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2 Stroke exhaust tuning...

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Feb 3, 2005
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#1
I have an after market silencer on my Husky with the stock pipe. The FMF silencer is MUCH quieter(good), but the top end power has fallen flat(bad).
I met a guy on the trails recently who had the same bike as mine. He said he had cut about four inches off his stock silencer and it had really "woken" the bike up.
So I started wondering exactly how the silencer affects tuning. I think I understand how the "echo effect" in the expansion chamber keeps the fuel in the cylinder longer thus increasing burn efficiency. What perplexes me is how the sound waves at the point of exit affect performance.
More specifically, I'm thinking of experimenting with my silencer packing. It's seems to me that I could maybe just pack the forward half of the silencer then try the rearward half all while keep my perfectly good FMF hardware intact.
I know, I know... "Louder isn't better", but I do have a little leeway to play with.
So, does this sound practical?
Is there a more scientific approach?
What do ya'll think?
 

Eric Gorr

Engine Builder
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#2
The silencer simply bleeds-off pressure from the system, thats why silencer tuning is unpredictable. I have a spark arrestor on my big bore 125 and I think it has better transition from low to mid, plus I can sneak up on loud 250Fs on an mx track.
 
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#3
I guess a better question for me to ask is whether packing half of the silencer and maybe wrapping the other half with aluminium foil would have a similar effect as cutting the silencer down shorter? Or does the length of the tube have a significant bearing on the overall function with respect to tuning?
 

RM_guy

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#4
It's the length of the tube. And if you did wrap part of it with foil it would be noiser.

I wonder if you could shorten the stinger on the end of the pipe to obtain the overall shorter length but still have the same, quiet silencer.
 
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#5
RM_guy said:
It's the length of the tube....
But the FMF silencer is the same length as the stock...
Well, as I write that, I realize that the shop may have cut a section off the end of the pipe to make the overall length the same, but since the mounting brackets lineup the same, it couldn't have been much.
 

RM_guy

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#6
The length of the tube can be affected by a shorter (or longer) silencer, a totally (or partially) saturated packing, or very loose (or tight packing). I suspect that the FMF silencer is quieter than stock because it is better at absorbing the sound waves which would imply that the packing is more "open" in that is lets the exhaust bleed off sooner. That would make overall tube length seem shorter which typically helps low end and hurts top end. As the packing becomes oil soaked some top end may come back but it would be gradual and you may not notice it.

I have noticed that after changing completely drenched packing I get a huge boost in low end. It can be pretty dramatic.

Does that help?
 
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#7
RM_guy said:
Does that help?
It certainly does! (That is... if I comprendo.)
My bike has, and is designed with TONS of low end and the FMF did in fact increase it. From what I've gleaned from the last post, if I wrap foil around the first half of the silencer it will effectively increase the tube length and move the power more toward the top end. No?
 
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#8
Oldtimer,
On that 360, there's a mod to the boot between the carb and reed block that will open up that topend for ya. I haven't done it but its been tested by a few with good results. The thread is on thumpertalk in the Husky 2 stroke section titled WOW WR 360 started by wallybean. Its a couple pages back from the front page but it has some good info.
Thanks,
Bob
 
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#9
Colemanapp,
Interestingly, I'm in the middle of a topend rebuild and noticed the restriction caused by the shape of the reed block and was kinda thinking about what would happen if I wallered that thing out a little bit.
What a timely tip!
 
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#10
Only proving once again that everything happens for a reason. :nod: Mod away. Let us know what you think.
 
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#11
Old Timer,

I think in your case, you are not dealing with silencer tuning but silencer restriction.

I had an old Dyno Port straight through silencer for my Husky 360WXC and it did make the bike much more powerful. However, I could not stand the noise. So I did some serious mods to the OEM silencer, including using larger internals.

I ended up with:

1) a slightly larger perforated tube in the first 10 inches of the silencer.
2) An open chamber in the middle of the silencer, with a small sheet metal disc centrally located.
3) a much larger perforated tube aft of the open chamber.

The modified muffler was much quieter than the Dyno Port unit, and it was just as powerful.

In my case, the OEM unit had very poor flow numbers on the flow bench. I more than doubled the flow rate.

Chris
 
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#12
cujet said:
...I think in your case, you are not dealing with silencer tuning but silencer restriction...
You may have a point there...
I suspect that the silencer may be the same as the one FMF uses for the 250. I don't have any real basis for this, but the opening seems smaller than the stock silencer. Other than that, you're in WAY over my head! :whoa: