The stock fan clutch isnt helping this at all, but you can adjust the limiters in the tune. My 02 and a few trucks Ive tuned will rev to almost 5k in P/N at a stop. Over that it they dont seem to rev anymore, not sure why.Ours is electronically controlled on our trucks so when in park/neutral it won't rev over 4,000. That's why the rev is always really weak. If you are driving sometime, say at least 10 mph or 20 mph, bump it into neutral, then rev it. It will go all the way to redline, and you can hear it suck in all that extra air. Sounds pretty badass when you hear it rev the way it does for the first time, depending on your mods.