Elevation - hp loss ??

[unclemark]

I heard that the 2smokes lose like 20-30% hp at 8000ft (can someone confirm this?)
Do 4stroks loose power also ?
And if so how much ?
Or it don't matter with the turbo ?
I ride 0-3000 ft mostly so I'm just curious if I will be having the same results as the guys that ride at 8000-10000 ft ?
Or what should I be expecting !

Thank

 

[diggerdown]

Dyno guys would have the exact figure but I believe it comes out to 2.8% every 1000 ft. Of course barimetric pressure and humidity come into play also. Four strokes seem to lose a little less, but there is still a loss.

 

[Turbo11T]

Theoretically if you maintain the same total pressure you should maintain the same hp. Say you loss 4 lbs of pressure at 10000ft that is 10.7psi verses 14.7 at sea level. You see a 28% loss in air pressure. With a turbo you can maintain sea level pressure. So say the same engine now with a turbo on it your at 10000ft but at running 4lbs of boost you should be seeing stock na sea level hp at 10000ft. After that every lb of boost increases power.

On the 4 stroke it has the advantage of intake and exhaust valves that close to prevent some loss but it is likely very small and still loses power at nearly the same as the a 2 stroke.

In the instance of the 1100 turbo from cat the turbo makes about 11-13 psi at sea level and makes about 16-18 at 10,000 ft stock to maintain the same power.

 

[madmax]

Normally aspirated engines lose around 2% per 1000 feet increase in elevation, depending on who you get the info from. Like mentioned in the above post, a boosted engine easily makes up for it by increasing boost, so you can maintain the HP as you inceprease in altitude.

 

[JBIZ]

Important to note that you still require clutching changes from sea level to high elevation because even though the turbo maintains your hp at high elevation the motor still has to swing the weight before boost is built. I run 93 grams at home( 1500 ft ), if I ran that much at elevation it would be a dog.

 

[JBIZ]

The more efficient the turbo is, ie super chute. The less sensitive to clutching at elevation. Boost comes in way earlier on the super chute vs stock sled

 

[sled_guy]

In the instance of the 1100 turbo from cat the turbo makes about 11-13 psi at sea level and makes about 16-18 at 10,000 ft stock to maintain the same power.

Really? I didn't think any of the turbos were variable. Their waiste gate is set a xx lbs of boost. So at sea level or 10,000 feet they boost the same amount... say 11lbs.

The advantage at altitude is the total loss is must less because the boost number remains constant while the base number decreases the same amount whether you are boosted or not.

All the waist gates I've seen are a differential system... they start with current ambient pressure and then allow for a fixed amount of boost over that. Some of the aftermarket turbos have a variable boost control, via a knob you turn, but all that is doing is changing the differential amount... but that would allow you to go to higher boost to compensate for the altitude loss.

sled_guy

 

[Turbo11T]

Really? I didn't think any of the turbos were variable. Their waiste gate is set a xx lbs of boost. So at sea level or 10,000 feet they boost the same amount... say 11lbs.

The advantage at altitude is the total loss is must less because the boost number remains constant while the base number decreases the same amount whether you are boosted or not.

All the waist gates I've seen are a differential system... they start with current ambient pressure and then allow for a fixed amount of boost over that. Some of the aftermarket turbos have a variable boost control, via a knob you turn, but all that is doing is changing the differential amount... but that would allow you to go to higher boost to compensate for the altitude loss.

sled_guy

They use a ecu controlled electronic boost controller to "fool" the wastegate actuator. It bleeds boost on the boost ref to the actuator to "fool" the actuator. It is a fairly simple idea. I beleive that MCXPRESS has been doing it on their turbo setups as well for years. Also car manufactures do it as well. This allows the same hp at sea level to 10000+ feet as long as the turbo is efficent enough to do it. Obviously pre spool the hp will be less but on a sled that is not really an issue as the clutching handles that just fine.

 

[snow4shover]

Horsepower Loss due to Elevation

I would use the following formula to calculate your horsepower loss.

HP Loss = (elevation x 0.03 x (horsepower @ sea level))/1000

I believe this will be the same for N/A or Turbo Applications.

 

[Arctic Thunder]

So considering we are talking about this explain to my why some say you can get away with 91 oct at 8000' but would need higher oct fuel below 8000' with a turbo setup. Pump gas M8 for example

If you are running 10lbs at 5000' running 100LL av gas, then you go to 10000' you could run 13lbs on the same 100LL. correct? or if you went down to 3500' you would need to run the boost down around 8lbs on the same fuel.

