Blaine B. said:
Not true, K&N filters better than most filters.
It's all a compromise, the paper filter, filters better, but also looses flow volume fast from fine dust, pollen, moisture, oil and whatnot. The paper filter gets plugged fairly easy. The Renix has a habit of spitting oil onto the paper filter. When the paper filter gets oiled, it is effectively plugged. A little oil on the K&N doesn't seem to hurt much.
The Renix has some room to tweak, but really not that much. More air and a little more fuel. Intake and injectors are probably going to give you the most bang for the buck. Getting the heads to flow better is likely the next step.
IMO tweaking the 4.0 is tricky, the air to fuel ratio is just about as lean as she will stand. Adding fuel isn't going to increase HP, if at all. Adding a little air flow and fuel while keeping the ratio constant (near stock) is likely the answer.
The formula is fairly simple, the higher in the RPM band the motor hits peak torque the more HP. Figuring out how to do that without reprogramming the fuel management system is the trick. Just adding air and fuel without messing with the management system isn't going to get you any dramatic increases.
A Paxton pump, an intercooler and larger injectors would likely pump up the horses dramatically, the question is how long would the motor last. The brute force approach.
If I was looking for horses, I'd call Paxton and ask if anybody has installed a pump on a 4.0 and ask how it turned out and how long it has been running. If Granatelli is still around, he always seemed fairly customer friendly and answered all of my questions.