Here is the more common answer (or way of putting it) -the heat of your exhaust (header) is related to the exhaust gas coming out of your engine. The temperature of the exhaust gas is related to the design of the engine and/or how the engine is tuned. If its running lean, the exhaust gases will be hotter and therefore lead to blueing due to chrome oxidation. If its running super-rich then this can also lead to blue-effects.
Here is a more technical explanation from a Japanese patent application (inventor was suggesting that silicon oxide film on the surface of an exhaust pipe would prevent the blue-effects.
Enjoy,
Andy
In many motorcycles or the like incorporating an internal combustion engine, the internal combustion engine is exposed in such a manner that the exterior appearance of the internal combustion engine can contribute to the aesthetic design of the motorcycle.
[0005] FIG. 14 is a side view showing an example of a sports-type motorcycle. A motorcycle 200 shown in FIG. 14 includes a V-type engine 201 and an exhaust pipe 202 for guiding along exhaust gas. Two cylinders of the V-type engine 201 become united into the single exhaust pipe 202, which extends toward and above the rear wheel so as to allow exhaust gas to be discharged at the rear portion of the body. In the present specification, an "exhaust pipe" means the entirety of any structure which composes a channel for guiding along the exhaust gas from the internal combustion engine, and may include any portion defining a muffler 202a or the like.
[0006] In general, the exhaust pipe 202 must have a certain thickness for allowing the exhaust gas generated in the internal combustion engine 201 to be efficiently discharged. Moreover, the portion constituting the muffler 202a has an increased diameter in order to accommodate the muffling structure. For this reason, the exhaust pipe accounts for a relatively large part of the exterior appearance of the entire motorcycle, and thus, the shape and color of the exhaust pipe have an impact on the entire motorcycle design.
[0007] For these reasons, the shape and color of the exhaust pipe are important factors in determining the entire motorcycle design. Therefore, it is common to construct the exhaust pipe in a smoothly bent shape to present a strong and voluminous impression, or finish the surface of the exhaust pipe with a lustered metal color or finish for an enhanced contrast against the other component parts. It is also common to finish the exhaust pipe surface with a similar color tone to those of the other component parts to obtain a consistent look with the surrounding structures.
[0008] In general, an exhaust pipe is formed of carbon steel tubes for machine structural purposes (STKM), stainless steel (SUS), titanium, or the like. In the case of an exhaust pipe composed of STKM, the surface of the exhaust pipe is often plated with chromium for better esthetics.
[0009] However, the exhaust gas traveling through the exhaust pipe, which comes directly from the internal combustion engine, has a high temperature. Therefore, when the exhaust gas passes the exhaust pipe, the exhaust pipe reaches a high temperature, as a result of which the chromium-plated pipe surface may be discolored, oxidized, or otherwise deteriorated. In particular, some regions may suffer from blue-violet or reddish-brown discoloration. Moreover, even in the case of an exhaust pipe which has been formed of SUS or titanium without being chromium-plated, the surface may still be colored blue-violet or reddish-brown due to high temperature, the generated oxide film may peel, and/or change in color tone may result due to oxide film formation.
[0010] Thus, due to blue-violet or reddish-brown discoloration or surface deterioration of an exhaust pipe having metallic luster, there occurs a problem in that the exterior appearance of the entire motorcycle design is degraded. This problem has become particularly significant in the recent years because the performance of internal combustion engines has been improved, resulting in an increased temperature of the exhaust gas.
[0011] This problem may be addressed by preventing the temperature of the exhaust pipe surface from becoming too high, by adopting a two-fold or three-fold cylindrical structure for the exhaust pipe. However, even by using a two-fold or three-fold cylindrical structure, the temperature of the exhaust pipe surface will not be adequately reduced, and surface oxidation or deterioration due to heat will not be completely prevented. Moreover, in this case, there is another problem in that the outer dimensions of the exhaust pipe are increased.
[0012] Discoloration, oxidation, or deterioration of the exhaust pipe surface may be addressed by covering the exhaust pipe with a cowl or a protector so that the exhaust pipe will not show on the exterior. However, in this case, the exhaust pipe does not contribute to the entire motorcycle design, thus making it difficult to pursue a desired characteristic beauty and aesthetic quality of a motorcycle.
[0013] In order to solve the above-described problems, the inventor has proposed forming a silicon oxide film on the surface of an exhaust pipe in order to prevent discoloration of the exhaust pipe (Japanese Laid-Open Patent Publication No. 2002-332838). As disclosed in Japanese Laid-Open Patent Publication No. 2002-332838, by forming a silicon oxide film having a thickness of about 0.05 .mu.m or more on the surface of an exhaust pipe through sol-gel processing, it becomes possible to obtain an exhaust pipe which is unlikely to suffer from discoloration even when heated to a temperature of about 400.degree. C.
[0014] However, it has been found that an exhaust pipe which is obtained by the technique disclosed in Japanese Laid-Open Patent Publication No. 2002-332838 may suffer from drastic surface discolorations due to exposure to a temperature exceeding 400.degree.
