So I have been thinking about the subject of stock headers versus equal length headers for the XB12 for some time now. I will likely need/want to replace my headers within the next year.
I like the way the stock headers fit tightly to the bike and don't stick out like some of the equal length headers.
The stock header also allows the use of a nice big RSS, which is important to me.
I have always felt that Buell must have looked closely at the performance afforded by the headers they used and done a lot of testing to determine the best design.
This feeling was also expressed by someone on the site (I'm sorry I can't find your name or your post though I have searched) who wrote what I had been thinking, namely that the performance of headers in scavenging exhaust gases is not only a function of the length of the headers but more importantly of what the engine 'sees' when it operates.
We know the rear cylinder runs hotter than the front one and the front header is more exposed to cooling air than the rear one. These factors affect the temperature and velocity of the exhaust gases in the pipes and thus the scavenging and performance of the headers.
Ideally the headers are designed so as to create the greatest scavenging effect on the cylinder whose exhaust valve is opening at the time. In order to do this the length of the header pipes before they merge is chosen through calculation and testing such that the exhaust pulse from the previously exhausting cylinder creates a lower pressure area around the exhaust port of the cylinder which is just starting to exhaust. This helps improve the scavenging of exhaust gases from that cylinder and allows more of the fresh A/F mixture to be drawn into the cylinder thereby increasing power.
So I agreed with the other Bueller who made the comment. But it got me wondering if our theory was correct and in fact the reason Buell chose the stock header design for our bikes.
So I asked the question:
"…I imagine that Buell did a lot of testing of various header and exhaust designs during the development of the XB series bikes and settled on the design of the stock headers as the most efficient design, based on what the engine 'sees' as opposed to the theoretical advantage of equal length headers over unequal length ones. I theorize that the difference in operating temperature between the front & rear cylinders results in different exhaust gas velocities which would require slightly different header lengths for optimal scavenging. The fact that, to my knowledge, only the Micron headers ever produced a significant gain in HP seems to support my theory.
My questions, if you can answer them, are:
1- Is my theory concerning the reason for the unequal length of the stock headers correct?
2- Am I better off staying with the stock headers design or will I realize a power gain with any equal length headers?…"
I got the following answer in return:
"The different length headers were determined by a very specific formula, and are absolutely superior. These do exactly as you suspect.
FYI, the 1190RS also has unequal length headers."
So there you have it.
I am not saying, nor do I suspect that Buell is saying, that it is impossible to design an equal length header which could produce more power than the stock headers, just that the stock headers are the best compromise (all design is compromise) for what our bikes were intended to do and you won't gain much if anything, HP wise, from an equal length header over the stock header.
The HP increase claims made by Micron were always specified as being tested with the muffler and headers together, so it is hard to tell how much each contributed to the overall gain.
It would be interesting to dyno various different headers using the exact same bike and muffler, kind of like the ASB muffler shoot-out, to determine actual performance gains, if any, over stock.
Still, there is the aesthetic factor as well as the 'cool' factor and some of the equal length designs out there look very elegant - even if they do stick out a little further than I like and reduce the efficiency of a RSS
Just thought I'd pass the info, and Buell's take, on why our stock headers are the way they are and that our stock headers are probably the best option we have performance-wise in headers.
I like the way the stock headers fit tightly to the bike and don't stick out like some of the equal length headers.
The stock header also allows the use of a nice big RSS, which is important to me.
I have always felt that Buell must have looked closely at the performance afforded by the headers they used and done a lot of testing to determine the best design.
This feeling was also expressed by someone on the site (I'm sorry I can't find your name or your post though I have searched) who wrote what I had been thinking, namely that the performance of headers in scavenging exhaust gases is not only a function of the length of the headers but more importantly of what the engine 'sees' when it operates.
We know the rear cylinder runs hotter than the front one and the front header is more exposed to cooling air than the rear one. These factors affect the temperature and velocity of the exhaust gases in the pipes and thus the scavenging and performance of the headers.
Ideally the headers are designed so as to create the greatest scavenging effect on the cylinder whose exhaust valve is opening at the time. In order to do this the length of the header pipes before they merge is chosen through calculation and testing such that the exhaust pulse from the previously exhausting cylinder creates a lower pressure area around the exhaust port of the cylinder which is just starting to exhaust. This helps improve the scavenging of exhaust gases from that cylinder and allows more of the fresh A/F mixture to be drawn into the cylinder thereby increasing power.
So I agreed with the other Bueller who made the comment. But it got me wondering if our theory was correct and in fact the reason Buell chose the stock header design for our bikes.
So I asked the question:
"…I imagine that Buell did a lot of testing of various header and exhaust designs during the development of the XB series bikes and settled on the design of the stock headers as the most efficient design, based on what the engine 'sees' as opposed to the theoretical advantage of equal length headers over unequal length ones. I theorize that the difference in operating temperature between the front & rear cylinders results in different exhaust gas velocities which would require slightly different header lengths for optimal scavenging. The fact that, to my knowledge, only the Micron headers ever produced a significant gain in HP seems to support my theory.
My questions, if you can answer them, are:
1- Is my theory concerning the reason for the unequal length of the stock headers correct?
2- Am I better off staying with the stock headers design or will I realize a power gain with any equal length headers?…"
I got the following answer in return:
"The different length headers were determined by a very specific formula, and are absolutely superior. These do exactly as you suspect.
FYI, the 1190RS also has unequal length headers."
So there you have it.
I am not saying, nor do I suspect that Buell is saying, that it is impossible to design an equal length header which could produce more power than the stock headers, just that the stock headers are the best compromise (all design is compromise) for what our bikes were intended to do and you won't gain much if anything, HP wise, from an equal length header over the stock header.
The HP increase claims made by Micron were always specified as being tested with the muffler and headers together, so it is hard to tell how much each contributed to the overall gain.
It would be interesting to dyno various different headers using the exact same bike and muffler, kind of like the ASB muffler shoot-out, to determine actual performance gains, if any, over stock.
Still, there is the aesthetic factor as well as the 'cool' factor and some of the equal length designs out there look very elegant - even if they do stick out a little further than I like and reduce the efficiency of a RSS
Just thought I'd pass the info, and Buell's take, on why our stock headers are the way they are and that our stock headers are probably the best option we have performance-wise in headers.
