Caito wrote:Hi guys. I know that exhaust need to be tuned to specific frequencies.
In tuning to that specific frequency two parameters come into play, length and diameter.
I imagine a bigger diameter would be less restrictive, but would cause a long pipe. Meanwhile a longer pipe would generate more losses too.
How do they reach a final value? Since, by looks, all teams use the same diameter, it must be the best solution.
It's not really about frequency, or flow restriction for that matter.
When the exhaust valve opens a pressure pulse (compression) will form in the exhaust port. This pressure pulse will travel downstream the exhaust pipe with roughly the speed of sound. When the pressure pulse is subjected to a change in area, which is what happens in the collector, a reflection pulse will form and travel back to the exhaust port with roughly the speed of sound. Since the area in the collector (area of secondary pipe + area of the other primary pipes) is greater than in the primary pipe the pulse is coming from the positive pressure compression pulse will become a negative pressure expansion pulse. When the exhaust has the correct length, the pulse will travel from the port to the collector and back to the exhaust port and arrive when the exhaust valve is about to close to cause a low pressure in the port. In principle this is only dependant on gas properties and the length of the pipe.
The diameter of the pipes is a different issue which is related to how a pulse is affected by a change in area. The starting point is the port area of the exhaust (valve seat area minus valve stem area). If the primary pipe have an area that is smaller than the port area it will form a convergent duct and if the pipe area is larger than the port area it will form a divergent duct. Most engines use divergent ports, although there are some turbocharged engines using convergent ports. For most engines this gives a port-to-manifold area ratio between 1.2 and 1.6 where engines with higher bmep tend to use a higher ratio. The purpose of this area ratio is basically to prevent the loss of pulse amplitude and energy in order to maximize the pulse tuning effect. The secondary pipe usually have a diameter to provide an area ratio in the collector of about 6, which gives a good reflection pulse.
In practice the exact dimensions require a lot of optimization work using either engines running in test cells and/or engine simulation software such as GT-Power or Ricardo Wave.
Read Blairs "Design and Simulation of Four-Stroke Engines" for more detailed information.
Edit: In F1 it's the teams (or a supplier to the team) that make the exhaust manifolds to the dimensions given by the engine manufacturer.
