THIS IS NOT MINE BUT FIGURED PEOPLE NEEDED IT SO HERE YA GO
This is a do-it-yourself on modding your intake manifold on a 5.2/5.9 Magnum V8. This is done to allow it to breath a little better, escpecially at mid to upper RPM range. First, some terms that will be used. The stock intake manifold sits directly beneath the air cleaner assembly and throttlebody. It is often referred to as a kegger because it looks like a small beer keg cut in half the long way. The kegger is a plenum design intake. This basically means that it holds a large volume of air inside. This air comes into the intake through the throttlebody, then into runners that then feed air into the cylinders on each intake stroke. On the bottom of the kegger is steel pan called the belly pan. A common problem with the kegger/belly pan design is that the gasket between the kegger and the belly pan can leak. This causes a vacuum leak and oil can enter into the intake plenum space. You will often hear people call this the plenum leak or plenum repair.
If you've decided to mod your kegger, you have two choices. You can either use the intake that is already on your truck, or you can buy a spare one and mod it on the side at your leisure. If you use the one that is on your truck, a good time to do this mod is if you are repairing your plenum gasket. If you buy a spare one, make sure you buy the correct one for your truck. There are two things to look for before buying a used kegger. First, if your truck has EGR ('94-'95, some '96, and some '97), make sure you get an intake that also has the EGR port. If your truck is '98 or newer without EGR, any '98 or newer intake should work, and one without a center divider would be the preferred intake to buy (more on that later). Second, see how many coolant temps sensors are on your truck. Some have one (some '97 and later), and some have two (some '97 and earlier). If your truck uses only one, you can use an intake that was made for either one or two (just plug the extra port if it has two ports). If your truck uses two, it is best to look for an intake that has two ports already. You can try taking one that only has one port, and drill and tap your own port, but this would not be the desirable method. Just keep looking for one with two ports already drilled out by the factory for you.
Using a spare intake has some advantages over using the one on your truck. One is that you can be working on it while still driving your truck, and at your own pace. Then it is ready to install when you want. Secondly, once you install the spare intake on your truck, you have the original one from your truck to either mod again, sell, or just keep as a backup. Thirdly, once you are done modding your intake, you can install the belly pan. This lets you have plenty of time to let the gasket sealer used on the belly pan gasket (ultra copper is what I used) cure before installing the intake in your truck.
Ok, on with the mod. My truck is a '99 with a 318. The stock intake is non-EGR. I bought an intake from a '97 that was also non-EGR to use as a spare intake. My truck only uses one coolant temp sensor port, but this intake has two. I'll just plug the second port later. The intake cost me about $25 shipped from a guy selling it on the forum. The steps below outline how I modded the intake. I will mention the tools I used, but it is really up to you and the tools you already have or are willing to buy. From start to finish, these steps took about 7 hours of total labor. I did it across the course of three evening.
1) The first thing I did was to remove all sensors and coolant lines from the spare intake. I also removed the belly pan that was still on it.
2) Next, I took the intake to a machine shop to have them check all the surfaces for trueness, and to machine as necessary. You don't want to waste time modding the kegger if it is not a good straight peice to begin with. A bad intake will result in leaks after it is installed. I also had them clean it up in a dunk tank so that it was free of oil and coolant debris. This cost $25 to have done.
3) Now it was time to start cutting it. I highly recommend that you do this on a wood work surface of some other soft surface. Since the intake is aluminum, it is easy to damage the surfaces as you are working on it on a work bench. A wood surface will not hurt the aluminum.
The first thing I did was to mark and cut the runners. On a 5.2, 1"-1.25" off the runner is considered by many to be appropriate. On a 5.9, you can cut the runner all the way down. Here is the mark I used as a guide. Use a sharpy and a straight edge to make your marks. Notice that I decided to make my cut at an angle to the existing opening edge of the runner. This opens up the runner more to the center of the plenum space, but keeps the backwall of the runner almost at its stock length. Another possible way is to make your cut parallel to the existing edge of the runner. Doing it this way would keep the opening direction of the runners the same as they are from the factory. It is up to you how you want to make the cut. No one has done enough testing of this mod to now yet which way works the best.
4) I then used a sawzall to cut the runners. I recommend using eye protection and even ear protection as you cut. It is very loud, and small slivers of aluminum can fly up. Cut slowly, so you do not overheat the blade. Use a blade rated for metal 3/16"-1/4". Be very aware of your blade as you cut. It is easy to cut up the belly pan mounting surface if you don't watch were your blade is. This could cause oil leaks into the plenum later. Also be aware of the clearance around the tip of your blade. As you can see, the cut is not quite in line with my mark. This is because the angle of my line was too sharp for the sawzall. I would have cut into the belly pan mounting surface if I had stayed on my line. You can leave it like this, or get back on your line using other smaller cutting tool (Rotozip, Dremel, or even a hand file for example). As you near the end of your cut, slow down so that you don't bust through the cut and hit your blade on something you didn't intend to.
7) If your intake has the center divider, it is time to remove it. Chrysler engineers figured out after a few years that the removal of the center divider promoted better air flow. Most '99-'01 intakes do not have the center divider. Since the intake I am modding is a '97, it has is. First, I tried to use a dremel to cut along the bottom. Even though this was easy to do, the blades broke very easily and did not make much progress.
Notice you also need a very small diameter cutting wheel to fit in the confined space, so the EZ-Lock blades can not be used here becasue they are too big around.
At this point, I decided to use the sawzall to remove as much of the center divider as possible. I figure this would make removing smaller pieces of it with the dremel easier then trying to cut the whole length of the divider with the dremel. I used a combination of angled cuts and straight cuts as you see here:
Then, I used a hammer to remove smaller pieces that were between the cuts. This worked well and in no time I had the vast majority of the center divider removed. Make sure the peices you are trying break with the hammer are not too wide (the one pictured here was almost too wide). This might require additional cuts with the sawzall, but better that then breaking the intake by trying to snap off too big of a piece.