Two Stroke Motocross
Two Stroke Motocross Forum => Technical => Topic started by: KXwestYZ on January 05, 2010, 02:05:40 PM
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Hi I have a KX 250 yr 2000 and my brother has a YZ250 yr 2004. I like my engine powerband to come on like a light-switch so there is little power down below then it suddently kicks in with a crazy hit and a hugh power surge in the higher RPMs. I was just wondering if anyone knew the best pipe to get the engine tuned like that? I know the gnarly FMF type pipes are supposed to give more bottom end torque at the expense of higher end revs - but does anyone know a pipe that would do the opposite?
Thanks!
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FMF makes pipes that will do what you want. A V-Force reed system will put "hit" in your powerband as well. Also, you can narrow the squish band on the cylinder head as this will increase top end as well.
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If you are a fan of FMF go with the SST with a short silencer or go with a works procircuit with a shorty silencer. The main thing being the short silencer for the top end. You will still need motor work done and it would help if you did reeds as well. correct me if I am wrong but a good set of stiff reeds would work good for a hard hit. Don't bother with a power now valve, but I would also look into a new throttle tube so you can turn that into a "light switch" as well.
This is just my opinion and I am by no means a SME (subject matter expert).
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I forgot about the throttle. Good idea. I would ask Donny Emler to be 100% correct, but the silencer makes no difference regarding length. The shorty silencer is for weight.
Personally, Mitch Payton has pissed me off so bad I won't buy, nor do I reccomend anyone else to buy from him. He is slurping up 4T's to save his bacon. I don't care what he thinks of me, nor should he care what I think of him. But I WILL NOT support anyone who doesn't emphatically support 2T's (like Mr. Emler).
Just Mad Scientist spewing his .02 >:D
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but the silencer makes no difference regarding length. The shorty silencer is for weight.
I'd have to disagree.Tuned length always makes a difference,though small and it is noiser.
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There is a book by Gordon Jennings where he gives the math for 2T exhausts. He also explains the function of the system as well. In that discussion, he explains how after the convergent cone length makes no difference. I think Jetzcorp downloaded a copy of the book. Maybe he can copy and paste that section so everyone can read it.
Calling Jetzcorp................................................... ;)
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I just had another thought. You are thinking in terms of 4T exhaust. Gordon Jennings explains it better than I do.
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http://2stroker.net/jennings/ (http://2stroker.net/jennings/)
Do all the reading and copying here^
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I can get the page up but can't open the PDF file. If I look up the page numbers do you want to copy/paste the thing for us???
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I got the PDF to open, but it was copied sideways(I have linux). Try downloading the .zip version it's a word file, but at least it won't be sideways.
If I look up the page numbers do you want to copy/paste the thing for us???
Sure, it would help the new guys a lot. :)
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Hmmm, i was under the impression that a short silencer aided bottom end and a longer one produced more top end. That's certainly what it feels like when i swap silencers on my CR, with the shorty fitted she's more snappy and comes on the pipe quicker. To me it's a noticeable difference.
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When I get some time on Sunday, I will find the texts in Jennings' book and have my wife type them out.
I hope you guys appreciate Mad Scientist sleeping on the couch.
Yes, I'll take the bullet for you guys. >:D :o ???
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Tuned length always makes a difference,though small and it is noiser.
The last I checked, only the pipes made the difference. But the silencer is a noise reducer rather than a performance improvement. If most people had a choice they would remove the silencer because the silencer is additional weight and cuts a little bit of power(not much though). Look back on 50's and 60's 2 stroke bikes and what do you see......... no silencers, I wonder why? I'm not telling you to remove the silencer because it is loud and you WILL!!! have a massive headache (I have done this on my KX125). :D
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It appears I should have expanded my comment a little.I never meant to suggest that a silencer offers a performance improvement,only that different lengths affect it in different ways.A shorty skews the power more to the bottom,a longer one more to the top.As we all ride with a silencer,the length you need depending on what you are after is the pertinent question,not the fact that it would be faster if you took it off altogether.A shorty is noisier as well due to the fact it has a shorter core and less packing,assuming all else to be equal.
On a noise note,a few years a go a friend got his can am 400 finished and came over for a ride.Now,say what you want about 4t's being louder than 2t's and while this is true in most cases,I've never heard such an ear splitting,brain rattling sound as what came out of this.He had it fitted with the can am accessory performance silencer-a "Disco-Jet"(it was stamped right on it)likely a cool name in those days.Anyway,he replaced it with the stock one after one ride.Whether it was because of the noise or he got tired of questions about his riding style mimicking John Travolta,when he was getting all white gear,his mid hesitation being due to a case of night fever,or having to listen to butchered versions of "you're the one that I want" complete with actions whenever stopped, I never found out.
I do remember thinking at the time that a track full of likely sounding machinery would have been as brutal if not more than the fourstrokes of today.Maybe people were a bit less sensitive in those days.