I just don't see how elevation changes the oct requirements when under boost if the boost is adjusted to the same level at 3500, 5000, or 10000 ft.

Last question. your boost gauge is reading 14.7 (zero) at see level. So at 5000' your gauge would be reading 12.2 absolute. So if you set up your sled at 5000' at 10lbs of boost on the gauge, then at 10000' your gauge is still reading 10lbs but you have now lost another 2.2 psi. Your down to 10.1psia.

So you say, I can run 12psi at 10000ft on 100LL compared to 10psi at 5000'. When actually you are really running the same boost to the motor but your absolute pressure has dropped so your gauge reads lower at higher elevation. Does that make any sence at all?

I am sure that was confusing as hell. But I am confused.

Thunder

 

[Turbo11T]

So considering we are talking about this explain to my why some say you can get away with 91 oct at 8000' but would need higher oct fuel below 8000' with a turbo setup. Pump gas M8 for example

If you are running 10lbs at 5000' running 100LL av gas, then you go to 10000' you could run 13lbs on the same 100LL. correct? or if you went down to 3500' you would need to run the boost down around 8lbs on the same fuel.

I just don't see how elevation changes the oct requirements when under boost if the boost is adjusted to the same level at 3500, 5000, or 10000 ft.

Last question. your boost gauge is reading 14.7 (zero) at see level. So at 5000' your gauge would be reading 12.2 absolute. So if you set up your sled at 5000' at 10lbs of boost on the gauge, then at 10000' your gauge is still reading 10lbs but you have now lost another 2.2 psi. Your down to 10.1psia.

So you say, I can run 12psi at 10000ft on 100LL compared to 10psi at 5000'. When actually you are really running the same boost to the motor but your absolute pressure has dropped so your gauge reads lower at higher elevation. Does that make any sence at all?

I am sure that was confusing as hell. But I am confused.

Thunder

Im not sure what your trying to get at?

If your running 10lbs at 5000ft with a 100ll and your running 13lbs on 100ll at 10000ft. That puts your sled at relatively the same absolute pressure.

If you r gauge is liquid filled and sealed crack the seal at 1000 ft to get a accurate measurement. If not liquid filled it should not be sealed and it should read zero without boost applied to it because the pressure is the same on the boost reference to it and to the gauge as well.

 

[sledneck_03]

yes to run pump at 1000 ft you would need to run a very low psi almost not worth it, Silber turbos offers a stock 7psi waste gate in their normal kit and you can purchase 3psi for lake guys that want to run pump....

 

[diggerdown]

What the guys do not take into account is the lack of oxygen at elevation. You can pressurize your camber, which is good, but you still don't have the same amount of oxygen as you do at sea level. Oxygen is the variable that affects the way and speed fuel burns at. Turbos make more sustained HP because they pack all the air in that you can use, but the percentage of it that is oxygen is a little less making for a less volitive mix so there is less chance of pre-ignition.

 

[backcountryislife]

What the guys do not take into account is the lack of oxygen at elevation. You can pressurize your camber, which is good, but you still don't have the same amount of oxygen as you do at sea level. Oxygen is the variable that affects the way and speed fuel burns at. Turbos make more sustained HP because they pack all the air in that you can use, but the percentage of it that is oxygen is a little less making for a less volitive mix so there is less chance of pre-ignition.

Digger, the percentage of oxygen doesn't change as you go up in altitude. It stays static at just under 21%.

 

[diggerdown]

BC is right, I was wrong in my assesment. Oxygen % stay about 21% until 10.000 miles not feet. But you still end up with less oxygen in the cylinder to burn. Just like hypoxia, the lungs are full but still not enough oxygen to function like dense air. I would like to see someone set a dyno up in the back of a truck and run some test at 10,000ft. and compare them straight out to one run at sea level. I've run a few million miles driving turbo charged trucks over about every mountain they built a road over and there is a difference once you gain enough elevation. Same boost but less HP.

 

[Arctic Thunder]

BC is right, I was wrong in my assesment. Oxygen % stay about 21% until 10.000 miles not feet. But you still end up with less oxygen in the cylinder to burn. Just like hypoxia, the lungs are full but still not enough oxygen to function like dense air. I would like to see someone set a dyno up in the back of a truck and run some test at 10,000ft. and compare them straight out to one run at sea level. I've run a few million miles driving turbo charged trucks over about every mountain they built a road over and there is a difference once you gain enough elevation. Same boost but less HP.

Bingo!!!!!

That was my point. Rather than side track this thread completely I will start a new one just to get the ideas flowing. All I am trying to do is get my hands around the concepts and what is "really" going on.

Thunder