Shearly out of curiosity. I recently bought an 04 Subaru Impreza Outback and this is my 4th Subaru so im decently familiar with the engines. I took a look after I purchased it and noticed the intake manifold was black plastic and have never seen that on an 04/05. Typically i believe they are a silver aluminum color. For those years up to 08+. Wondering if it's factory or not and why it would be plastic rather than the aluminum.
The plastic intakes apparently flow better than the original aluminum ones. Plastic = less heat soak, easier to make, less top-weight, no need for welding and the shape is more flexible to manage vs. metal. Most pictures I can dig up for the 04/05 has the aluminum.. I would guess there were issues with the one that came factory and the P/O just went with the plastic mani. Another soundbite I dug up "intake manifolds became plastic around 04/05ish on some engines and gradually spread across the entire lineup."
No, it should have had an aluminum one. Like this: The only non-turbo EJ plastic manifold I know of, is on the '10-'12 Legacy/Outback, which looks like this: The port shape, size, and spacing is the same between the gaskets (plastic housing is basically o-ring style, and does not have bolt holes in it, so I can't compare that). So I suppose it's possible the manifold was swapped, but it seems more likely to me that the entire engine was replaced.
04 Impreza is the only GD year that the sohc came with the plastic intake manifold. 05+ changed to the big equal length intake manifold and 04 and below are the regular ones.
I stand corrected, I looked again, and there was one part number that I assumed was a supersession. But cars manufactured 10/02-3/03 (listed under MY 2004), I show a 14001AB540. And a google image search of that shows this: From 04/03-03/04, it switches to a 14001AB541, but I only have line drawings of that (and the drawing says '02-'04). No indication whether it's plastic or aluminum.
