KNOW WHY THERE IS BULGE IN EXHAUST PIPE OF DIRT BIKE !

Dirt bike engine uses two stroke gasoline engine.The exhaust pipes of dirt bikes are shaped strange. They expand into a wide diameter and then shrink down to a very small diameter where the exhaust gasses emerge into the environment.In this article we will understand why these exhaust pipes are shaped in that manner.

That bulge is called an expansion chamber, and it is used to increase the horsepower of the engine . This technique only works on two-stroke engines, which is why you see it on a lot of dirt bikes but not on street bikes. Most street bikes use four-stroke engines.

The basic idea behind an expansion chamber is to use the momentum and pressure of the exhaust gases to create a pump that squeezes more air and fuel into the cylinder during the intake stroke. It does the same sort of thing that a turbocharger does, but it does it without moving parts.

How it works:

While moving downwards after ignition, the piston exposes the exhaust opening, and the burned gas streams into the exhaust pipe like a (high pressure) shock wave.

Due to the inertia, this gas will create a slight vacuum wave behind it, which helps to suck out more burned gas, but also fresh gas as soon as the intake opening is uncovered. 

The first cone helps to increase the vacuum: When the gas travels through the pipe with a certain speed, it travels through a certain volume per time. If the cross section is increased and the speed remains the same, the wave travels through a larger volume. This creates more vacuum volume behind the wave.

Now we have fresh gas in the cylinder, but also in the manifold. The shock wave now hits the right cone and gets reflected.That is, you now have a shock wave running towards the cylinder. It hits the fresh gas right in the moment when the intake opening is covered by the piston, and presses this gas into the cylinder. When the piston covers the exhaust opening, too, the fresh gas already is under some pressure.

This way, the exhaust forms some kind of compressor, increasing the volumetric efficiency / power of the motor.