Newbie Intake Manifold Question

Better for the flow to change out the plenum.. usually accompanied by a TB change as well.

Changing the lower intake isn't generally a big mod unless you're going from NON-PI to PI, or upgrading for strength (read: our plastic intakes have been known to give out under nos, boost.)

Do a search for intake setups, and see what you come up with. It is typical for most guys looking for some extra power to buy a new intake elbow and throttle body setup from some company like Dragon, Accufab, BBK, C&L, or Trickflow. The typical power gain is usually 5-8rwhp across the power band, so it's nothing big. As for the lower intake, there are a couple of companies (Purple Racing and HPS) that are working on an aluminum lower intake for the Modular 4.6ltrs. So far, neither have any dyno results that prove they make any power. The only one of the two that is in production is the one offered by Purple Racing, but it's almost 900 dollars shipped for one of those. Hopefully, someone will get some dyno results on them soon, and we'll know whether they make any power.

As for proven intake manifolds, there are very few fully assembled aluminum intake manifolds that actually make more power than stock, and all of them are very expensive. The Bullit intake is very popular, but the full kit will cost around 1600-1800 dollars before it's all said and done, as you have to buy a dual bore throttle body and new alternator to go with the manifold.