iab's are the secondary butterfly valves in the gsr's dual plenum intake manifold. if you use a p28 and still have the stock intake manifold, the secondaries wont open at 5800 and could cause a lose in power.

another thing i read somewhere is that vtec wont engage below a certain throttle position. so even if you reach 4400 rpm but are under a certain throttle position, vtec wont engage. just something i heard. :dunno: