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Currently have a 2015 3.6 OB. No eyesight (darn). We live in OR & do a lot of driving in mountainous terrain. 3.6 does great going up several mile 6%+ grades. Also when going to Yellowstone thru ID east of Boise the speed limit on I84 is 80mph. Car does great at those speeds & holds speed going up steep grades.

So my question is: Will the 2.4 turbo do as well? I'm not a mechanic but it seems to me that there would be more stress put on the 2.4 than the 3.6. Therefore putting more wear on that engine in relationship to similar miles driven. And also not be able to do as well speed wise going up long steep grades.

I've never had a turbo so am green as grass regarding that type of engine.

One last thing, we have owned 4 Subarus, so I'm a fan. Would appreciate an honest positive discussion about my question.
 

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We have owned many Subaru's 2.2, 2.5 2.0 turbo,2.5 turbo, 3.6. The turbos excell in the higher altitudes. I suspect the 2.4 will be a gem. But the power delivery is different. I prefer the smooth, quiet, efortlessness of the 3.6. There is no replacement for displacement.
 

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Own a 2015 3.6, no eyesight (darn). Do a lot of mountainous freeway & secondary hiway driving. 6%+ grades for several miles. Also go to Yellowstone & east of Boise the speed limit is 80mph. OB does great holding speeds up grades & at 80 - 85mph.

My question is: What would be the experience with the 2.4? I'm not a mechanic but it seems to me that there would be more wear given similar miles & type of driving.

I'm a Subaru fan (have owned 4). So would like a good positive discussion about this
 

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In order to find out the actual info that you are looking for as opposed to what people might guess about the performance of the 2.4Turbo FA24 engine - I suggest that you go to the Ascent forum and ask that same question. There you will find folks with actual experience with this engine. The SuperAdmin there Robert Mauro has 30K+ miles with this exact engine on his Ascent and just returned from a cross country journey raising money for the National Parks and meeting other Ascent owners. He also pulls an RV. Here is where you need to go: https://www.ascentforums.com/
 

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At higher elevations the air is thinner and due to this engines will lose power and typically run in a reduced capacity. A turbocharged engine uses forced induction which effectively powers more air into the engine, counteracting the issue.

Aircraft engine use turbocharged engines for this very reason, I think you would be at least as good and more likely better off with a turbocharged engine in the circumstances you describe.
 

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I have owned two 2.5 non turbo liberty/legacy’s, one 2006 2.5 turbo Forester and now a 2011 3.6 Outback. As I have mentioned elsewhere on this site I prefer the non turbo 2.5 over the 3.6 but that says more about my personal choice than the engine types.

If you drive the turbo vehicles as designed (not accelerator to the floor all the time) they are actually very capable vehicles with so much torque that driving becomes a pleasure. Being able to lightly accelerate up a hill with no need for a gear change is what makes driving a sheer pleasure. It allows you to concentrate on the drive without worrying about whether you are over stressing the engine or gearbox or using too much fuel etc etc.

The power and torque specifications of my 2006 turbo Forester are so close to the power and torque of the 2011 3.6 (the 3.6 is just a little higher) that the drivability of each vehicle is very comparable. The advantage of the 2011 3.6 is that is uses a lower octane fuel which means that here in Australia I am saving around 70 cents per US Gallon every time I fill up with fuel.

In summary I think that you would be more than happy with either engine choice and both vehicles would be more than capable of what you want them to do.

Seagrass
 

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The 2.4 in the mountains will be like Aquaman in the water. The 3.6 in the mountains will be like the Incredible Hulk running.

I think the formula to roughly calculate the HP lost in elevation is 3% lost for every 1k feet. The turbo doesn't have that concern.
 

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The 2.4 in the mountains will be like Aquaman in the water. The 3.6 in the mountains will be like the Incredible Hulk running.

I think the formula to roughly calculate the HP lost in elevation is 3% lost for every 1k feet. The turbo doesn't have that concern.
in theory. But unless you are using some aftermarket boost control, the ECU in the car will begin to limit things to keep the turbo within whatever range subaru thinks is safe.

