AS POSTED BY billyburger :
I too have the Flow master 40/K&N FIPK II combo...but to be honest, I haven't noticed that big of a noise difference as you have. It does give it a little better throttle response but that's been about it for me.

You know what I think it is .
I think that if you have a 5Spd you make more hp when using this two modes (cold air intake + flow master) than automatic transmition because they go through higher rpm`s and more often. You got 5spd vs 4spd which go gradually through the rps .