hehe i mean 'before the charger' . I think if you imagine the airflow as a stream, the origin of the air is the atmosphere (like the top of a mountain for a stream of water), as it flows towards the engine it passes for example the MAF (a dam in our water example), then through the charger, those two items are then upstream, (once water's gone through the dam, the dam is upstream of the river). that is my understanding.