geojunkie
Guest
I have been exploring my options for exhaust systems on the 1200 and was surprised that most of the dyno tests are not showing much improvement over the stock system. For example, in the link attached only a couple of silencers showed to be better than the stock throughout the rpm range and many were actually worse at one end or the other. I can't read German, but click the "Leistung und Drehmoment" button for each and you will get the dyno results (the calibrated sound byte buttons below that are worthwhile too). I believe the tests are only the cans and not full systems. If someone can read German and confirm that please let me know.
http://www.motorradonline.de/static/specials/Auspuff_1200GS_2005/FrameSet_auspuff.html
Having just read about how the stock "silencer valve" works, I got to thinking that perhaps the dyno tests are not reflecting what happens in real riding. The BMW shop manual states that the valve opens up on the stock can only upon hitting a certain level of backpressure and after a period of time, due to its highly dampered actuation. Under a dyno test, the throttle is wide open the whole time, so this valve is likely always wide open for the entire dyno run.
That to me would say that the dyno comparisons are to a stock can with valve wide open. On the road, the valve takes time to open and maybe this is why people reporting on their aftermarket systems say they notice much better throttle response, even if they are not sure how much more power they have (louder sound does alter your perception of power). I am wondering if this valve opening is contributing to the surge in power that comes on in the 1200 (with the stock exhaust) at about the same time as the sound gets much throatier, and it is not just the cam profile doing this.
In summary, my theory is that the aftermarket systems may fare much better in the real world that the dyno charts show. I have to be well above 5000 rpm at lower throttle settings, or right near there wide open before it sounds like the stock valve is opening, and of course most of us let off to make each shift. So, the valve may be closed a lot and the backpressure increase may serve no useful purpose other than sound reduction, at least the BMW manual says that. It makes no claim that it improves bottom end torque, just that it lowers sound levels for city conditions. And I believe the dyno charts we see show the results with the valve open anyway, so this would reflect the stock can wide open performance. I would be interested if others concur with this, or have different thoughts.
BTW, it looks like the SR Racing can was the best of those tested, almost filling the power dip at 5000 rpm. But, that was can only I think. Are there tests showing it also has the best full system results out there? If someone found a way to get the stock valve to stay open all time, perhaps that would be the poor mans way to an upgrade. I noticed in the posts that one fellow just cut it off and welded the hole shut. Perhaps he was doing the right thing?
Dan
http://www.motorradonline.de/static/specials/Auspuff_1200GS_2005/FrameSet_auspuff.html
Having just read about how the stock "silencer valve" works, I got to thinking that perhaps the dyno tests are not reflecting what happens in real riding. The BMW shop manual states that the valve opens up on the stock can only upon hitting a certain level of backpressure and after a period of time, due to its highly dampered actuation. Under a dyno test, the throttle is wide open the whole time, so this valve is likely always wide open for the entire dyno run.
That to me would say that the dyno comparisons are to a stock can with valve wide open. On the road, the valve takes time to open and maybe this is why people reporting on their aftermarket systems say they notice much better throttle response, even if they are not sure how much more power they have (louder sound does alter your perception of power). I am wondering if this valve opening is contributing to the surge in power that comes on in the 1200 (with the stock exhaust) at about the same time as the sound gets much throatier, and it is not just the cam profile doing this.
In summary, my theory is that the aftermarket systems may fare much better in the real world that the dyno charts show. I have to be well above 5000 rpm at lower throttle settings, or right near there wide open before it sounds like the stock valve is opening, and of course most of us let off to make each shift. So, the valve may be closed a lot and the backpressure increase may serve no useful purpose other than sound reduction, at least the BMW manual says that. It makes no claim that it improves bottom end torque, just that it lowers sound levels for city conditions. And I believe the dyno charts we see show the results with the valve open anyway, so this would reflect the stock can wide open performance. I would be interested if others concur with this, or have different thoughts.
BTW, it looks like the SR Racing can was the best of those tested, almost filling the power dip at 5000 rpm. But, that was can only I think. Are there tests showing it also has the best full system results out there? If someone found a way to get the stock valve to stay open all time, perhaps that would be the poor mans way to an upgrade. I noticed in the posts that one fellow just cut it off and welded the hole shut. Perhaps he was doing the right thing?
Dan
