[chuckm]

I'm not a diesel expert, so correct me if I am wrong on this:
The reason why a diesel engine will not respond well to a WAI relates to how engine speed and power is regulated. In a diesel engine, there is no throttle per se. You push the pedal, more fuel is added, increasing the downforce, thereby increasing power (whether translated to torque or rpm). The air intake is not throttled, meaning that the engines air is regulated by it's pumping. The AFR is highly variable, but typically run much leaner than stoich (28 - 32:1 rather than 14.7:1). Since the engine is pumping a set volume per revolution (rather than a set mass), a colder (ie denser) air charge means more power as the gases expand. Pumping losses due to increased viscosity are mitigated by the lack of a throttle plate.

In a conventional gasoline engine, you push the pedal and the throttle (air) is opened. Sensors then adjust the amount of fuel injected to acheive the proper ratio. A warm air intake, because of modern MAF sensors, will translate into less fuel injected. Pumping losses due to the throttle plate are slightly offset by the lower viscosity of the air.