How does high altitude decrease horsepower?

Less O2. Lean out the carbs if you have them, otherwise the computer should do it.

Engines need a lot of air and fuel to make HP

 

technically , its because the air is less dense at altitute.

its also like running turbocharged cars in the hot afternoon , the cars
are noticeably slower.

 

I live at 6500 feet myself. The general rule of thiumb is that the air thins at the rate of 3%/1K feet with a corresponding loss of power in IC motors. It should be pointed out that air also thins out with higher temps which is why some people notice more power on very cold days. It is defintiely seat of the pants noticeable, I can sense more power just going down to about 5K feet, or less power at 10K. I have driven as high as 14K where the power dropoff gets REALLY obvious. At higher speeds, the loss of power is offset somewhat by less drag. I have also noticed fuel mileage is better at altitude. I have always really noticed the difference in low end torque especially. It is a treat to take a car to sea level once in a while. Newer cars all have computers which precisely adjust air/fuel mixtures. Older cars with carbs can benefit from some leaning of mixture and some increase in ignition advance which is possible because the motor is making less compression due to the thinner air.

Dave

 

For Detroit pig iron we used to change the jets.


I had not noticed less power in my Porsche 944 Turbo.
In 86 we hooked up with a 911 Turbo (930) and a 928 normally aspirated.
We were going to Munich from Hockenheim after the German GP.
On the level Autobahn the 928 pulled us. When we got to the mountains
the 930 and us left the 928. When we got to Munich the 930 was ahead
only because I refused to pass the slower traffic on the right. The 930 was ahead by the with of the drive way at the Shell station in Munich after you get off the Autobahn.
Later that year when were bringing it home to Kansas (it was shipped to the west coast) going into Denver I didn't downshift to 4th and we were still
showing 2 bar on the turbo gage.

stephen

 

I moved from Colorado Springs (7,000 feet) to Anchorage (sea level) and noticed an increase in power in my truck (1995) and Ferrari 328 (1989). The computers compensate some, but the cool dense air here is great. I drove the Ferrari one morning at 50 degrees and it loved it.

Probably faster than Hardtop's F430, 328 and STi in Colorado....

 

As was mentioned, cars lose power for the same reason you lose breath at altitude. You need the O2 present in air to burn and make power. Less dense air, less O2 in each cylinder charge, less power.

But as for thinking that 328 at sea level will be faster than an STi at altitude... As has been mentioned anecdotally in this thread, turbo cars don't have to lose (much) power at altitude because the control system can be tuned to maintain constant absolute pressure in the intake. The turbo can just be asked to make more boost to make up for the lack of ambient pressure but you'll maintain essentially the same performance level at sea level as altitude. This does mean the compressor is working harder, so the charge temperature will increase, and you can run into hardware limitations, so a turbo may not maintain full power at altitude, but it'll usually be close, and since it's SOOO much closer to nominal than a NA car you don't notice.

 

A couple things need to be corrected here. First of all, there is no less percentage of Oxygen at 8,000 feet than exists at sea level. Its a loss of pressure. There is considerably less pressure at 8,000 feet than at sea level, and less atmospheric pressure makes it harder for a normally aspirated engine (non-supercharged) to suck air into itself. If you understand that even with your throttle wide open at 7000 rpm, that there is still a positive pressure inside the intake manifold, compared to say the vacuum of space, then you can understand that higher pressure air outside the engine will attempt to "force" its way into your engine. A passenger jet airliner, uses the compressor sections of the jet engine, to bleed off air to pressurise the cabin. By being able to pressurise the entire inside of the plane to 4 psi, they can provide an atmosphere at 35,000 feet that feels like 8,000 feet. The only reason a pilot would breath pure Oxygen at high altitude is in a non pressurised environment.

On a small airplane, some have a guage called manifold pressure, which is connected to the intake manifold, downstream of the throttle butterfly. This guage is generally calibrated in inches of mercury. Sitting on the ground with the engine not running, the guage will read the same as barometric pressure. Lets say that today, for example, that would be 29.5 inches barometric pressure. When the engine starts and is idling, the guage will read lower, perhaps 14-16 inches. At wide open throttle it may drop to 2-4 inches, but still not a perfect vacuum. with 29.5 inches of pressure outside, and 2-4 inches of pressure inside, the higher pressure air outside will try to rush in to fill that space. Obviously, if you have followed me on this, less outside pressure, from being at a higher altitude, will not be as forceful to fill that lower pressure insde your engine. All a supercharger is doing, is forcing a larger quantity of air into the engine in an attempt to raise manifold pressure.

But one caution on turbochargers at higher altitude. As the air thins out, the turbine has to spin ever faster to maintain the same manifold pressure it provided at sea level. If the output is not cutoff in some way, its possible to overspeed the turbo to the point of failure. And shattered turbine blades can do a lot of damage inside a combustion chamber. I recall Audi had a turbo controller to protect the turbo overspeeding. The higher the wastegate is set, the higher the turbine will spin up at higher altitude attempting to maintain that pressure.

 

Just a joke between friends...

 

Krobar had it right, the percent of O2 in air does not change with an altitude change; to sum it up, what changes is the density of the air. The molocules in the air are spread further apart as altitude increases. The "compression effect" caused by gravitattional pull at a lower altitude packs the molecules closer together. At higher altitudes the molocues are further apart. At higher altitudes there is less wind resistance and if an engine can compresss the intake charge enough; a vehicle will perform better. This is why a jet airplane, even on a short trip, will generally "climb until it needs to descend for landing". Another interesting point is the airspeed indicator (essentiall a ram pressure gauge) of a jet airplane flying at 35K feet, will indicate about have of the airspeed it would read at sea level.

