I've just fitted a K&N FIPK air filter to my E46 330CI and I'd like to share my thoughts on this product.
The reason I decided to get an aftermarket cone filter was for the reported extra induction noise. If there was going to be any performance gains, that would be a bonus. Out of curiosity, using an AP22 performance meter, I measured the car's BHP via its acceleration before and straight after the filter fitment.
The filter kit is a K&N FIPK model no. 57-1001 and cost me £222 inc. postage from an ebay trader. K&N claim that it gives an extra 11bhp and 10 ft lbs of torque throughout the rev range. The kit is very comprehensive, includes a heat shield, is well made and comes with clear instructions that make fitting it an absolute pleasure. It took me about 1.5 hours to fit at a very leisurely pace.
A subtle increase in induction noise is noticeable from half throttle upwards. It's a pleasurable kind of sound that greatly improves my enjoyment of the car.
The car feels more responsive and seems to rev more easily. For example, if I'm driving fast, I'll change up at 6000RPM instead of 5800RPM.
The harsh truth:
The performance meter recorded a 5 BHP reduction in power throughout most of the rev range after fitment of the filter. During the fitting, I had unplugged the battery. This had probably reset the ecu and so I drove the car for a couple of weeks to allow it to re-teach itself. When conditions were similar i.e. air temp, fuel load etc, I repeated the acceleration test for a third time. The result was that the power had improved over the previous run and was now identical (within 1 or 2 bhp) to that of the car with the original filter.
No power gain or loss with new filter.
Well, I hope this is of some interest to anyone considering buying an aftermarket filter. Perhaps the std airbox is a particularly good design which can supply more that enough air to the engine. Or, does the ECU adjust to provide less emissions from a more efficient intake design and therefore give no net gain in power? I've no idea and would like to know if an ECU upgrade would be able to take advantage of a better intake design (assuming that something like a K&N is a better design). It's interesting to note the incorrectly perceived extra power owing to the new filter's noise. If I'd known all of this before buying it, I'd still have bought one because of the sound. Regarding insurance, it's cost me about an extra £30 to have it as a notified mod.