Regarding underdrive/overdrive, Al explained it thoroughly, and is right on. The larger pulley on the accessories will make them turn slower. (It's the exact same reason your speedometer indicates slower with larger tires).
Regarding the electric pump, there are horsepower gains to be had... The parasitic loss due to turning a mechanical water pump is significantly greater than the parasitic loss due to generating the electricity needed to turn the electric pump. It's the exact same benefit derived from electric cooling fans, and on newer cars, electric power steering. Even as inefficient as electric motors can be, it's still less parasitic power loss than a mechanical setup.
My question would be: if you could replace the mechanical pump with an electric one, how would you route the belt around the accessories to compensate for the water pump pulley no longer being there? Seems like there might not be enough contact area on the crank pulley after that... Just something to think about if you ever got to that point.
It's going to take a paritcular flow rate through the engine to prevent hot-spots in the heads/combustion chambers. Drop below that threshold, and the coolant will begin to boil locally, and that's not good... Keeping the flow rate up with an electric pump won't decrease the coolant temp, but it will prevent increases in the temp.
Also, Al, the temp increase at idle should be expected... It's not necessarily a bad thing. Because the coolant is moving through the head more slowly, it's absorbing more heat from the head. As it flows past the thermostat housing (where the sensors are located), the coolant is hotter, yes. But that doesn't mean it's hurting the engine. You're still well below the boiling point for the system, and it's not likely the engine is running significantly different.
The UD pulley is still providing enough coolant flow to keep the engine cooled at freeway speeds, as evidence by the solid 190 degree temp on the freeway.
Where you'd probably see a difference would be in the spark trim... If your scanner can monitor it, if the heads/combustion chamber are, in fact, running hotter, the engine won't be able to dial in as much spark advance as when it's cooler because of the detonation caused by the hot-spots. If the spark advance (under load, of course, so like leaving a stop light) is close to the same as with the stock pulley on it, and you're not seeing any significant performance loss off-idle, I wouldn't worry about it.
Now, all that being said, running near WOT, up a grade, pulling a trailer, the UD pulley may not be able to provide enough flow, and the temps might begin to rise beyond the heat carrying capacity of the system. If you think of it as an equation, the amount of heat the system can remove from the system always needs to be greater than the heat put into the system. (otherwise, it'll boil over). If the heat you're putting in can be carried away, you're fine. But the maximum heat put into it isn't going to be at idle... it'll be WOT, AC running, trans fluid warming up, 105 degrees, humid, running up-hill. The engineers design, test, and re-test the system for those operating conditions. That results in a vehicle that's got a massive cooling capacity at idle, but not so great of a margin under the worst-case conditions. Yes, it'll be over-cooled at idle, or at 20-30% power on the freeway, but it'll still be overcooled under the worst case scenario too.
Make sense? I'm rambling... sorry... just some food for thought.
-Joe
