Hi All,
I was pondering the whole jetting situation the other day after reading a few posts on the subject, suggesting that the 07's have some sort of jetting guide, and pm'ing with a few members who had some questions and problems.
I used to fly small planes on a regular basis, and part of the preflight work you had to do was calculating density altitude, more so in planes for the rate of climb, and runway distance required for take off. This was especially important at higher altitudes on smaller runways often associated with general aviation airports in those area's.
Density altitude is just what it implies, how dense the air is at a given altitude. One uses pressure altitude (altitude indicated by an altimeter set at 29.92 HG (inches of mercury) ), temperature, and dew point to arrive at the density alititude for a given fixed altitude on a particular day using the forementioned varialbe criteria. And thus, density altitude determines how much oxygen, and pressure is present which directly affects jetting and the resulting engine performance.
I had got out my old Jepsen slide rule computer that I used to calculate these figures years ago and in the absence of an altimeter, but armed with the knowledge of a known altitude and temperature, and by using the NOAA website was able to obtain a barometer setting for the area in mind I found the following.
At my summertime riding area in the Sierra's @ 8400'msl (mean sea level) @ 89 degree's F, local barometric pressure of 29.96HG, with a dew point of 63 degree's, the density altitude is 12,109'. In other words, your motor will think it's at 12,109, and will perform accordingly. That's quite a difference eh? Which will also explain why you get short of breath at high altitude during the summer, and not so much during the winter because during the winter the temps are colder and thus the density altitude is lower than the known altitude. For example, if we use the above criteria but just change the temperature to 20 degrees the density altitude decreases to 7827'.
So it becomes very evident that one would need to correlate density altitude with changes made in ones jetting. Once this has been recorded it will be quite easy to jet a bike for a given riding area/time of year, using the readily available weather data, and known altitude.
I did some poking around on the net and found this cool calculator page that I'm sure many of you will like http://wahiduddin.net/calc/calc_da.htm It not only has a density altitude calculator, but also max speed calculator, dyno correction calculator etc... There is also a link to a pdf file for Rotax jetting calculater based on mmbar/temperature. And as it notes this chart is only appliclable to carbs that use jet sizes that are related to jet size diameter.
Go and check it out,
I was pondering the whole jetting situation the other day after reading a few posts on the subject, suggesting that the 07's have some sort of jetting guide, and pm'ing with a few members who had some questions and problems.
I used to fly small planes on a regular basis, and part of the preflight work you had to do was calculating density altitude, more so in planes for the rate of climb, and runway distance required for take off. This was especially important at higher altitudes on smaller runways often associated with general aviation airports in those area's.
Density altitude is just what it implies, how dense the air is at a given altitude. One uses pressure altitude (altitude indicated by an altimeter set at 29.92 HG (inches of mercury) ), temperature, and dew point to arrive at the density alititude for a given fixed altitude on a particular day using the forementioned varialbe criteria. And thus, density altitude determines how much oxygen, and pressure is present which directly affects jetting and the resulting engine performance.
I had got out my old Jepsen slide rule computer that I used to calculate these figures years ago and in the absence of an altimeter, but armed with the knowledge of a known altitude and temperature, and by using the NOAA website was able to obtain a barometer setting for the area in mind I found the following.
At my summertime riding area in the Sierra's @ 8400'msl (mean sea level) @ 89 degree's F, local barometric pressure of 29.96HG, with a dew point of 63 degree's, the density altitude is 12,109'. In other words, your motor will think it's at 12,109, and will perform accordingly. That's quite a difference eh? Which will also explain why you get short of breath at high altitude during the summer, and not so much during the winter because during the winter the temps are colder and thus the density altitude is lower than the known altitude. For example, if we use the above criteria but just change the temperature to 20 degrees the density altitude decreases to 7827'.
So it becomes very evident that one would need to correlate density altitude with changes made in ones jetting. Once this has been recorded it will be quite easy to jet a bike for a given riding area/time of year, using the readily available weather data, and known altitude.
I did some poking around on the net and found this cool calculator page that I'm sure many of you will like http://wahiduddin.net/calc/calc_da.htm It not only has a density altitude calculator, but also max speed calculator, dyno correction calculator etc... There is also a link to a pdf file for Rotax jetting calculater based on mmbar/temperature. And as it notes this chart is only appliclable to carbs that use jet sizes that are related to jet size diameter.
Go and check it out,
