Ok i am a little confused, why does the 2.5 liter engine have such a small increase in horsepower and torque. I mean I dont really understand the choice, but then agian I dont really understand the pros and cons of turbocharging as opposed to increasing engine size. Does this increase reliability or, abitity for aftermarket mod?

I'm going to assume you're talking about the 2006 WRX?

The 2.5 gives a small increase in power, it gives a large increase in torque.

Forced induction gives larger power gains than adding displacement, take a look at the power numbers on any V8 compared to a turbo charged 4 cylinder and you should be able to make the association.

Subaru have increased the displacement, but not changed the turbo on the engine. Hence you've not seen any major power gains, the STI uses a different turbo setup, notice how much more power it generates for the same 2.5l of displacement.

Oh ok my question though way why would they want a larger engine if they had no intention to to increase horsepower. so what i wanted to know was why make that change, i cant seem to figure out why.

From a production standpoint it makes sense, they’ve increased the production volume on the 2.5 turbo engine which thereby decreases production costs.
It’s basic logistics, the more of a product you produce, the more you reduce your production costs.
Keep in mind, they were already making a 2.5 turbo engine for the Legacy GT, Outback GT and Baja so extending it to the Impreza helps increase production volume and reduce costs.
Subaru are still producing the WRX and STI with the 2.0 for other markets, just the US market gets the 2.5.

From a production standpoint it makes sense, they’ve increased the production volume on the 2.5 turbo engine which thereby decreases production costs.
It’s basic logistics, the more of a product you produce, the more you reduce your production costs.
Keep in mind, they were already making a 2.5 turbo engine for the Legacy GT, Outback GT and Baja so extending it to the Impreza helps increase production volume and reduce costs.
Subaru are still producing the WRX and STI with the 2.0 for other markets, just the US market gets the 2.5.
I also heard for the dealer that they also used the 2.5L to appeal to the tuners. Obviously the 2.5L will be able to handle a bigger turbo and things like that.

Oh ok my question though way why would they want a larger engine if they had no intention to to increase horsepower. so what i wanted to know was why make that change, i cant seem to figure out why.

The production standpoint that was mentioned above makes sense, plus the power gains and performance are significant in the increased displacement, as there is significantly more low end torque needed to get the car off the line quicker, requires less boost to achieve the same acceleration and less issues with turbo lag. Driving a 2.0 turbo myself, the lag and the peakiness is annoying, even with some aftermarket tuning.

Ed