 Hey guys, have you ever wondered why two-stroke exhaust pipes look like this? It's definitely not a random choice. They are shaped that way for a reason. Now I'll explain you why and how it affects the performance of your bike. The popularity of the two-stroke bike has spiked in the last decade, mainly because Hard Enduro is getting more and more fans every day. In Hard Enduro races, riders tend to choose the two-stroke bike for its agility and power output. However, two-stroke bikes have a huge downside. It's fragile and exposed exhaust pipe, which is frequently seen tented or even damaged at the end of a race. The two-stroke engine uses an exhaust pipe with that shape solely to increase its power by increasing its compression. The way the engine is built simply doesn't allow such a higher compression compared to a four-stroke engine. This means that the engine uses the pipe to help it push more air and fuel mixture into the cylinder. So let's see how that black magic works. Well, when the exhaust port opens, the high pressure exhaust gases create a high pressure wave along with the exhaust process. The gases travel along the exhaust pipe at a constant speed and when they reach the divergent section of the pipe, the gases are no longer at the same high pressure, since the cross-section of the pipe gets a lot bigger. After this, the pressure wave and the gases travel along the pipe at the same speed but at the lower pressure until they reach the convergent section of the pipe. The gases get all squeezed up and this amplifies the high pressure wave which bounces back. But the exhaust gases continue their path along the pipe and at the end the silencer. You can understand this by picturing yourself going up an escalator while the escalator is going down. The escalator or the stairs are the exhaust gases and you are the pressure wave. When the high pressure wave travels back to the beginning of the pipe, it pushes some air and fuel mixture back into the cylinder that otherwise would not stay in the combustion chamber if it wasn't for this high pressure wave created by the pipe. However, this only explains half of why two stroke pipes are shaped this way. As you can see, the only thing that matters is the moment the high pressure wave pushes the new air-fuel mixture back inside. That is why a 125cc bike has a smaller pipe compared to a 500cc bike. The pressure wave needs less time to travel in a 125 compared to a 500 since the stroke and the bore of the engine is way smaller and that also applies to 200, 250 and 300cc bikes. But this is where things get really complicated. The big problem is that for every different RPM, there's a different timing for the pressure wave to push the new air back in but the pipe's length remains exactly the same. Ideally, a pipe is built for one single RPM, which simplifies its shape. For example, small moped pipes are built for one single RPM, full throttle, and that makes its shape a lot simpler. Instead of having a simple exhaust pipe with a divergent engine version built for one single RPM, you have to build one that can increase the compression of the engine throughout the whole rev range. That is one of the reasons why the pipe is shaped in such a funky way. The pipe will never increase the engine's compression to the fullest in every RPM, but will increase the compression of the engine, let's say from 10% to 80% throughout the whole rev range. But now you can ask, ok, that explains the length and the bulkiness of the pipe, but what about the curves? Since it's in a neurobike, you'll take it to pretty nasty and rugged up places, so the chances of the pipe getting damaged are quite big. Because of that, the engineers have to keep the pipe the furthest away as possible from the ground without shortening the length of the pipe. That's why it bends, simply to make it more compact. And do those curves affect the performance of your bike? Not so much, as long as the curves don't interfere with the flow of the exhaust gasses, so tight curves are a no-no. This means that only considerably large dents, such as this one, will affect the way the exhaust gasses flow inside the pipe. And this was another episode of Offroad Engineer. I hope you learned what two stroke exhaust pipes are for and how they affect the performance of your bike. And before you go, don't forget to subscribe.