| Author | Message | ||
     Thirty2vsvt |
hey guys, i'm new to the board and just bought an 05 xb12 lightning last week. so far i love it. it is completely stock and just under 5k miles. eventually i will buy a louder muffler or get a used stock one and modify it, but until then i was wondering if there is any harm in wiring the interactive valve open. earlier i unhooked the cable from the servo and held it open. it sounded pretty good at idle. anyone done this on their bike? | ||
| Tdiddy |
Don't do it. You need the back pressure this valves creates at different RPM speeds. You will notice a little more sound but your new bike have flat spots in the power curve. Just roll down the road 50 mph in 5th. gear and wack it open. You'll hear the valve open and the bike will start to pull hard. As the valve is opening it sounds good. After a few seconds, the valve will close, the tone changes and the bike still pulls hard. Just under 5000 miles? Your motor should start to feel as if it has more power as it fully broken in. Congrats on your new bike, You'll love it! | ||
| L_je |
If you are using the stock exhaust and ECM, you'll be worse off if you disconnect the servo. The interactive exhaust has a job to do. http://www.badweatherbikers.com/buell/messages/142 838/298923.html?1187104940 | ||
| Blake |
"Back pressure" is rarely if ever helpful to modern engine performance. I wish folks would stop using that mythical rationale. "Vacuum" at the exhaust valve is what is desirable, not "pressure". The negative pressure pulses, vacuum pulses, from the exhaust tract arrive at the exhaust valve depending upon two major factors, engine speed, and exhaust tract geometry. So the optimum exhaust tract geometry for any given engine speed is that which puts a negative pressure pulse at the exhaust valve just as it is closing and while the intake valve is opening, the intake/exhaust valve overlap event, thus helping to draw additional fuel/air charge into the combustion chamber, thus increasing volumetric efficiency, meaning increasing engine performance, HP. The interactive exhaust provides a variable exhaust tract geometry tailored for optimum performance depending upon engine speed. It has three ranges of engine speed for which is provided different exhaust tract geometry. The low and high speed ranges happen to use the same geometry, while the mid-range speed uses a differing geometry. Each geometry is designed to put a negative pressure pulse, a vacuum pulse at the exhaust valve during the intake/exhaust valve overlap event. | ||
| Tdiddy |
Thanks for the clarification Blake. Everyone learns from this board.  | ||
| Jaimec |
Hmmmm... I do seem to remember Kevin Cameron explaining it quite differently, and it did involve a back pressure wave (that moved at the speed of sound) from the exhaust back to the engine, and that this was desirable for stuffing an extra charge into the combustion chamber. That article was written MANY years ago, though, and I no longer have it to quote precisely. I do know that what Blake describes SOUNDS like it would result in unburnt fuel getting sucked out the exhaust before there is a chance for combustion to occur... a DEFINITE no-no these days. (Message edited by jaimec on August 15, 2007) | ||
| Dre99gsx |
So... does this mean all other exhaust upgrades (which don't include a flapper/valve) will induce torque flat spots? If this valve is desirable, wouldn't the stock exhaust system be the best for this bike? I have been considering upgrading the exhaust but from the americansportbike review, none of the upgrades seem to make much of a difference from the stocker. Otherwise, I'de just open the valve as well for the sound. | ||
| Djkaplan |
"Hmmmm... I do seem to remember Kevin Cameron explaining it quite differently, and it did involve a back pressure wave (that moved at the speed of sound) from the exhaust back to the engine, and that this was desirable for stuffing an extra charge into the combustion chamber." You can experiment and induce this pressure wave yourself in a small diameter straight tube by blowing through it while you're humming. You can actually feel how when you approach resonance, it will help or hurt flow through the pipe as you modulate the humming and airflow through the pipe. It has nothing to do with backpressure; it hase everything to do with the resonance of the sound waves produced by your vocal cords. These helpful pressure waves doesn't occur over a wide range of parameters, so Buell used a servo to create different parameters for it to happen. The washer trick that old school Sportster riders used to smooth out and fill in dips in the powerband actually worked on this principle. A lot of people thought it was the backpressure that helped, but it was the washer changing the rpm where these pressure waves would occur. Granted, there is little connection between your vocal cords and the sound and pressure wave (sound is a pressure wave) created by a combusting medium under compression, but the principle is the same. Try it and prove it to yourself. You should hear my explanation of how the Helmholz tube works in the breadbox on tubers. (Message edited by djkaplan on August 15, 2007) | ||
