How hot is too hot?
In my opinion, the dipstick gauge should not be allowed to get over 260 degrees. How did I arrive at that number? There is a long explanation below. If I am doing sustained highway riding on a very hot day, my gauge maxes out at 260. All is fine, but if I then immediately slow down and start idling, the engine seems to be a little "loose" until it cools down. I have done this a lot of times, and I don't think 260 does any permanent harm, but I think that is the practical limit for these engines. However, at 260, I do think there is some break-down of the oil, which I think is a really good reason to change your oil every 1,000 miles,
which is what the manual calls for.
I know for a fact that lots of XS650s can easily exceed 260. So, what to do if your cruising along and you start exceeding 260? You simply lower your RPMs. You might think that the more RPMs, the more speed, and therefore more air-cooling. It doesn't work that way at highway speeds. What else can you do to lower engine temps? You can fix your lean condition -- try increasing the size of your main jet. Or you can install an external oil cooler.
I have done three side-by-side comparisons, running my bike against other XS650s, both bikes equipped with ThermoDipSticks. In all three cases, my bike ran 10 to 20 degrees cooler than the others. Thermometers were swapped in mid-ride, results were the same (these are very accurate and high-quality thermometers). Why am I cooler? Well, for one thing, I spent a LOT of time dialing my bike in, and the jetting is perfect. I use BS38s, and two of the "hot" bikes had BS34s, which came lean from the factory due to EPA requirements. There are probably other differences, such as octane used and riding style. For example, I run premium, and one of the bikes I ran against was using regular, and that rider liked to keep his RPMs high to avoid knocking
http://xs650temp.proboards.com/index.cgi?board=EngineStock&action=display&thread=16693&page=3
The above thread shows what temperatures I and others are getting, beginning on page 3. If you go to the beginning of the thread, page 1, you can see how the ThermoDipStick was developed.
How hot is too hot?
This is a really complicated issue. The following really simplistic answer says that motor oil begins breaking down at temperatures above 260 F:
http://www.answerbag.com/q_view/281441
On the other hand, Wikipedia says that in petrol gasoline engines, the top piston ring can expose the motor oil to temperatures of 320 °F:
http://en.wikipedia.org/wiki/Motor_oil
I pored through a lot of articles on the various XS650 forums, and I found where someone said optimum running temperature is 200 F, but that is certainly not a maximum, and it also implies that below 200 F is too cool for a number of reasons. For example, fuel does not burn as efficiently when you are below an engine's optimum temp.
This picture was made by the late xsjohn, who used an infrared thermometer for readings. His sump temperature of 140 is extremely low, but note that he is using an external oil cooler AND a big ol' automotive oil filter. Also note the 85 degree difference between the cylinder and the sump. I also have an infrared thermometer, and I intend to conduct my own cylinder temperature tests, which I will be reporting here.
BTW, my MikesXS oil filter/cooler kit just arrived this minute. I am going out right now to conduct before and after temperature comparisons, which I will also be reporting here.
