Hi m3!!! ^_^. You see I didn't use the performance air filter, b'cause my 660 have very limit of torque, I have borrow and try the "ah beng brand" performance air filter from my friend, result is; sound like racing, easy to rev and only present the power at hi rpm range...
As I told you before, the way to my working place is a up hill road, when climing up hill it feel harder then before, must rev "gao gao" to give more power, after I have changing to the zenden extractor and try again, it go more worst. But when runing at high way, ha ha!!! feel like 850. So I have no choice to advance my inigtion timing to make the torque output early, and change back to original air filter.
If I'm right, sekigun is using the A'pexi air filter, compare to the VISION air filter, it is two diffrence design. Take a closer look at the top of the VISION's air filter, there is hole at top of it, when runing at low rpm, the air are flowing in at the side, when hi rpm, the air flowing in both direction, they call it "2 way flow design".
This design is to prevent the air flow loseing the flow speed, at the same time allow engine suck in more air. So it can give better pick up and top speed at the same time. Just like I told jimmy before ^_^
Here is my 660 engine bay again ^_^!