That's all it does, make the vtec engage at the rpm level you set it to. You may get an extra 1-2 hp out of this, but without mods, it just gives you the hp you get from vtec for a longer period of time. (and kills your gas mileage) They are fun though, not having to wait so long for vtec. I had a chip in my B16 4G hatchback with vtec coming on at 4200, rev limit at 8400, and no speed limiter. Ahhh, good times, good times...