The most accurate place is at ther outlet of the intercooler. By putting it in the engine bay, near the manifold, the sensor itself starts to heat up due to the radiant heat coming off from the engine. Very expensive sensors are specially designed to combat this - what u are trying to measure is temperature by conduction.

If you place it in the vacume pipe, u will also get dodgy readings. Air here is moving slowly, and easily picks up heat. The very fast moving large volume of air at the outlet of the intercooler does not pick up any radiant heat on it's way to the manifold. The senor is kept at air temperature, and can respond much more quickly to changes.[/quote]