Filter prefilters are a long conversation but in short, if they are on an oiled foam filter they suck-down onto the surface getting the oil on them which makes all that should pass through them to the filter stick on its surface blocking air. The more it gets blocked the more it restricts. Better to put a large prefilter away from the foam filter like on the airbox's intake. If you are not using an oiled dual stage filter like the Unifilter, and you are using the CFM filter, your engine is at risk of a short life from particle ingestion.