Where to start.. It's a fairly simple and straightforward procedure. If you do it, I recomend getting a good grinding bur(for a rotary air tool). The cheap ones work but don't last(in my humble opinion). You can pick up a good cast-iron grinding bur for a air dremel for around $30 at a tool shop(I borrowed mine from a buddy who bought it so I could do his heads). You only will need the 3" piece as you don't need to go very far in to port the mani's on the car.
Start off by dropping off the exhaust(disconnect all the o2's). You may have to disconnect the shift cable bracket to get the piping off. I found it helps to remove the resonator to put it back up and would probably have helped when taking it down. Undo your four bolts/nuts that hold the downpipes to the collectors. Mine requires a 15mm deep swivel socket for the drivers side outside bolt. A swivel socket does help on the passenger side as well, but is not required. Remember when doing this, you're dealing with cast iron. And when in the car, you are looking up at them. Get a set of the full surround goggles. Most people only have two eyes, and cast iron splinters just don't feel good in them.
Once you get the exhaust down and out, you can see right up in the manifolds. In the pictures that follow you can see the lip in them suckers and see why it's not bad to do this. You are more than welcome to take out as much as you want, but I strongly advise you keep your grinding greed under control. If you take out too much and remove too much of the ball seal thingee that the down tubes hook up to, you won't get a seal.
Now with a good bur, this won't take hardly any time at all. I spent about 30 minutes doing the grinding I did.
I only took the lip off of the inside of the flange for the ones that are in. I believed that a nearly stock NPI 4.6 like mine wouldn't need any more. It might have been able to take more, but someone else can try that.
While I can not provide any dyno numbers before/after for this, I can testify that afterwards my car shifts 100 rpm later than before(5500 to 5600 now). Possibly a minimal loss in the bottom end.
I have a set fully ported for another engine that I did myself as well. Here are the pictures:
So... overall, I give this a two beer
do it yourself
Author Douglas Sajak