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October 7th, 2011, 03:20 PM   #1
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FE570S Exhaust Restriction

I purchased the FE570S this last week, brought it home, and I have been installing parts since then. Today, I finally finished and buttoned everything up. All week while I would occasionally start the bike, it sounded extremely choked up on the exhaust note. I even started it once without the can on the exhaust to see what the exaust pressure coming out of the un-restricted pipe was like. As I suspicioned, the exhaust was normal coming out of the engine.

This concerned me because of the many reports of excess heat in the pipe causing fuel pump issues and fuel in the tank to boil. I figured any restriction that would make a 570 sound like a 250 and barely put out any felt breeze from the pipe had to be choking off the engine and contributing to the back pressure and heat build up in the engine.

I decided to dismantle the exhaust can and see how the internals were built. Boy am I glad I did this. The removal of the end cap immediately revealed a reduction in the exhaust that is not visible from the exterior. The exhaust outlet in the end cap is choked down to probably less than a 1/2 ID hole (I didn't measure it). I could not place my pinky finger past the first knuclke into this hole. Seriously, it was pitiful. I decided a little work with a hacksaw would fix the problem, and I cut off the restrictor which took approximatly 2 minutes. I dressed everything with a file, and now the actual size of the exhaust matches the exterior diameter of the outlet on the end cap.

I also located a diffuser in the interior of the pipe which necked the pipe down by about half of it's original volume. The diffuser has a pointed cap in it's center that pushes the exhaust to the outsides of the pipe and blocks the center of the pipe. I took a 1/2 inch drill and bored a nice clean hole through the center of the pointed cap and I was finished. I re-installed all of the pieces and started the bike back up. The exhaust note is very nice, not loud. The exhaust now sounds about the same as the factory exhaust on my KTM 525, and it is expelling spent gasses well enough that it will pop your hand away if you try to cover the end of the pipe. Until I can afford a new aftermarket pipe, this modification will have to do, but I can guarantee I've removed a huge amount of heat and back pressure from the engine.
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October 7th, 2011, 06:48 PM   #2
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Re: FE570S Exhaust Restriction

An excellent write-up. Some photos would be greatly appreciated.

I wonder if you have inadvertently defeated the spark arresting features of your muffler, though.
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October 8th, 2011, 08:40 AM   #3
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Re: FE570S Exhaust Restriction

I considered that I might be defeating the spark arresting features of the muffler, and I knew what time of year I am entering into here. In the spring, I might go back into the muffler and install a piece of screen which is what I have seen most often used as a spark arrestor in mufflers. The screen will not affect the flow nearly as much as the garbage that was in there.

Also, because the flow has to reverse direction to exit the tip and is swirled by the remaining diffuser in the can, I seriously doubt the ability of any spark to exit the can. But just in case, I'll install a screen in the pring time. There is a perfect place to attach a screen directly in front of the tip. The exhaust is probably two inches OD at this point giving the screen lots of surface area without increasing back pressure.
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October 8th, 2011, 12:51 PM   #4
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Re: FE570S Exhaust Restriction

There's supposed to be a two-layer screen in the tip, held in place by a multi-pointed star shaped screw-in fastener. I have read that it's the internals of the muffler that constitute the spark arrestor and not the screen, though.

The mods I've made on my FE570S muffler are more conservative than what you've done. I cut about 1/4 off the end of the small exhaust pipe that points forward inside the tail cap, rounded the end of what remains, drilled four holes around the small pipe that serve as a kind of bypass, and ground off the inner layer of the screen. I'm taking it a little at a time to see what each mod does, because there's no reversing the process.
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October 9th, 2011, 07:32 AM   #5
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Re: FE570S Exhaust Restriction

I'd cut the whole end piece off if I was you. If you don't like it after you cut it off, you could always weld it back on. That little itty bitty pipe you cut off and tapered continues up inside of the endpiece and tailpipe about three quarters of an inch where you can't see it. By cutting off the external section, you really didn't do anything at all.
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October 9th, 2011, 11:15 AM   #6
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Re: FE570S Exhaust Restriction

Actually, shortening the small pipe did matter. As you mentioned in your original post, the gasses have to change direction twice to exit the muffler. (The small pipe extends well into the concave pocket of the fan-shapped diffuser.) Shortening the small pipe reduces the effective length of the exhaust tract inside the muffler. I noticed a change in sound for sure, but I don't have a calibrated butt and can't swear to any change in effective power.

The four holes I drilled around the perimeter of the small pipe also had some effect. Since gas that passes through the holes completely bypass the small pipe, less gas travels the small pipe route. I think I notice less heat on the inside of my right leg as a result.

The reason I'm being so careful about this is because I really do want the bike to remain street legal. I'm doing one increment at a time, gauging the effects with each step.
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October 9th, 2011, 04:45 PM   #7
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Re: FE570S Exhaust Restriction

I think you'll like the results when you go all the way and remove the entire reducer. The sound is quite mellow, and I'm entirely sure you will not get any added attention from the law for noise. I guarantee it will still pass the 94db test. I don't have a db meter to test this, but it is still as quiet as my KTM which does pass the 94db test. So, I understand you not wanting to change the sound and make it obnoxiously loud. Be lieve me, I hate loud bikes, I believe you will be pleasantly surprised at the nice mellow tone it has with the entire reducer removed.
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October 10th, 2011, 04:45 AM   #8
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Re: FE570S Exhaust Restriction

Thank you, Brian. I will certainly keep your recommendation in mind. Where did you cut? I'm guessing right behind the weld where the large pipe meets the small one.
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October 10th, 2011, 10:17 AM   #9
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Re: FE570S Exhaust Restriction

That is exactly where I cut the restriction off. Be fore warned though, if you cut straight through the pipe, as I stated earlier, the nipple extends inside of the pipe 3/4 of an inch or so and you will cut through the nipple again. If you want to leave the nipple intact so that you could weld it back in place if you are dissatisfied with its removal, be careful to cut the outside in a circular manner (like a tubing cutter). You can just rotate the tip while you are cutting it, or you could use a die grinder to make the cut.
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October 10th, 2011, 04:39 PM   #10
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Re: FE570S Exhaust Restriction

Gotcha. Thanks, Brian!
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