Elevation and horsepower loss

BigBore · Dec 26, 2001

We all know that our bikes lose power at high elevation. I'm wondering, is there any formula or equation to figure out exactly how much power you lose? Lets say, you're riding at 6,000ft above sea level, on an 80 degree day....what kinda power loss are we looking at?

yzeater · Dec 26, 2001

7, but you can make up for it if you put on a shiney new pipe:)

Just kidding. I don't think that elevation will mess up power if you're jetted right. If your bike is jetted for Death Valley in July, but you're riding on Everest in January, you're gonna have some problems. If you fix your jetting right, you could make the same hp at both locations.

hoosierf · Dec 26, 2001

You definately will lose horsepower and you cannot get it back through jetting for the altitude. The old saw is that you lose something like 3% for every 1,000 feet of elevation.

My Checkmate 242 Convincor went 70 mph in chicago and 55 mph in Pueblo, Colorado.

KYHU · Dec 26, 2001

Originally posted by yzeater
If you fix your jetting right, you could make the same hp at both locations.

It's unfortunately not the case. The higher the altitude, the lower the atmospheric pressure. That means less oxygen molecules in a given volume than at a lower altitude. Even if your jetting is perfect, less oxygen molecules mean less fuel, hence less power from your engine.

But don't forget that when you go higher, the temperature decreases. This compensate in part for the lower atmospheric pressure.

david

yzeater · Dec 26, 2001

Sorry, I thought that if you leaned out your jetting, you'd be fine. I didn't realize I was wrong:(

Tony Williams · Dec 27, 2001

That 3% per thousand feet power loss is probably in the ballpark.

The loss of air pressure with increase in altitude is not quite linear, but roughly a loss of 1 inch of pressure per thousand feet.

If you use 30 inches as a baseline sea level pressure, you can see that at Leadville, Colorado (about 10,000 feet high), you'll have 20 inches, or about a 1/3 loss or available air. Or about 3% less per thousand feet.

Remember, the fuel mixture ratio needs to be consistent with the amount of air. What affects the amount of air (and the part we're interested in, O2) that will exist in a given cubic unit is air pressure, temperature, and moisture content (water displaces what could be O2).

Soooooo, you will produce less horsepower as you get "high, hot, and humid". And you will need to adjust the mixture accordingly for optimum performance, albiet diminishing in power.

Tony Williams · Dec 27, 2001

I forgot to tell you what "hot" would mean at a given altitude....

Basically, a standard sea level day pressure is 29.92 inches of mercury at 15 degrees Celcius (59 F).

For each 1000 feet above sea level, standard pressure falls one inch and temperature falls 2 degrees C.

0 feet - 15 C
1000 feet - 13C
2000 feet - 11C
etc.

So, a standard day at Leadville, Colorado has 20 inches of pressure and is negative 5C temperature. The elevation at Leadville is 9,927 feet, therefore, on a 18C/65°F day, the "density altitude" is an astounding 12,350 feet.

Check out:

http://rshelq.home.sprynet.com/calc_da.htm

to figure out the density altitude for your ride. You can carefully adjust your jetting for all occasions.

Tony Williams · Dec 27, 2001

Percent Horsepower Calculator

OK, I like this better:

http://rshelq.home.sprynet.com/calc_hp_dp.htm

which has a % horsepower calculator.

"This calculator shows you how much the air conditions on any given day will alter the horsepower of a normally aspirated internal combustion engine. The Relative Horsepower Calculator determines the relative torque and horsepower available at any temperature, barometric pressure, dew point, and altitude. The calculations are derived from SAE J1349 Revision JUN90."

weekendtoy · Dec 27, 2001

I race a 99 SS Camaro.I post on a huge owners forum and the guys in Colorado do not run anywhere near the ET's that we do here on Long Island (if the motor mods are equal of course).This whole subject is why they have a correction chart for dyno readings.

