The STi put down a lot of power because of its turbo. If it didn't have the turbo on it, it would most likely have the exact same HP rating that the 2.5RS does.

Cylinders are related to power output only because more displacement generally means more power (but this is not always the case as I'll try and explain). But, there is only so much displacement you can have per cylinder to be effective (FYI displacement is measured in liters or cubic centimeters, as in the STi's 2.5L of displacement). That is why it is generally true that an engine with more cylinders is faster then one with fewer.

But, a turbocharger or supercharger can make extra horsepower with less displacement by forcing more air into the cylinder. Thats why the STi can make so much extra power (nearly twice what it would be making without the turbo).

Its hard to compare engines directly across the board like the C230 versus the Impreza 2.5RS, especially when the engines are so close to the same displacement. The different companies will have different tunes on their engine. Generally, in a non-turbo or non-supercharged car, the more displacement means more horsepower, but that is not always the case are there are many other factors involved.

Google says that the naturally aspirated engine (meaning, non-turbo/non-supercharged) 2.3L MB motor put down about 148hp. The Impreza 2.5RS puts out 165HP. But, you also have to factor in drivetrain losses. All of these HP measurements are done at the crank, and every car loses some horsepower through all the different gears and and whatnot that it takes to get the power from the engine to the wheels.

AWD takes a huge chunk out of the power and the 2.5RS will probably lose 65-75HP in the drivetrain, and will be putting down around <100HP to the wheels. The N/A 2.3L MB motor, on the other hand, is probably still putting down ~125hp to the wheels. This would make the Mercedes slightly faster (although from a stop the 2.5RS might have a slight launching advantage).

Okay, I hope that made sense to you, as I'm a bit tired right now. If not, ask more questions, someone will be happy to answer them.

 

In short, no. Displacement (How many cc's or liters) and horsepower cannot be directly related..

The horsepower and torque an engine makes depends on many (many!) different variables other than it's displacement. The STi produces such awsome power due to the fact that it's turbocharged (more air is getting forced into the engine) and intercooled (the air intake is getting cooled, which makes the air mass denser), among many other things (ECU Programming, Compression Ratios, Exhaust system, etc). However one of those things is that it has half a liter more displacement than the standard WRX, so yes displacement does have something to do with the horsepower/torque of an engine, but they're still not directly related.

In general terms, more displacement is never a bad thing except when it comes to the physical size and weight of the engine. As you can imagine, the more displacement there is, the larger, heavier and more rigid the block has to be to account for it. That's one of the reasons that companies like Ferrari, Porsche, Maserati, etc decide to stay with reasonably low-displacement engines tuned to thier limits, while companies like GM, Ford, BMW, Mercedes, etc use larger engines with more displacement, but are not worked so close to thier limits for reliability.

 

The engine is a "pump". It will burn the air mixed with fuel and make power. A smaller displacement engine can "suck" in less air than a larger displacement engine. However, a turbocharger or a supercharger forces the air into the engine by pressurising it. In other words (kindergardenly speaking) a "cup" of pressurerised air has just as much air as a "bucket" or non-pressurised air. So where a small displacement engine can only "suck" in a "cup" of non-pressurised air, a turbocharged car (e.g. STi) can "suck" in a "cup" of pressurised air which is more or less equals to a "bucket" (or the "sucking" ability of a larger displacement engines). Thats where the power is coming from.

How is that for a technical explanation...

 

lol that's pretty darn good

 

short and to the point, and easy to understand

the other two explanations were quite good too...two thumbs up