Short Description: Allow more air through your stock air box, and let your engine breathe.
This is a really simple way of gaining a few extra hp and keeping the cost of buying a whole new intake down. Plus if you get daring, and decide to go through water higher than the bottom of the deck plate, you can just close it off. However, thats pretty high up here, and you'd be foolish to atempt something like that with a stock air box anyway.
Take a trip to your local marine hardware shop, and buy a 4" deck plate, or "inspection hatch", and "sensor safe" sealent of your choice. You also need 6 bolts, washers, and nuts to secure the deck plate to your air box.
Start by removing your stock air box. There are four bolts holding the box to the engine bay, and a hose clamp holding the intake tube to the MAF. Use either a philups screw drivers or a 10mm socket to loosen the clamp. There is also a connector to your O2 sensor, disconnect that. Carefully pull the box outta your rig. Take out the air filter as well...
Use a rag or something to cover your O2 sensor. You don't wanna mess that up with plastic shavings. Take the plate outta the deck plate, and use the edge of the plate to trace a line where you wanna cut your whole. Out of personal experince, I highly suggest you place it in the upper right corner. This way you have "clearence" for your hand when taking it on and off. Make sure that the outter edge of the deck plate is on the flat surface so you get a good seal with the sealent.
Use your cutting tool of choice to cut out the circle. Once you have cut it, dry fit the outter plate, and make sure it fits. If it fits, align your outter plate however you see fit, and drill the holes to fasten it down. I suggest you take an air-hose to it once you are finished cutting and drilling; you don't want plastic shavings in your engine...
(hole cut, and holes drilled)
Once the all the holes are delt with, it's time to seal everything up. Place a bead of sensor safe sealent around the edge of the deck plate. Put the deck plate in it's whole, and before the sealent gets a chance to dry, put the bolts and nuts in, and tighten them down.
(deck plate installed and sealed)
Give it a few minutes to dry. I suggest while you are waiting, you take out your EFI fuse, and relay in the fuse box. This will reset your EFI computer, so it doesn't throw a CE light when you turn your car on. Once it has dried, put everything back together. Now once it's all back together, replace your EFI fuse and relay.
Now, if you didn't use sensor safe sealent, and you turn your rig on, expect a check engine light. Your O2 sensor is picking up the sealent, and tossing the error. Turn your rig off overnight, and in the morning you will be fine. At least give it an hour to cure or something.
When you drive it, you will really notice a difference in induction noise. It's louder, much louder. Now, will all that noise comes about 7 or so hp. People have dyno'd it at about 158hp at the wheels with the deck plate mod, and an aftermarket drop in filter (k&n or amsoil). I myself cannot say that it for sure does, but it sure feels like it. Throttle response is improved, along with power around the 3k to 4k range.
As for MPG, some people have reported a gain, some have reported a loss. Nothing major. It really depends on the way you drive. You will drive around for the first week or so with the throttle wide open... Then after that you will return to your normal driving habits, whatever they may be. After I laid off the go-pedal alittle, I did experience a slight gain in MPG, nothing major, but every little bit helps.
I really like this mod since you only hear the induction under high acceleration. So you don't hear that much of a difference while you are crusin' on the freeway. Over all, I highly suggest this mod to anyone with an air box big enough to fit a deck plate on.
If you're in the South Metro area (Littleton, Highlands Ranch), there is a boat shop on S. Santa Fe just north of Highlands Ranch Parkway. That's where I got mine. I can't think of the name of the place, but it's tough to miss, it's been there for ever. If your going south on Santa Fe and you get to the light at HR pkwy you've gone too far.