I have never heard anything about the aerodynamic properties of the air dam. As far as I know its primary function is a skid plate/debris protector for the engine compartment and exhaust manifold. From my auto cross and track day experience a ton of people remove them for the after market aluminum ones for better protection. It is such a pain to remove it on my STi to change the oil but I am willing to say if you dont use one you wont hurt anything or notice a decrease in gas mileage.

 

I think it does have an aerodynamic purpose. In my experience driving without it has a negative impact on mileage.

 

Anybody else?

Also do you know where I can purchase one of the after market aluminum ones for a 1998 Forester?

 

Anybody else? Also do you know where I can purchase one of the after market aluminum ones for a 1998 Forester?

I think the plastic undertray primarily serves to keep the engine clean, and secondarily allows air to pass under the engine more aerodynamically. The car is designed for use in mud and snow, and the boxer engine hangs low with the convoluted headers and heat shields on the bottom. They are close to the road and exposed to road dirt and tire spray. That area will become a dirty rusty oily mess sooner without the shielding.

Over time, oil changers lose and/or damage the pop screws and plastic sheets of the OEM undertray, which begins to hang down and come off. To replace the whole system is over $200, and you can get a front skid plate for about the same.

Here is a source and a thread on skid plates:
http://www.get-primitive.com/
http://www.subaruforester.org/vbulletin/showthread.php?p=322382#post322382

You can probably find a used undertray, as people who install skid plates often set the undertray aside for sale.

 

Some people have also made custom skid plates using old metal street signs.
I highly recommend going with either the Primitive or Subtle-Solutions skid plates, the latter of which I don't believe is offered in SF-chassis fitment at this time.

 

Making your own...

Bottom pan/protector/skid plate isn't very hard.

If you have a state highway garage close, go ask if they have any scrap metal laying around. They probably do.

Even if you have to buy a sheet of aluminum, it isn't that much.

A reasonable drill of 8amp(more the better) and a drill bit for metal, even if you have to buy both...I'm sure the drill will come in handy later(I just bought a 10 amp DeWalt...giggity).

The biggest issue is what to cut the aluminum with. Shears aren't exactly cheap...but...

Even if you have to buy a sheet of aluminum, the shears to cut it with...and the drill bit. You are still less money than buying one and having it shipped.