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Hi,

It is commonly acknowledged that the way to obtain better fuel economy is to use turbocharging, hence the recent trend towards smaller displacement engines with turbos to get higher power.

The FB engine in the 2014 Forester is rated at 32 MPG on the highway whereas the 2.0T engine is rated at 28 MPG. With a typical EPA cycle involving some acceleration and deceleration, it makes sense that the turbo engine would get poorer economy despite its smaller displacement (more power = more fuel consumption).

What happens in the real word, however? One would think that in steady-state cruising, the smaller displacement engine would have a significant advantage due to lower pumping losses. From what I've read, that's not the case, with reported consumption pretty close to the EPA rating.

Interestingly, on the city cycle, both engines are rated within one MPG of each other.

Obviously, I'm missing something. Thoughts?
 

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2013 Outback 3.6L Limited with EyeSight
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Subaru's powertrain with a turbo has a beefed up CVT and lower final drive ratio than the naturally aspirated 2.5L FB engine. Engine RPM's are higher on the smaller 2.0L turbo unit during freeway driving.
 

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Lawn ornament XT
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The promise of better fuel economy with a turbocharger is predicated on the idea that boost will only be used during acceleration, to overcome the small displacement of the engine. While cruising, the transmission will work to use the right gear ratio to keep the engine RPMs low enough that the turbo de-spools.

This all goes away as soon as you pick a cruise speed that is higher than what the transmission can map to pre-boost engine RPM. A final ratio that keeps you out of boost at 55 will have you turnin' and burnin' at 75. While the automakers do need to be realistic, they are allowed to target any cruise speed they want. No legal standard involved. So they can make it pass a test, get a great score, then when nobody hits that score they won't get in trouble.
 

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The promise of better fuel economy with a turbocharger is predicated on the idea that boost will only be used during acceleration, to overcome the small displacement of the engine. While cruising, the transmission will work to use the right gear ratio to keep the engine RPMs low enough that the turbo de-spools.

This all goes away as soon as you pick a cruise speed that is higher than what the transmission can map to pre-boost engine RPM. A final ratio that keeps you out of boost at 55 will have you turnin' and burnin' at 75. While the automakers do need to be realistic, they are allowed to target any cruise speed they want. No legal standard involved. So they can make it pass a test, get a great score, then when nobody hits that score they won't get in trouble.
As a turbo guy, I have always thought that the EPA tests for turbo cars were kind of biased against. I know that a lot of people who drive turbo cars report very poor fuel mileage because they cannot keep their foot out of the car-they don't know how to use lower boost to accelerate.

With my turbo cars, when I am trying to get better mileage, I usually don't have very much low end power. I can make the turbo spool up off the line, but I know that the ECU will respond by throwing lots of fuel into the mix, and I know that will kill my mileage. That's from a 2.5 turbo.

My NA car, a 4.0 Xterra, is way easier to tool around town at low RPMs because it has much more torque below 2000 rpm.
 
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