Mobile Newsletter chat avatar. Mobile Newsletter chat subscribe. Under the Hood. Why is there a bulge in the exhaust pipe of my dirt bike? As the exhaust gases expand into the expansion chamber, they create a vacuum at the exhaust port. This page has a nice graphic that describes the process. Cite This! Try Our Crossword Puzzle! The expansion chamber relies on diverging and converging diameters to create an acoustic wave.
Keeping your chamber undamaged is crucial to its operation. Unfortunately this is easier said then done since there are tons of opportunities for damage on a dirt bike. These dents can affect the direction of the acoustic waves and even the timing. That is why it is crucial to keep your chamber safe. While your bike can function with a dented chamber, its best to pull out the dents or replace the chamber if its unrepairable. The expansion chamber plays a critical role in your bikes performance and efficiency.
Its size and shape is matched specifically for your engine. Be sure to protect your exhaust and add a protective plate if you plan on doing serious off-roading. Two Stroke Engine Ports Two stroke engines have an inlet port for the fuel and air mixture, and an exhaust port for the gases to escape after combustion. Early Two Stroke Engine Tuning When two stroke engine tuning first started, the exhaust was just a cylindrical tube with no changes in diameter. How Does an Expansion Chamber Work?
Diverging tubes allow exhaust gases to expand This is an important feature of the exhaust, because the increasing diameter of the tubes creates a longer returning sound wave and increases suction in pulling exhaust gases out of the cylinder. Crude drawing of an expansion chamber The converging cone reflects positive waves back towards the exhaust port. Signs of a Bad Expansion Chamber The expansion chamber relies on diverging and converging diameters to create an acoustic wave.
Conclusion The expansion chamber plays a critical role in your bikes performance and efficiency. Share this: Twitter Facebook. This website uses cookies to improve your experience. We'll assume you're ok with this, but you can opt-out if you wish. Close Privacy Overview This website uses cookies to improve your experience while you navigate through the website.
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Without it the piston will push intake charge out the exhaust port as it rises. Doing both things increases engine power at high RPM.
But if you want an engine with explosive power on top then you have to have a strong baffle return wave. The baffle should be designed to match the bikes RPM range of the needed powerband which takes precedence over the peak power you want.
A longer baffle due to a lesser angle will allow a slightly wider pipe powerband, but another advantage is less power loss when the engine RPM is just below the RPM range of the powerband. Tuned length The perfect location for the baffle, in distance from the piston, needs to be set to match the desired peak RPM of the desired powerband.
The header, diffuser, and belly set its length from the piston. But if the baffle uses many cones of differing angles then using a formula can't accurately determine the tuned length because a formula doesn't take into account the modified baffle wave shape due to multiple cones.
Angle For a single angle baffle cone the steeper the angle the stronger the baffle return wave and the stronger the supercharging effect. Lesser angles cause less peak power but don't restrict pre-powerband power as much as a steep angle and so are better for trail bikes and street bikes although you can eliminate most of that power dip with a boost bottle. Making a 3 cone baffle with increasing angles gives more power at the beginning of its pipe powerband.
A 3 cone baffle with decreasing angles gives more power near the end of the pipe powerband. It is normally better for race engines. Click here to read more on this subject. Ratio of Diffuser Angle to Baffle Angle Others have written that the diffuser angle needs to be half that of the baffle angle but that is a crude generality that doesn't apply here because all the best of my pipe designs have multiple cones for diffuser and baffle which complicate the situation. All that really matters is the return waves and so I will speak in regards to them.
If the diffuser wave peak strength is too weak in relation to the baffle wave peak strength then you will have that horrible pre-powerband engine drop in power that is common with race bikes that are intended to only be ridden within their pipes powerband. I had a pipe do that on my cc street bike and it was completely annoying. It ruined my riding experience until I redesigned the pipe to not do that by increasing the belly diameter and the diffuser angles. I wouldn't recommend allowing that to happen with any pipe design for an enduro or street bike.
If your ride has that problem then you can also try lessening the pipe back pressure caused by the stinger if it's smaller than what ECcalc recommends on sheet 1 by increasing the stinger diameter. Back pressure adds to the baffle wave and detracts from the diffuser wave strength which increases the pre-powerband power dip.
It controls the amount of average pressure in the pipe. So too narrow or too long a stinger will increase the back pressure and decrease the diffuser wave too much. The pipe pressure caused by the stinger increases with RPM so that the highest pressure is at top RPM which is why it hurts the diffuser wave the most there, more so than at mid RPM.
I never believed that the length of a tube would have much affect on its ability to flow air through it till I blew into two tubes of the same diameter but different lengths and could feel the difference of resistance to flow between the two. Then I looked for a good flow calculator and found one that works good. Experimenting with stinger diameters and using the flow calculator I concluded that a good range of stingers would provide about.
If you want to double check the results of my pipe calculator just go to the flow calculator site. For more on stinger sizing click here. Flow restriction from the silencer, if not a completely flow-through design, contributes to total exhaust restriction and to back pressure which restricts the engine a bit.
For example my Suzuki gained 10kph more top speed when the silencer was removed. Excessive exhaust restriction also adds to engine heat. Although my computer mic doesn't capture the sharpness of the exhaust note exiting a standard stinger, you can watch this video to see and hear how quieter the exhaust is when it exits the belly.
Gordon Jennings wrote that a belly stinger adds a little bit of mid range power without losing any high RPM power. His belly stinger was the stinger pipe stuck up into the pipe so that its beginning was at the same distance from the piston that the beginning of the baffle was. Here is what he wrote on the subject: Click here to see a belly stinger "channel" before being welded to my pipe. I cut off the previous stinger and had it welded to the belly stinger channel. Then the end of the baffle cone was welded shut.
Click here to see the end product. The graph below shows how too much pipe back pressure can elevate the whole system of return waves. This is from one of Blairs research papers with a cc engine. Note also how the EGT at the belly changes with stinger differences. D3 is the stinger diameter in inches. My experience tells me that if the intake of an engine is restricted with too small a carburetor or too small a reed valve then it's best to have a big diameter stinger for no back pressure.
Also a free flow silencer. Click here to go to my page about carb sizing. Secondary Waves When the returning diffuser wave and baffle wave gets to the cylinder some of their energy is expended if the exhaust port is open and the rest bounces back into the exhaust pipe. Those sound waves return a second diffuser wave from the diffuser and a second baffle wave from the baffle. And then the process repeats over and over many times till all the waves energy is expended. All these waves following the primary diffuser and baffle waves I call "secondary waves" because they are of secondary importance to the primary diffuser and baffle wave.
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