I have had my K&N set-up on my car for a few weeks now. Went in today to get the car dynoed. Horsepower was DOWN 18, and torque was DOWN 33 compared to the stock airbox and filters. The graph was not smooth at all, and was all spiky. Something definitely not right.
So, remembering the threads on blockages and drilling, we pulled the K&N filter housing to check the openings. The crank case vent hose filter was completely blocked. There was a hole maybe as big as a pin, the rest was obstructed by a big gob of plastic. There was no way that any amount of air was getting through that vent hole. Out came the drill, and we opened the hole up. Then it went back on the dyno.
Numbers were now much better. Horsepower was now 7 ABOVE stock baseline (to 440) and torque was at stock baseline (476). This was after five pulls with a very very hot engine on a very hot day, so perhaps the numbers may have been a little better if I'd waited and cooled things off a bit.
The moral of the story is, make sure to drill your K&N filter housings. I hope that they will pay a little better attention to quality control from here on out. Seems a bit shoddy for a company with such a good reputation.
Kai