If you are in an airplane (especailly the cockpit) the noise from the air molecules hitting the airplane gets much greater as the plane descends.

 

rho = p/RT pretty much says it all.

Turbos are more adversely affected by temperature or pressure than normally aspirated engines. I have not run any numbers about so I cannot verify it but I have read that a turbocharged engine loses 2% of its peak power per 10 F change in temperature. A normally aspirated engine loses about 1 % per 10 F temperature change.

But I can say this. I frequent Road Atlanta monthly. It has a long straight section. In the cool mornings my turbo will creep away from certain cars on the straight. In the afternoon when it gets really hot there, my slight advantage goes away.

 

There is an urban myth that altitude does not affect turbo'd motors, but that is not true. There is still less air(pressure) for the turbo to work with. I can tell you that my STI is noticeably less powerful at 10K feet than 6K feet with a good deal more turbo lag as well. It may be true that one can compensate by increasing the boost, but this will produce more power at all altitudes, not just high altitude and as pointed out above, turbo speed may be too high, or, at lower altitudes, compression may be too high, unless the computer dials back the boost or timing.

Dave

Mule,
I'm sure your 328 feels like it's on steroids in Anchorage after gaspinig for air those to years in the Springs.

 

you are kidding (??) I think so, because the difference in altitude between Hockenheim and Munich is barely 400 mtrs.

Best Regards from Germany

Martin

 

It makes everything better. Car and truck run cooler and feel stronger. I did not know it could be this good, after being at high altitude all the time.

 

Not to be pedantic but here is some qualitative information.

For a reversible adiabatic compression of air, the final pressure can be related to the initial state by the relationship.

P2 = P1 * (V1/V2)**k

V1 and V2 are the volumes before and after the compression. k is the specific heat ratio, 1.4 for air. A compression means V1 > V2, so with k being positive, the quantity (V1/V2)**k must be greater than 1.

Qualitatively speaking, the above formula indicates the final pressure is directly proportional to the initial pressure when compressed. So if you reduce the initial pressure P1 (such as a change in altitude), your final pressure is also reduced in a linear relationship assuming the compression ratio remains constant. Less pressure means less oxygen means less power developed because the engine management will introduce less fuel.

 

You overstate your case, I think. I can tell you that the turbocharged engine program I worked on running the dyno program made 190hp at 7000 feet and 200hp at 1300ft. That's pretty flat, and the loss was actually related to some other system components that needed to be upgraded.
Lag will be increased because, as you state, the turbo has to work harder to make the same manifold pressure. I'd say that you're probably running into a hardware limitation, most likely turbo speed limits as referenced above. Subaru wouldn't increase the boost pressure in that situation...

And Krowbar, didn't you cross some units between MAP and Vacuum when presenting your airplane analogy? I'm pretty sure you meant to say that Manifold Pressure would approximately equal atmospheric pressure at WOT. With the restriction of the throttle plate essentially removed, the manifold can now easily match the pressure available to it from outside even though those cylinders are sucking air in.

 

Less pressure means less oxygen means less power developed because the engine management will introduce less fuel.[/QUOTE]


Unless you have a carb. Carbs have no way to automatically compensate for altitude/barometric pressure changes. Now, carburators on an airplane do have mixture control, which we have to keep leaning as we ascend, or on those hot days when mr motor just needs a leaner mix for full takeoff power.

You guys at higher altitudes with carbs need leaner jetting. And a word of caution: make sure any leaned high altitude cars are rejetted richer before driving at lower altitude. That can and will burn pistons.

 

I'm with Don on this one. The variations in turbo-engine performance has a lot to do with the sophistication of the engine-management system. A lot of people run manual boost-controllers on their cars for adjustability and these do not compensate for altitude. These are spring-loaded controllers that work on pressure-differential between outside atmospheric air and boost-pressure in the manifold. The pressure that opens the valve is then based upon this pressure-differential vs. the surface-area of the check-ball which against the spring to open the valve. You end up getting a boost-level that's a fixed amount above atmospheric. So a valve that gives +15psi over atmospheric at sea-level will also give +15psi over atmospheric at 6000ft. The difference then looks like:

Manual Controller
0-ft: 15psi atmospheric + 15psi boost = 30psi absolute (2-bar)
6000ft: 12psi atmospheric + 15psi boost = 17psi absolute (1.8-bar)

So you end up losing about -10% HP at altitude with this particular car. Still better than a non-turbo engine.

Even better is an electronic boost-controller that self-adjusts to maintain constant manifold-pressure.

Electronic Controller
0-ft: 15psi atmospheric + 15psi boost = 30psi absolute (2-bar)
6000ft: 12psi atmospheric + 18psi boost = 30psi absolute (2-bar)

In this case, the controller adjusts the wastegate to give higher boost over atmospheric at altitude vs. sea-level to maintain the same air-volume going into the intake-manifold to give close to the same power as at sea-level. However, the turbo may have to spin at 120,000rpm to move this much air as opposed to 90,000rpm at sea-level. This moves it into a less efficient region of the compressor-map and end up heating the air up more than at sea-level. This may be the minor difference that Don noticed on his car.

So there's no hard & fast rule about turbo cars in relation to altitude, it all comes down to "it depends" based upon your particular configuration.