All you need to know about 2-stroke exhaust systems

We're now going to take a look at how the exhaust system of a 2-stroke engine works. Back to the basics guys ;)

"The muffler also improves your engine's performance".

No, the muffler doesn't just extract burnt gases and reduce noise, it also improves your engine's performance! But how? Ixtem takes a closer look ☺

How does the muffler improve your performance?

Many of you discovered your passion for two wheels on mopeds such as the famous 103 sp or 50 box bikes of the good old days... And how many of you started tinkering with these engines to gain a few horsepower!? Many of you! Yes... Not necessarily homologated... But the acquisition of a "racing" exhaust will have thrilled many.

Here's a simple explanation of how it works, for both small and large displacement engines. As already mentioned, the muffler on a 2-stroke engine is extremely important to the engine's smooth operation. Its primary function is to expel spent gases from the cylinder (more precisely, from the exhaust port). It consists of two parts:

• The expansion chamber
• The silencer

I rode 125 YZ and 125 KTM bikes for 5 years when I took part in Junior events. Suffice it to say that I tested several types of muffler from different brands.

Why did you change the original pot?

Several reasons:

For a start, the original pot on these bikes is extremely thick, with a paint finish designed to resist abrasion. These two factors provide good protection for the pot's longevity, but don't allow for a good resonant frequency. In terms of performance, fitting an exhaust line is essential, especially when preparing the engine cylinder to improve compression.

I essentially used two different mufflers:

One gave me greater power at low revs (more torque) but with reduced extension, while the other gave me more linear power at low revs with much greater extension.

Depending on the type of terrain and discipline, it was possible to modify these criteria quite quickly (between a supercross-type terrain with little recovery or, on the contrary, a motocross-type terrain with big climbs).

Technically speaking, the difference between these two types of pots was mainly in the divergent and convergent cones with different angles, but also in the chamber length (e.g.: greater chamber length = lower power but increased range of use, lesser chamber length = greater power but reduced range of use).

The disadvantage of this type of pot is its rigidity. In order to achieve the best possible resonance, the material used is extremely thin. This means that after a good fall or a deep rut, the pot bends very quickly! Within reason, it can be repaired...

Relaxation pot

Depending on its volume and design, the expansion chamber allows fresh gases to be returned to the cylinder via the exhaust port. To achieve this, the expansion chamber features a divergent cone (closed angle), a chamber and a convergent cone (open angle).

To be more explicit, taking the 2-stroke engine's combustion cycle as a starting point, when the intake and exhaust ports are opened to allow fresh gases in and used gases out, some of the fresh gases are expelled directly via the exhaust port (without being used as they pass through the combustion chamber). This phenomenon reduces the amount of fresh gas in the cylinder, and consequently engine power (less gas compressed during the explosion = less power).

When the gases are expelled (used gases first, followed by fresh gases), they release a residual combustion wave (from the cylinder). Thanks to its angle, the divergent cone facilitates the exit of the gases, while the convergent cone allows the used gases to exit and the fresh gases to bounce back into the cylinder in the opposite direction. It's all a question of resonance!

The angle of the converging cone, depending on its inclination, can facilitate gas rebound if it's high, or on the contrary, facilitate gas exit if it's low. The important thing is to find the right compromise so that the cone fulfils its two functions: evacuating used gases while returning fresh gases to the engine for better explosion and performance.

Finally, there's the muffler, directly connected to the expansion chamber, which attenuates noise thanks to its perforated, rockwool-coated inner tube. Once again, different silencers are available.

A short silencer improves pick-up and acceleration at low revs, while a long silencer favors mid-range revs and increases your bike's length. The diameter of the tube reduces or increases engine temperature (large diameter = lower pressure, lower temperature).

For crossmen looking for performance! Before you go and prepare your engine or change your muffler (while breaking the piggy bank), invest in a good suspension preparation!

"A well-prepared engine will save you 10ths. Well-tuned suspension...seconds!"

PS: Thank you for taking the time to read this article. I'd be delighted to respond to your remarks and comments. If you liked it, please share the article with friends who might be interested, it counts for IXTEM MOTO. Thank you ;)