In this article I will discuss the basics of resonance and manifold design. Is it the best place to start talking about how engines work? Hell no, I just think it's one of the more interesting places to start.
The goal in designing a vehicles intake and exhaust system is to allow the most amount of air flow through while trying to achieve certain goals. In my opinion, the best designed set of manifolds will not produce the most amount of power possible. The best designed manifolds will make the engine as predictable and confidence inspiring as possible.
By predictable, I mean by giving the engine as flat of a torque curve as possible. For example with a flat torque curve if you get 300ft-lbs of torque at 2500rpm, you'll also get 300ft-lbs at 4000rpm and 6000rpm. In a driving sense that means that no matter what speed you are going, you know exactly how much force the engine is going to put out based on what gear you are in and how much throttle you give it. Entirely ruling out trying to keep track of what speed the engine is operating at.
The key to designing a torque curve lies in resonance. Resonance refers to the pressure shock waves that pass through the exhaust and intake. By changing certain parameters of the manifolds (like tube diameter and length) you can control when and where those certain pressure waves will be during each cycle of the engine.
Resonance works roughly as follows.
When the exhaust valve opens a pressure wave is created as the hot gases escape the cylinder into the exhaust. This pressure wave travels down the exhaust until it reaches an opening or void (usually the first void is a merge where multiple exhaust pipes meet). When the pressure wave reaches this point part of the wave reflects and travels back up the exhaust pipe towards the engine and part of the pressure wave carries on. The part of the wave that travels back to the engine generally will reflect back and forth until the next exhaust valve opens, depending on the lengths of the exhaust pipes it could travel back and forth 2 3 or even 4 times depending on what speed the engine is running and what lengths you use. Each time it reflects though it loses some of its pressure.
Where the benefit of all these waves running around is when you design the system so that the pressure wave is just leaving the exhaust valve and creates a small vacuum in the exhaust pipe. This vacuum helps suck extra exhaust gases out of the cylinder. This extra empty cylinder then sucks in extra air do to it's slight vacuum. Extra air means you can use more fuel, and more air and fuel means you get more torque/power.
Now for the intake. When this extra strong suction comes from the intake valve into the intake manifold another pressure wave is formed. This pressure wave will travel back and forth in the intake system much like the exhaust resonance. However instead of creating a vacuum outside of the cylinder like on the exhaust side, if the intake is designed right it will then create a high pressure zone exactly when the intake valve opens, this will help force extra air into the cylinder.
Unfortunately however, it isn't this simple. The main problem lies in the fact that engines operate at a variety of engine speeds. Tuning the lengths and diameters of the manifold will only change the rpm at which the resonance effect works.
It should also be noted that outside of that resonance level the resonance works against the engine by doing exactly the opposite of where the peaks are. So say if you wanted to tune for peak torque at 5000rpm, and you set your intake lengths/diameters to resonate at that as well as your exhaust system. You would gain a very large boost in your torque curve at 5000rpm and you would lose a lot of torque everywhere else.
If however you design your intake and exhaust to resonate at different engine speeds then you can minimize these peaks and valleys in the torque curve and thus get a very flat level torque curve that is easy to use.
Some Porsches, a couple motorbikes, and some other high end cars I have seen have what is best described as variable runner lengths. In Porsche's system they have a couple valves that let the exhaust travel down different lengths of tube at different engine speeds. Thus Porsche is then able to get multiple resonance spots. With some of the motorbike systems I have seen, they have sliding collars on the intake runners that extend or contract based on engine speed. This is a far superior method of resonance tuning because they provide consistent resonance rather then a few different resonance spots.
On one last note, there are different structures of tuned exhaust pipes, on most 4 and 8 cylinder engines, the 4-2-1 or 8-4-2-1 exhaust routing is perhaps provides the best performance. 4-2-1 means that 4 pipes merge into 2 pipes, which then merge into one final pipe. Each length in the system will provide resonance, so you can tune the first set of exhaust pipes at a different engine speed then the second or final exhaust lengths.
Have a good weekend everyone :)