When Turboed why don't sleds lose power at altitude

[tkuss]

When Turboed why don't sleds lose power at altitude when running the same boost levels do they as compared to sea level? Or do they I dont really know. I would think they would considering there is less oxygen in the air for the turbo to throw in the engine.

 

[Auskins]

Higher the altitude the less dense the air but it has the same percentage of ozygen. When you compress air to so many pounds its basically like density, so 10 pounds at 7500 feet is the same as 10 pounds at sea level. (Same density)

 

[nate007]

A turbo can produce sea level- like atmosphere at altitude much better that a supercharger, because it is free to spin the rpms needed to maintain consitant boost.
I fly a turbo- equiped piston powered airplane, and it will perform much better than it's normaly aspirated counterparts, because it is capable of putting the oxygen back into the motor that would be lost from altitude.
Turobs do have their drawbacks though. Heat will be much more of an issue, and protecting surrounding components will be very important.
Check the turbo/ supercharger forums. Those folks over there know their business...

 

[tkuss]

Higher the altitude the less dense the air but it has the same percentage of ozygen. When you compress air to so many pounds its basically like density, so 10 pounds at 7500 feet is the same as 10 pounds at sea level. (Same density)

If there is the same amount of oxygen why is harder to breath the higher up you go?

 

[bdm]

http://anthro.palomar.edu/adapt/adapt_3.htm

 

[LazyBastard]

Its not the same amount of oxygen *per volume*, its the same amount of oxygen *per mass*. I.e., 1 Kg of air contains xx grams of oxygen, but consumes more SPACE at higher altitude. This means that since your lungs are of a fixed capacity, when you inhale, you are inhaling the same volume, but a smaller MASS, which means that you are inhaling less oxygen overall.

Now, to say that a boost of 10 psi is the same at sealevel as it is at high altitude is technically INCORRECT. The pressure at sea level is about 14.7 psi. At 5000 feet (same temperature), it drops down to 12.2 psi. At 10000 feet, its all the way down to 10.1 psi, so 10 pounds of boost at sea level is 24.7 psi, at 10000 feet, its only 20.1 psi, thats a difference of 4.6 pounds!

Now what you do with turbochargers is this; you COMPENSATE for altitude. If you like to run 10 pounds at sea level and are going to run at 10000 feet and want the same power, you turn up the boost to 14.6. It is no harder on the engine as it gives it exactly the same amount of air. Well, not exactly, as it takes more power to make 14.6 pounds of boost than it takes to make 10 pounds, so you need just a little more than 14.6 to make the equivalent of 10 pounds at sea level.

 

[tkuss]

Gotch Ya, Thanks for a good explanation.

 

[Mikey]

LB yer head is gonna explode with all that info you have crammed in there.

 

[IFlyEm]

nate007

nate007,

What do you fly? I have an airplane hangared in Iowa City.

 

[ahicks]

If a waste gate is set to let go at 10psi at sea level, would there not be 10 psi when (if?) it let go at 12,000 ft.?

Agreed the density of the air drawn into the compresser will be less at altitude, and the compressor will be spinning faster because of that, but the manifild pressure would be the same. No?

 

[NytroSx]

Just to add to what LB said but to compare a turbo to non-turbo...

Calculate the absolute pressure/air that you are pushing through the engine:
For example:
-At sea level:
run 10 stock psi + 14.7psi (atmospheric pressure) = 24.7psi total.
-At 6000ft
run 10 stock psi + 11.4psi (atmospheric pressure) = 21.4psi total.
-At 10000ft:
(Figure about .55psi atmospheric drop for every 1000ft) So 10psi + 9.2psi (atmospheric pressure) = 19.2psi total

To calculate the % of air loss you experience compared to sea level (aka roughly the amount of hp loss you will experience):
-at 6000ft 21.4 / 24.7 = 87% so you have 13% less pressure going into the engine
-at 10,000ft 19.2 / 24.7 = 77% so you have 23% less pressure going into the engine

This is interesting:
Comparison for a non- turbo engine in terms of how much % less air:
-at 6000ft 11.4 / 14.7 = 78% so you have 22% less pressure going into the engine.
-at 10,000ft 9.2 / 14.7 = 63% so you have 37% less pressure going into the engine.

 

[LazyBastard]

If a waste gate is set to let go at 10psi at sea level, would there not be 10 psi when (if?) it let go at 12,000 ft.?

Agreed the density of the air drawn into the compresser will be less at altitude, and the compressor will be spinning faster because of that, but the manifild pressure would be the same. No?

I don't know the construction of your typical waste gate. Most pressure sensors will only indicate RELATIVE pressure, ie so many pounds over AMBIENT pressure, HOWEVER, a barometric pressure sensor will read ABSOLUTE pressure. This type of pressure sensor has a sealed section with a fixed volume of gas to act as an absolute reference point. The problem with this type of pressure sensor is that in order to get an accurate reading, it also requires a TEMPERATURE sensor (since volume changes with temperature as well as pressure). The ideal waste gate would have an electronic valve and take input from a barometric pressure sensor AND a temperature sensor. In addition to being able to provide a constant absolute intake pressure, it could also be used to compensate for temperature, i.e. at lower temperatures, air is more dense, so to achieve the same power, less boost is required.