gripper wrote:Will not a over advanced spark give red exhausts too?
I have to disagree with others on this .... over advanced spark will make the internal (inside the cylinder) temperature higher than normal, but the exhaust temperature may be less than, normal, or higher than normal.
The reason overly retarded timing affects exhaust gas temperature is because fuel takes a finite
time to burn. With overly retarded timing, fuel is still vigorously burning as it passes into the exhaust ports and headers, raising the exhaust gas temperature, but the internal cylinder temperature is lower than normal, since less fuel was burned in
the cylinder as compared to normal timing.
With overly advanced timing, virtually all the fuel burns in the cylinder. This causes a slight increase in cylinder temperature as compared to normal, because a small amount of burning fuel leaves the cylinder with normal timing. However, a larger temperature change occurs in the cylinder with overly advance timing because the first fuel to burn is working against the crankshaft rotation. In effect, the other cylinders must put work into
the cylinder to turn the crank past TDC. This additional work input raises the cylinder temperature higher than normal. The exhaust gas temperature is not so predictable.
In short, with late timing the header pipes run very hot because fuel is burning get in the headers. With very advanced timing, the exhaust temperature is reflecting the events inside the cylinder.