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Question about 2 stroke pipe shapes

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Does anyone know the science behind why 125 pipes look like this ImageUploadedByThumper Talk1452742939.551714.jpg

and 250 pipes look like thisImageUploadedByThumper Talk1452742970.365281.jpg ??

Specifically the part where the 250 does a special bend near where it attaches to the cylinder and the 125 doesn't.

Just curious about the reasoning why. Assuming something about performance but not sure.

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Larger engine requires more capacity, a 500cc 2-stroke's pipe is even longer and snakes around more than a 250.

 

Principals of 2-stroke expansion chamber:  https://en.wikipedia.org/wiki/Expansion_chamber

 

In short, versus a strait pipe it's a tuned pipe & chamber that 'sucks and pushes' to improve the efficiency of a 2-stroke engine within a certain rpm range.

You can also say it creates a forced induction, ramming more air/fuel mixture into the combustion chamber that the engine could have sucked in by itself.

 

Example the FMF : Gnarly, Fatty & SST pipes have different shapes and lenghts and are tuned to improve efficiency in different power/rpm ranges.

 

Also notice on some 'torquey' enduro engines (like the BETA 300 X-Trainer or the KTM Freeride 250)

that pipe's diameter is also much smaller, better tuned to the desired low rpm caracteristics wanted from those engines.

Edited by mlatour
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A very basic explanation (follow mlatour's link for a more detailed description):

 

The purpose of the expansion chamber is to create a controlled environment for expansion of exhaust gasses, and back pressure in the form of a pressure wave.  As the incoming fuel charge helps push exhaust gasses out the exhaust port, the expansion chamber allows the exhaust gasses to expand at a controlled rate and create a pressure wave that travels backward to the exhaust port.  This pressure wave helps keep the fresh fuel charge from exiting the exhaust port, if timed correctly.  Size and shape of the expansion chamber changes the timing.

 

As the displacement increases so does the exhaust gas volume.  A larger, longer pipe is needed to correctly time the back pressure wave when displacement increases.  The pipe on the 250 is bent that way because it needs a longer pipe, and they have to fit it in somewhere.

Edited by DeadSpider

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I used to race karts and did very well. The reason being is because I really understood pipes and clutches and not so much because I was a good driver. In kart racing it was common to have a couple of different expansion chambers. Also you kept a set of different length straight pipe to connect the expansion chamber to the header. They let you tune the power to what you needed for any particular track.

The expansion chamber shape determines the power band. Small diameter pipes with sharper angles where the gets larger and then go back to being smaller create very strong very narrow power bands. Larger diameter pipes with more gentle angles where the pipe gets larger and then go back to being smaller create wider softer power bands.

The total length of the pipe before the expansion chamber determines where the power band comes on. Shorter pipes before the expansion chamber have power bands higher in the RPM range. Longer pipes before the expansion chamber have power bands lower in the RPM range. This is because a short pipe (expansion chamber closer to the engine) reflects the pressure wave back faster than a longer pipe.

125cc bikes operate at higher RPM than 250cc bikes. That's why 250cc bikes need a longer section of pipe before the expansion chamber to move the power band down into 250's useable RPM range.

Experimenting with different expansion chamber shapes and pipe lengths on a 2 stroke kart is something that everyone who wants to really understand pipes should do. It's a ton of fun and amazing how much difference a pipe can make. The way karts work it's very easy to do a practice laps, change your pipe, do another practice laps and so on.

Edited by Doc_d

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Shorter pipes with steep cones are tuned for a strong effect at higher RPM.  Longer pipes with shallower cones are designed for a less pronounced hit spread over a wider range, and due to the length of the pipe they sign off sooner than a shorter pipe.

 

The primary factor in pipe design is the length.  This determines what RPM range the pipe works in.  The speed of sound is fairly constant so longer pipes are required to time the desired effect to a certain RPM. 

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As the cart racer pointed out stinger length (muffler) effects are often forgotten. Many here run top end pipes with shorty mufflers.

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