On my SOHC Vtec D16Y8, when I rev my engine to about 5500rpm, the sound coming from my loud intake changes tones. Is this the Vtec kicking in and why doesn't my friends exact same car and engine do the same thing. My car seems faster than his and all I have is intake. He has header, intake, exhaust, fuel crap, cam gear, and other stuff. Why is my civic stock just as fast ?