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Hopefully, I can clear up some confusion here. Our atmosphere is comprised of the following:
21% oxygen
1% inert gases
78% nitrogen
Whether at sea level, or on top of Pikes Peak this ratio changes little. What does change is pressure.The higher you go there is less pressure available to force said air into the motor. While a turbo may adapt to this change it is still affected. Modern EFI system only add and subtract fuel to maintain a 14.7:1 AFR, and cannot change pressure incoming in to the engine. All vehicles lose power proportionally.
Example, I moved from 400' to 5100' and modified my fuel injection fuel maps based on datalogging, so it runs smoother. I still have a loss of power, but my ECM is no longer taking wild swings in the memory learn function.
Should the OP choose a Suburu, over his present vehicle a power loss will be felt. If the 2012 F250 owner who feels no difference unloaded it's no surprised, but should he use the vehicle as intended with say a 12,000 load behind it would become quite apparent.
Turbos don't "adapt" to environment... They are hunks of spinning metal, and they work the same way no matter where they are.
The reason turbo cars work more efficiently than NA cars is because they have the ability to pressurize (and condense) some of the intake air. Every turbo has a flow map, specifying just how much air flow they can output, based on RPM and pressure ratio. Along with that, every turbo has an "efficiency island" where the turbo can operate at its peak ability, which is also based on RPM and pressure ratio.
At sea level, atmospheric pressure is highest because of the weight of the air above, it's the same concept as water... the lower you go, the higher the pressure. This helps a turbo engine, because all a turbo does is pump air into an engine, say 1 bar, which is about 15psi. At sea level, the atmospheric pressure is already 1 bar, so a turbo has an easy time of exceeding that pressure.
At higher altitudes, pressure falls below 1 bar, and a turbo has to overcome the loss of pressure AND still be able to stay in its efficiency range. NA cars just don't have an ability to do this, they are stuck with the loss of pressure, and thus, air that can be moved through an engine, resulting in a loss of power.
The only time you'll notice power loss is if you go to the drag strip at sea level and then to the drag strip at Bandemere and compare numbers. Otherwise, it is all relative: everyone around you on the highway is also experiencing power loss.
I'm curious why you want to go to a Forester. The new Focus is pretty badass.
So we want to run the same pressure through the engine at 6000ft as we did at sea level right?
-Sea level total pressure = 29.2psi
-So at 6000ft 11.4psi (atmospheric) + x = 29.2 where x is the amount of boost your turbo has to run which would be 17.8 psi.
example
-At sea level:
lets use 14.5 as stock psi + 14.7psi (atmospheric pressure) = 29.2psi total.
-At 6000ft run 14.5 stock psi + 11.4psi (atmospheric pressure) = 25.9psi total.
-At 14,380ft ~I have noticed about 1.1psi atmospheric drop for every 2000ft you go up (6000ft ~ 3.3psi pressure drop). So 14.5psi + 8psi (atmospheric pressure) = 22.5psi total
2ND - calculate the % of air loss you experience compared to sea level
-at 6000ft 25.9 / 29.2 = 88% so you have 12% less pressure going into the engine
-at 14,380ft 22.5 / 29.2 = 77% so you have 23% less pressure going into the engine
NOW same comparison for a NA car in terms of how much % less air:
-at 6000ft 11.4 / 14.7 = 77% so you have 23% less pressure going into the engine.
-at 14,380ft 8 / 14.7 = 54% so you have 46% less pressure going into the engine.
Most automotive manufacturers steer clear of altitude compensation in turbos due to the additional loads imposed. However, a few have played with it. It is used in aircraft though. Therefore, I stand by my statement that turbo's don't magically adapt. As Snofarmer has pointed out, it is not as bad with the turbo, but still suffers from the effect.
I've always felt that pressurization will reduce service life over NA engines due to additional loads imposed. For some this loss is acceptable. My guess is most don't even think about this and probably just buy into the whole more power idea.
I've been looking into a new car, currently leaning towards the 2014 forester (non turbo). One thing I am concerned about is various comments I've seen with it having problems going up hill at high altitudes. I live in Colorado and am a skier, so traveling to high altitudes will be a must for me.
Will any of the other crossovers (ford escape, honda cr-v, etc) do better at these altitudes?
It sounds like getting the turbo engine is the only solution here, but I am trying to avoid that as I'd prefer to not have the additional cost. I'm trying to get a idea of how bad the slow down is so I can see if it's tolerable or not.
Great question re: CRV & Forester - I've been looking at both of them and will be doing more mtn. driving.
The whole idea of a wastegate on a turbocharger is to eliminate overboost. Most turbos with a wastegate are calibrated to prevent a turbo from boosting over around 15-17 psi. At altitude, that means that the wastegate may not open as far as it does at sea level, but the turbo should have enough boost to maintain that 15-17 psi at altitude. Some of the earlier turbocharged gas engines (say, in the late 1980's) had turbos that were not wastegated and were sized for sea level--those would lose some boost at altitude. Intercooling is also much more common on today's turbocharged engines, which further improves their performance (because cooler air is denser).
Whether one wants the extra high-altitude performance that a turbocharger brings is a personal choice. I drive both turbo and non-turbo vehicles--all at altitudes over 5,000 ft. and much of that driving in the mountains. Each type has its advantages and disadvantages. A non-turbo vehicle will often get better fuel economy at altitude compared to sea level--odd as it seems--because the electronic engine controls will "lean out" the fuel mixture to compensate for the higher altitude, but that power loss will be there. A turbo engine will tend to get worse fuel economy because, for most people, when the power is available, they will use it. Many, but not all, turbocharged gas engines also require premium fuel--another added cost. Engine longevity may also may not as great with a turbo gas engine compared to a normally aspirated engine, though that gap seems to be narrowing all the time.
Truly the ideal engine for a Subaru in high-altitude areas would be the turbocharged diesel engine that Subaru sells in the Forester and Outback in Europe, but, unfortunately, we may never see it here in the US. Subaru has been rumored about thinking about building a turbodiesel hybrid XV Crosstrek in a few years (they are putting their toe in the water with a mild gasoline hybrid version later this year). If they did bring us a turbodiesel hybrid, I would be looking very seriously at that.
I live at 8,500' and our HEMI Durango does just fine. Definitely slower than at sea level but it's hardly noticeable.
Only is a big deal if you do a lot of heavy towing. Then a turbo diesel is the way to go up here.
Last edited by PokerMunkee; 04-13-2013 at 02:18 PM..
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