The exhaust system seems like it's not liked by many.. Tire rubbing, very difficult to service the middle box. I've been studying a bit in preparation to service mine. It occurs to me that the system is quite a bit different than other two stroke systems I've seen. The middle box is full of packing with just a perf tube running thru so it doesn't really seem to be much of an expansion chamber. It's almost like there is a middle silencer and an end silencer and no spans ion chamber. Can anyone shed some light on this? Has anyone tried gutting the middle box so it would have an expansion chamber. It seems like if a guy did that and then put a light weight aftermarket silencer (fmf or something), would be a big improvement in power and weight.

Edited January 14, 201511 yr by tahoebrian5

Most guys are looking for less power so modifying the exhaust is not a good idea unless you want more high end power at the expense of low end which will make it less like a trials bike. Trials bikes don't have expansion chambers because they are tuned for trials.

My middle box is packed with coarse stainless steel wool so it is not easily repackable. I made a door of sorts and can open it and burn out the oil. My silencer bent so badly I had to cut it up and have it welded to stop tire rubbing.

That does make sense but it brings me to my next question. Why have a middle box at all if no expansion chamber is needed? it seems like weight could be reduced by just having a 4 stroke style system and maybe a bit of a brace for the fender.

That does make sense but it brings me to my next question. Why have a middle box at all if no expansion chamber is needed? it seems like weight could be reduced by just having a 4 stroke style system and maybe a bit of a brace for the fender.

Why? To reduce the noice, that's obvious.

Seems like an odd system. Other 2 stroke engines get by on a much smaller single silencer. Is the higher compresdion engine louder and therefore needs more silencer length? Seems like something else is going on

Edited January 14, 201511 yr by tahoebrian5

Trials bikes are among the quietest two strokes. I think there is a regulation in Europe where trials bikes have to be legal on the road which is probably why they are quiet and have lights.

Trials bikes must pass noise limit tests for FIM / ACU competition as well as road use.

The perforated tube is a bit more purposeful than it first appears. It is significant with regards to 2 stroke power characteristics and silencing. The perforated section gradually removes energy from exhaust pressure waves. If this energy were not remove the noise level would be unacceptable and the engine would have a sudden step in power (grip breaking) over a narrow RPM range.

As far as I can tell, the power delivery for a 2 stroke trials bike is fairly unique when compared to other 2 stroke vehicles. Low down, evenly spread torque is the primary requirement; mainly aquired by exhaust design. No bulbous MX style expansion chambers for us, thank you very much.

I ride trials stuff on my MX bike and when the power valve kicks in it get scary! Fantic had an exhaust valve and they went out out business shortly after. It seemed to just add weight with no benefits.

http://www.motorcycle.com/how-to/how-twostroke-expansion-chambers-work-and-why-you-should-care-3423.html

In the past i have cut the rear muffler in half making it as long as the end of the seat/airbox. Slightly louder but not too bad. That really eliminates the exhaust rubbing for the most part. My older TXT edition would pull the air box from the exhaust bracket. Yamaha's will rub through the exhaust, Montesa's will too. I think mx bikes would as well if the tire was not as far away with no sub-frame.

If you really want to eliminate your center box issues on a 2004-2009 GG pro get the 2002/2003 Steel mid box. I did on my 04 and sure it weighed a bit more but it was bullet proof. You have a slightly different rear muffler connection. If you try and go to the single piece system that came in 08 you have to modify the forward bracket at the frame. Eventually however if you go to the steel mid muffler you have to replace or repair the rear because the center section of the rear muffler will break out. I have never seen anyone sucessful going to the larger mid center most current system on the older bikes because i think the frame is not wide enough on the older ones.

Running any trials 2 stroke with out a mid muffler is not adviseable. Its just way too poppy and loud. My Dad's greeves are a great example of that.

--Biff

What I gather from 2 stroke exhaust theory is an expansion chamber consists of a divergent cone followed by a throat, then a convergent cone. I can't seem to find it now, but I saw a picture where someone had cut nearly the entire back side of the middle box out, instead of the two window method on motomerlins blog. It appeared that the perf pipe inside was a constant diameter in an S shape. If this is correct than there is no true expansion chamber unless an S pipe has similar properties due to the bends. Unfortunately I can't find that picture now so let me know if anyone remembers what it looks like. I'll probably get going on mine next week.

Here's the exhaust from my old 2011 Sherco.

The exhaust lit itself up after a hard run so I decided it's time for a new one as I knew straight away what had happened inside.

The wire wool in the mid section turned to a molten state as wire wool does, It goes up like a christmas tree.

Anyone thinking of putting fuel down the exhaust and lighting it to burn out the oil think again!!

Looking at the exhaust it has a sort of expansion chamber design.

Well the sherco certainly appears to have an expansion chamber. Thanks for the pic Steve. It is interesting what dadof2 has mentioned. The perforations in the expansion chamber appear that they would soften the sound wave resulting in a less pronounced hit. Ok well I found the picture I mentioned. Take a look. There doesn't appear to be much of any expansion chamber in the gasgas

Here is a link to the thread. I'm now wondering if this is not how it looks in stock form.

http://www.trialscentral.com/forums/topic/49124-txt-pro-rebuilt-baffle/

Edited January 16, 201511 yr by tahoebrian5