we will continue with the second part of this article, talking about sound tuning.