Replacement

First thing you notice about the (cylindrical) K&N filter compared to the OEM one is the diameter. The K&N filter is about 5 inches diameter whereas the OEM is must be at least 7 inches. The OEM was fairly clean (done 14K miles) except for a circle on the back side (windscreen side) that was fairly dirty.

 

Fitment is quite easy. Firstly, you'll need to disconnect the big induction hose from the front of the resonator box. Just loosen the jubilee clip around the hose/resonator by a few turns. I found it easier to wait until I'd also removed the screws from around the resonator box before actually disconnecting the hose. Be careful not to stress cables, wires and small hoses that attach to the intake hose whilst disconnecting it. Some attach underneath and aren't easily visible from above.

 

There is another small 1 inch diameter hose from the cam cover through to the left side of the resonator box. I tried to remove this by loosening the clips but it still seemed to be firmly stuck in place. Anyway, I found I could swing the resonator box open enough, after removeing the screws (see below), without having to remove or damage the small hose, so, I left it attached.

 

Next, remove the five cross head screws around the resonator box itself. These have a loose washer each, so, careful not to drop these. The nuts are retained inside the lower half of the resonator box, so, no need to worry about losing them. A flexi or articulated shaft screwdriver is best to loosen the screws, some are a bit awkward. Note that there is a black rubber stopper about halfway along the right side (when viewed from fron of car) of the resonator box. Removing this exposes a large hexagonal nut. Ignore this, it secures the lower half of the resonator box to the chassis. If you wanted to remove the whole resonator box in one go, then, you'd remove this but we don't need to.

 

Next remove the old filter, give the resonator box a quick clean with a slightly damp lint-free cloth (i.e. not tissues) and insert the K&N replacement. Then, close over the resonator box, replace and tighten the screws, re-attach the intake hose and re-tighten the jubilee clip. As above, I found it easier to re-attach the intake hose whilst closing the resonator box but left tightening the jubilee clip until last.

 

So, now for the road test....

 

At < 3K rpm

At turn over the engine revs about 250 to 500rpm higher than normal just for an instant. However, it now seems to be able to idle about 100 to 200rpm lower once the engine warms up. Sound wise, especially with windows closed, there's very little difference in the quality or volume of noise. The slight "bogging down" of the engine at low rpm that I've noticed when using Optimax seems to have eased off but you can still feel it a little. Overall the engine seems to pull more smoothly and slightly quicker.

 

At 3K to 6K rpm

When at constant speed the sound quality and volume (with windows closed) is little different from normal. Slighlty louder and with a more pronounced "whine". Under acceleration you begin to hear the difference, more "parp" and "whine". Again, it seems to pull more smoothly and slightly quicker but not greatly different (much better with windows open). The most noticable difference to me is the nice little "whaa" you get when lifting off. Strangely, this sound seems to eminate more from the exhaust/exhaust manifold than the resonator. Nice nevertheless.

 

At >6K rmp (VTEC)

This is the strangest one. Perhaps it's beacuse there's more volume below 6K rpm now, but, the change in sound quality and volume across the VTEC transition seems less pronounced than with the OEM filter. You still get the power/torque transition just that my expectation of that change over "howl" isn't so apparent. Probably this rev range shows the least noticeable power/torque benefit.

 

Overall

For £54 (Halfords) I think it was a good buy. Modest gains in quality and volume of noise. Small gains in power/torque below 6K rpm. Makes me feel it's going faster even if it's not.