Yes, it will attempt to keep the MAP around 14.7 or whatever the PSI is for boost, but what you begin to see at higher elevations is the requested is higher than the measured... so less boost (this isn't the old mechanical wastegate version). If the car were able to provide extra boost, the turbo would be spinning at a higher rate as you increase altitude... extra head generated in the exhaust, much more heat to go through the intercooler (loss of efficiency).

It will lose power, just not as much as a naturally aspirated engine (and there are many limiting factors based on electronics and controls on the system)... then you have driveability - as the outside pressure drops, the turbo lag is more pronounced (these cars aren't using things to adjust for that, like a VNT system as an example).... so your lag starts to creep up in the RPM range. You'll notice it first off idle, then just a little higher. What does that mean for real world driveability of the 2.4T? Probably not much. Depends on what the car needs off idle for power, and really more of an annoyance for anything below 10,000 feet. Just ask turbo ascent owners, most will say "it's great at altitude". But it's not true that turbos are totally unaffected by altitude. The power delivery for the 3.6 is unaffected, but power is down more than on a turbo motor. Enough to bother you in the real world? nope - just ask 3.6 owners.

Here is one discussion (I realize it's old, but it's relevant)
 

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I had a 15 Forester XT and never fell for the turbo power. It was either on full boost or complaining about the heat.
I would take a 6 cylinder no questions in the mountains. My 18 Pilot EX-L crushes the 10,000 foot passes with ease, and I am sure the 3.6 would do the same.
 

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Took my 2001 3.0L outback up to Pike's Peak around 14k feet. Sure, I could feel some definite power loss, but it wasn't terrible and didn't affect its ability to climb the hills. Didn't do freeway speeds at that elevation though, obviously. Had a twin turbo'd 3000gt vr4 back in the day (first car that turned me on to the miracle of AWD and why I drive subarus today in fact...) that I used both at sea level and at around 4k feet (Salt Lake City), and even with fuel and boost controllers I could tell a difference, turbos definitley working harder with less power at the same max boost. So as Walker mentioned, you'll see a difference in both, though it'll be less dramatic in the turbo'd car vs the NA.
 

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Turbines operate over a pressure ratio. Typically assumed adiabatic (why the incoming heats and needs an intercooler).

In calculating the pressure ratio, we assume that an adiabatic compression is carried out (i.e. that no heat energy is supplied to the gas being compressed, and that any temperature rise is solely due to the compression). We also assume that air is a perfect gas. With these two assumptions, we can define the relationship between change of volume and change of pressure as follows:
P_{1}V_{1}^{\gamma }=P_{2}V_{2}^{\gamma }\Rightarrow {\frac {P_{2}}{P_{1}}}=\left({\frac {V_{1}}{V_{2}}}\right)^{\gamma }
= turbine Compression ratio (not the cylinder CR)

P2 is the engine pressure, p1 in this case atmospheric. As atmospheric drops P2 will drop, but not as much as a NA engine.

Atmospheric pressure at 10k ft is about 10 psi. P2 for a NA is also 10 psi. For a turbo engine, if you had a turbo with 8 psi of boost, you'd have a p2/p1 of 22.7/14.7 = 1.544 for a compression ratio. At 10k ft, P2 = 15.4 psi, which is better than the NA 10 psi, but still significantly lower than the 22.7 you'd get a sea level. Moral of the story both engines will lose power at altitude. IF you operate the turbo engine at low rpms where boost is low, it will seem like a NA engine and lose a lot. You'll have to use significant boost and even then you won't have the power you have at sea level, but better than the NA engine. You can't get the turbine to operate at higher pressure ratios just becasue you are at altitude.
 

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I never gave it much thought since I live at sea level, but it appears turbos work on PSIG (gauge; i.e. boost above ambient). If they worked on PSIA (absolute) then they'd lose even less power at higher altitudes.
 
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