| Djkaplan |
"I do know that what Blake describes SOUNDS like it would result in unburnt fuel getting sucked out the exhaust before there is a chance for combustion to occur..." During most of the exhaust stroke the intake valve is completely closed, not letting any atomized mixture into the combustion chamber. During this early phase of the exhaust stroke, any help you can give the piston to evacuate the products of combustion is a big help. It's only at the very end of the exhaust stroke that the exhaust and intake valves are open at the same time (this is called overlap). On turbo or supercharged engines, overlap is kept to a minimum to keep the mixture from blowing right on through the cylinder, but on naturally aspirated engines (only atmospheric pressure) you can use this overlap period to help fill the cylinder. Too much overlap and you can get the exact opposite: you can contaminate the intake charge with spent gasses because of reversion. I read Kevin Cameron too! I hope I remembered it right... | ||
| Jaimec |
Thanks for the explanation. I do remember the "wave" we're talking about is very different from the pressure wave that helps charge a two-stroke engine's cylinder but the principle was similar; stuff the combustion chamber with as much combustible material as possible. | ||
| Xl1200r |
An aftermarket muffler doesn't have to give a flat spot because of a lack of a valve, it just depends on how it's tuned. An exhaust system that cannot change can really only be optimally tuned for one RPM range and one throttle load position. Once you move outside that, there will be a better exhuast design that would yeild more power (BTW - the same is true for intake tracts, and many car manufcaturers have variable intakes as well). | ||
| Blake |
Following is a repeat of something I wrote a few years ago. It popped up via a general google on the issue. If by "back pressure" we are describing the resistance to flow inherent in the exhaust system then there is only one correct answer: The less back pressure the better. But that is not the only exhaust system parameter affecting engine performance; in fact in some cases it may not even be the primary factor affecting engine performance. An engine exhausts hot combustion gasses not in a steady state continuous flow, but in discreet violent pulses. Each exhaust valve opening and closing event defines the timing and duration of each exhaust pulse. At the front of each of those pulses is a high pressure wave front where extreme combustion chamber pressure first encounters the much lower pressure inside the header pipe. At the tail end of each exhaust pulse is a low pressure wavefront, a "trough" if you will. Whenever these pulses both high and low encounter a discontinuity (any change in surrounding geometry) a portion of their energy will be reflected back up the exhaust track towards the exhaust port. This is similar to the behavior of waves in a body of water that encounter obtacles or discontinuities in boundaries. The reflected portions of exhaust pulse energy can either assist cylinder evacuation (low pressur epulse) during the exhaust event or they can hinder it (high pressure pulse). But most importantly there is a tiny window of opportunity as the exhaust valve is about to close where the intake valve is opening. This is the "overlap" of the exhaust and intake events. If the exhaust tract is designed such that a reflected low pressure (suction) pulse arrives back at the cylinder head's exhaust port during the overlap event, it will actually assist in evacuating spent combustion gasses and even help to fill the combustion chamber with new incoming air/fuel charge. It acts like a vaccum cleaner sucking up the spent combustion gassses. If however a high pressure pulse arrives at the exhaust port during the intake/exhaust overlap event, it will hinder evacuation of spent combustion gasses and even push new incoming air/fuel charge back out of the combustion chamber. A high presse pulse then act like a blower trying to push spent combustion gasses back into the combustion chamber. Not good. So why all the hoopla and competition in aftermarket exhaust designs? They are all free flowing. They all seek to put the reflected low pressure pulse at the exhaust port during the overlap