[johngriff]

This should probably Get stickied.

First, your direct focus on EGT itself is a little misguided.

EGT

EGT can be seen as the byproduct of cylinder temperature, though there are quite a few different exhaust devices, such as headers, turbochargers, downpipes that tend to maintain a higher temperature than an aluminum combustion chamber, because they are typically iron/steel.

Because of this, EGT is not ALWAYS the most reliable tuning tool, and in theory, the closer you can get an EGT sensor to the head, the more accurate a reading of cylinder temp you will have.

Temperature is EVERYTHING

This brings the tuning discussion to the principal tuning factor, Cylinder temp, and attempting to avoid excessive heat.

Traditionally, gasoline is looked at as the "combustible" and air as the facilitator in the internal combustion process. In effect though, gasoline should really be looked at as the accelerant in the process of combusting air.

Taking this approach, the mixture of gasoline to air determines rate at which the combustion takes place, and more importantly the temperature of the combustion.

Traditionally, in 4 stroke gasoline tuning, the lean spectrum of AF/R will produce hotter cylinder temps, and the Richer spectrum of the AF/R will produce cooler cylinder temps.

Octane and Cylinder Temps Relationship

Octane ratings of Gasoline are an indication of the heat/temperature point at which the Gasoline will ignitie. The lower the octane rating, the lower temperature at which the gasoline will Ignite. The higher the octane rating, the hotter temp it will take to ignite.

This is why we traditionally prefer higher octane rated fuels for performance applications, because the likelyhood of "pre-ignition" of the fuel is less likely the higher the octane rating.

Cylinder Temp and Knock

With the above taken into account, it is important to control the Grade of fuel (octane) and the mixture of fuel (AF/R) to maintain acceptable cylinder temperature per your application. If the octane rating of the fuel is too high, or there is too high a mixture of fuel to Air, the combustion process will burn slowly, so slow that it typically will not burn all fuel, with that unburnt fuel exiting the engine through the tailpipe (Black Smoke Rich Condition).

On the other side of the equation, lower grade fuel, and higher cylinder temps result in the pre-ignition of fuel. This is known as Knock. Knock is bad because it is defined as an ignition of fuel mixture BEFORE the desired rotational point in the 4 stroke process. So when knock occurs, the energy of the combustion is met with the opposing force of the mechanical rotating assembly. At this point, the weakest material surrounding the combustion, or any gaps available will absorb the combustion process. This is why you see knock producing aggressively worn / broken rings, sleeves, valve seats and head gaskets, even taking pieces of pistons apart.

Boost and Heat and Knock

Going back to the rule that Air is the principal to combustion, adding more of it makes more power. The increased pressure of forced induction increases the likely hood of gasoline igniting. Typically this is the reason you will see "on boost" fuel maps AF/R result richer than "off boost" maps.


Summarizing the Tuning Process

First the engine must be able to adequately cool itself through fans, radiators and oil coolers. Once these basic systems are functioning, the tuner first adjusts the AF/R to a desirable ratios. This assures cool cylinder temperature throughout the map. The tuner then tunes ignition timing against tractive effort of the dyno so that for each degree of timing adding, at leas 2.5ftlbs of tq are gained. When additional timing nets less than this tq, ignition timing is very close to its knock threshold, or igniting the fuel/air mixture at a mechanically inappropriate period.

Fin

In the End, EGT is not a primary concern, if the foundation of the tune is correct. EGT is also a very poor indication tool to tune ignition timing against, though it can work as a very good alarm mechanism if a mechanical failure occurs.