Review on Enhance Your 2003-2004 TOYOTA 4 Runner's Performance with K&N Cold Air Intake Kit: Increase Horsepower with 63-9022 Compatibility! by Trevor Bannister

Easy installation. Took me about 30 minutes. Didn't notice a big difference. I hear a faint "whistle" when I step on it, but it's nothing critical. Overall I'd say it's not a worthwhile upgrade if you don't have the cash to toss around. In conclusion I would like to argue that this is not a true cold air intake as this filter is not really isolated from the rest of the engine compartment. So the air it brings is hot air. It's certainly counterproductive, or at least the air intake is slightly better than stock.