event. So why are the results different? The difference is in the intent of the muffler designer and where he/she seeks to optimize this low pressure pulse performance wrt engine speed. See, the exhaust pulses travel at virtually the same speed, no matter the speed of the engine. Regardless of engine speed (RPM) the pressure pulses of the exhaust take a set constant time to travel to discontinuities in the exhaust tract, and then be reflected traveling back to the exhaust port. It is only when the duration of the reflected exhaust pulse round trip coincides with engine speed such that the negative pressure pulse arrives during the overlap event that we obtain optimum performance. Muffler designers are basically configuring the geometry of their products to achieve that optimum performance for engine speeds which they feel are most important to their customers. They also may utilize varying geometry (megaphone, stepped pipes, muffler valves) to achieve a wider more consistent range of optimum performance across the operational range of engine speeds. That is the challenge. And no one system will provide optimum performance across the entire rev range of the engine. Not yet anyway. The optimum design would be a perfectly free flowing straight pipe that could change its length with engine speed, starting out longer, then getting shorter as engine speed increases, thus always causing the low pressure trough/pulse that is reflected back from the tailpipe opening to arrive at the exhaust port at the most opportune optimum time. Like a sliding trombone. Freer flow is a start. Optimized geometry is key. You can actually significantly improve performance of an engine at a certain engine speed by strategically placing a small obstacle (geometric discontinuity) within the exhaust tract. It is not the added backpressure (increased resistance to flow) that aids engine performance. It is the low pressure wave that reflects off that obstacle that is aiding performance. A stepped or megaphone style straight pipe is the absolute optimum configuration for an exhaust tract as they each provide a number of discontinuities optimally configured for the desired powerband while avoiding serious impediments to flow. They are also horrendously loud. Pretty sure there are animated gifs on the web that illustrate this whole situation. Maybe someone can find one and post a link. Some more excellent exhaust design and effect information... http://www.rbracing-rsr.com/exhausttech.htm Backpressure used to be vital in order to prevent a too efficient exhaust tract from burning up the valve seats and valves. Newer improved alloys that we now have in our commercial mass-produced engines are better able to withstand the higher temperatures of a freer flowing exhaust tract. | ||
| Diablobrian |
Great link Blake! Although they minimize the value of coatings and wrappings even for heat suppression which we have seen and felt to be capable of making serious changes to the amount of heat radiating from our bikes. Then again they are pretty Harley-centric (to coin a term) so I guess there could be some difference between the two brands. | ||
| Blake |
Thanks Brian. I agree on the exhaust wrap. It does make a difference. | ||
| Tom60 |
Good read.. I like the sliding trombone idea  I wonder however what the guys at RBR/LSR would make of our valve system - they ridicule the harley servo valve system  | ||
| Xl1200r |
I wonder however what the guys at RBR/LSR would make of our valve system - they ridicule the harley servo valve system Aren't LSR's pipes tailored after their Land Speed Bike designs? If so, they're really only looking for peak power - no valve needed because the pipe only needs to make good power in one part of the rev range. | ||
| Tom60 |
True, but they also sell pipes for stock or "streetable" harleys. You can have a look (the link above by Blake). | ||
| Mndwgz |
Is our exhaust valve restricting or redirecting flow in a stock can? Anybody have pics if the inside of the stock muffler? | ||
| Tom60 |
Fuell magazine, Nov/Dec 2003 (article by Dan Hurda, with pics) | ||
| Thirty2vsvt |
thanks guys. lots of great info on here. i'm not going to mess with the valve. i would love to get a buell race muffler or even a drummer, but the race muffler is hard to find and i really dont want to spend that much on a used muffler and then the $ for the drummer (i'd like to keep my stock muffler just in case) im leaning towards a jardine system. do these kits come with the brackets to mount the chin fairing? | ||
| Mndwgz |
"Fuell magazine, Nov/Dec 2003 (article by Dan Hurda, with pics)" Not on the buell site.... where can I find it? | ||
| Tom60 |
It IS there  MAIN MENU: The Buell way - Buell on Buell - Buell Tech |