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Removing the silencer will only give you a headache if you ride without a helmet. If you keep your helmet on (and your bike isn't giant) you should be okay. I never ride with a silencer on my '67 Kawasaki and the noise is perfectly acceptable to me as I ride it. Now, if you happen to be riding just behind someone running a straight stinger, then you're going to encounter some pain.
Back in the days when men were men, you would see 501 Maicos being run with no silencer at all. You want to talk about noise, Johasus! And yes, in the '70s when riding was starting to get popular, the noise complaint was exactly what it was today. Track owners and promoters were shutting down tracks, and no one wanted to put on a silencer because it robbed performance. Then, some of the magazines did dyno tests that showed that a good silencer doesn't significantly reduce performance. What they conveniently forgot to mention is that it also has to be looked after a lot more than most people are willing to, so in reality, 90% of riders will lose performance with a silencer. The difference in power for my 250 was insane once we got rid of the packing. It doesn't do much in the way of silencing anymore, but we don't have to worry about it getting gummed up.
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Hey Jet,
I took my silencer off my KX125 one day, just to see how loud it would be, and holy crap is it loud, even with a helmet on. Needless to say I put it back on after about 15 minutes of riding around.
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Again, Jennings says that because of pipe design, mufflers make no difference. Their length will affect sound output.
As for unmuffled 2T's being louder than 4T's...anything without mufflers is loud. But, a decibel meter does NOT "hear" things like a human ear. So, different sound (pressure) waves will affect the ear differently. A 2T has a different sound wave than a 4T, as does a nitromethane burning car does. We can't say what is loud relative to our ear, because sound is generally measured with the decibel "A" meter.
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I've got Gordon Jennings handbook but i can say with certainty that my bike doesn't run as well without the silencer and sounds like sh*t
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I think Jennings was comparing a pipe designed to use without a muffler verses a pipe designed to work with a muffler.
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Hi I have a KX 250 yr 2000 and my brother has a YZ250 yr 2004. I like my engine powerband to come on like a light-switch so there is little power down below then it suddently kicks in with a crazy hit and a hugh power surge in the higher RPMs. I was just wondering if anyone knew the best pipe to get the engine tuned like that? I know the gnarly FMF type pipes are supposed to give more bottom end torque at the expense of higher end revs - but does anyone know a pipe that would do the opposite?
Thanks!
Wire the powervalve open. Best pipe will be one designed to work like that, not a biscut cutter off the shelf pipe.
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Removing the silencer will only give you a headache if you ride without a helmet. If you keep your helmet on (and your bike isn't giant) you should be okay. I never ride with a silencer on my '67 Kawasaki and the noise is perfectly acceptable to me as I ride it. Now, if you happen to be riding just behind someone running a straight stinger, then you're going to encounter some pain.
Back in the days when men were men, you would see 501 Maicos being run with no silencer at all. You want to talk about noise, Johasus! And yes, in the '70s when riding was starting to get popular, the noise complaint was exactly what it was today. Track owners and promoters were shutting down tracks, and no one wanted to put on a silencer because it robbed performance. Then, some of the magazines did dyno tests that showed that a good silencer doesn't significantly reduce performance. What they conveniently forgot to mention is that it also has to be looked after a lot more than most people are willing to, so in reality, 90% of riders will lose performance with a silencer. The difference in power for my 250 was insane once we got rid of the packing. It doesn't do much in the way of silencing anymore, but we don't have to worry about it getting gummed up.
Silencers used to be considered a "neccessary evil" by the manufacturers, and they only included one because regulations forced them to. That is no longer the case. On modern competition bikes (two-stroke or four) the silencer is an integral component of the over-all tuned package. Removing the silencer from your bike will drastically change the wave tuning of the exhaust. It's a complete crap shoot as to what changes will occur to the power curve. In most instances you will drastically reduce the width and rideability of the power curve in exchange for a small boost at the very top of the power curve, not a worthy trade-off at all in my opinion. And the ramifications of riding un-silenced bikes is way too serious to be recomending such to other riders. Nearly every riding area and track that has close across the country in the last ten years has closed for one reason-sound complaints. It's both irresponsible and selfish to be recomending to others that they should remove their silencers.
And removing the packing from your silencer? I defy you to show me a dyno test that provides any evidence whatsoever that it does anything other than reduce peak power production. In an unpacked silencer, the normal smooth, laminar flow of exhaust is destroyed, and replaced by a very turbulent flow. Turbulence will have the exact opposite effect of what you claim. It will actually reduce flow, thereby reducing peak power. Your bike will perform better with a properly packed silencer. In most cases, when a rider claims their bike makes more power with an un-packed silencer, it's nothing more than a placebo effect caused by the increased sound. Sound does NOT equal power.
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X2.For me,it's time to repack when two things happen.1.power loss 2.noise. #1 is generally noticed first.
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Excessive noise can, will and has created more problems for motorcycles than any other reason.... bar none.
Riding or racing without a quiet bike in this day and age will do nothing for our cause.
While at the beginning the silencer was just a bolt on to reduce noise, over the years it has become an integral part of the whole exhaust system. In addition the newer machines have significantly more power and usually have a better power delivery as well.
Making your bike loud on purpose is downright stupid...