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Old 01-09-2013, 08:55 AM   #17
Disco Dean
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Joined: Jul 2007
Location: The Great White North
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Quote:
Originally Posted by roger 04 rt View Post
As I mentioned a couple posts ago, all R series motorcycles have a barometric sensor buried in the ECU and the GS-911 does report it, see the data from an R1150 below. Ambient air pressure is column F.

It's my opinion that changing the exhaust or intake changes the tuning dynamically of the air flow based on what I've read in numerous books. When an exhaust pulse leaves the engine a pocket of high pressure air leaves the cylinder when it hits a change in exhaust geometry (e.g. wider or narrower pipe, the catalytic converter, etc.) some of that pocket of air reflects and travels back to the exhaust valve, we're talking thousandths of a second here. If the reflected pulses pressure is high while the exhaust valve is still open, less air fills the cylinder when the intake opens. If the pressure from the reflectednpulse is low at that moment, more air goes in when the intake valve opens. These dynamic conditions change with RPM. When you put on an aftermarket exhaust or intake you may change these dynamics which BMW has carefully measured and accounted for the the VE (volumetric efficiency) also know as the Fuel table.

Very cool and yes I was sure that the ECU had those functions (especially in the newer models of R bikes) From my experience the comments you make about "pulse' etc. is correct and most times are accommodated but are most important in very highly tuned engines. Most specifically in engines that take advantage of megaphone exhausts - when building a cam or engine timing in conjunction with the properly calculated taper and exhaust length & carb intake length one can in effect use the shock wave/pulse to hold gasses in the cylinder a little longer allowing for more aggressive valve overlap etc. making it possible to change up and be more aggressive with your cam timing and profiles - many many different factors to think about. However that is some very fine tuning and normally only reserved for racing applications as well as older megaphone exhaust systems. ie. Norton Manx singles or Honda 5 cyl multies. The benefits of doing this on a modern bike are largely negated when you have cross over exhausts, 4 into 2 into 1, cats, and non-megaphone exhausts. Much why we see very short exhausts on modern bikes. The added control of the ECU and fuel injection, along with the sensor control make this kind of thing a non-issue on modern bikes - so it is my understanding that the exhaust pulse and management of that for tuning purposes in modern bikes is not an issue worth addressing as it really is insignificant due to the above.

If this was such a big issue the aftermarket exhaust industry and can industry would be non-existant. I know there is a lot of fashion and hype about that but in most cases a full exhaust will add hp without changing mixtures on a modern ECU bike. It of course changes the airflow and the tuning but the ECU accommodates for that appropriately in modern bikes. Maybe not bikes that are 15 or more years old but the R1200 for the most part (it is only my understanding) is not affected for mixture - It is affected and shows performance gains with better air flow (filters) =more oxygen and better exhaust flow = less back pressure more flow - easier to push that out the back end.

Like you said - none of us really know unless we get the data on the specific bike and do the tests right... so it is cautioned speculation I think based on our combined years of experience... wow kind of scary that actually. But in the long run I think we are all pretty safe with these things - maybe not perfect but I think... safe.
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