WoodsRider · Dec 27, 2001

I recently did some horsepower correction factors for two marine diesel engines operating at 12,000 ft. on a lake in the Andes Mountains. There is no "magical" number for every 1,000 ft. since you must factor in the average daily temperature, relative humidity and air density. The engines were de-rated from 3000 to 2300 HP, approximately a 23% loss in horsepower.

BigBore · Dec 27, 2001

Thanks for the replies. I kinda figured there isn't any way to exactly tell how much power you're losing, but it was worth a shot. I always re-jet for high altitude, though. All I know is my 650 starts to feel a little aneimic when riding up around 8,000ft!

Tony Williams · Dec 27, 2001

Originally posted by WoodsRider
I recently did some horsepower correction factors for two marine diesel engines operating at 12,000 ft. on a lake in the Andes Mountains. There is no "magical" number for every 1,000 ft.

As I mentioned, there are "magical" numbers under the ICAO for a standard day. A standard day at sea level is 15C (59F) and 29.92 inches of mecury. I'm not sure of the humidity standard off the top of my head, since it's affect is minor in comparison to temperature and altitude (air pressure).

since you must factor in the average daily temperature, relative humidity and air density. The engines were de-rated from 3000 to 2300 HP, approximately a 23% loss in horsepower.

My point exactly.... you factored in temperature, altitude and humidity from some standard. There are international standards that exist for that. Corrections are made from the standard. What you and your company use for those standards, I don't know.

Even a fixed altitude has fluctuations in air pressure (those high and low pressures the weather man tells you about). And there are internationally recognized standards for those corrections, also.

A very accurate measurement of air density can be calculated, and has a direct proportion to the horsepower produced. This, of course, is assuming an engine that is not turbo or supercharged. How 'bout that big boat? Is there any type of pressure augementation to the air intake?

Secondly, it sounds like your engine example is "rated power", and not absolute available power. For those reading who don't understand the difference, rated power is simply an amount of power that can be produced continously for a given amount of time, and with a certain life span. Great for boats, planes, and trains. Not applicable to drag cars and race bikes.

Off the previously quoted webpage <http://rshelq.home.sprynet.com/calc_hp_dp.htm>
I swagged some numbers from your example:

Air Temperature 70 degrees F
Barometric Pressure 29.92 inchesHg
Dew Point 30 degrees F
Altitude 12,000 feet

And calculated:

Relative Horsepower 61.1% (of course, no turbos, nitrous, etc.)
Dyno Correction Factor 1.637
Actual Air Pressure 19.03 inchesHg
Actual Vapor Pressure 0.166 InchesHg
Relative Humidity 22.5 %

Tony Williams · Dec 27, 2001

Originally posted by BigBore
Thanks for the replies. I kinda figured there isn't any way to exactly tell how much power you're losing, but it was worth a shot. I always re-jet for high altitude, though. All I know is my 650 starts to feel a little aneimic when riding up around 8,000ft!

Gee.... what would make you say that? Both Woodrider and myself have pointed out that it can be calculated.

Of course, nothing can replace that final fine tuning after all the calcs.:confused:

Tony Williams · Dec 27, 2001

Originally posted by weekendtoy
This whole subject is why they have a correction chart for dyno readings.

You better believe it!!! Ther are no wheels to be reinvented here. All commonly available information.....

BigBore · Dec 28, 2001

I guess I should have worded that different. What I meant to say was, I kinda figured there wasn't any really "easy" way to figure it, like saying you lose x amount of power for so many thousand feet.

Tony Williams · Dec 29, 2001

I'd use 3 1/3% per thousand feet as a good overall guide. That precludes super hot or cold temperatures. You can get altitude information from a portable GPS or toppgraphical and aviation maps. The figures assume a properly jetted machine for each altitude.

Altitude - Power produced
sea level - 100.0%
1000 ft - 96.7%
2000 ft - 93.3%
3000 ft - 90.0%
4000 ft - 86.7%
5000 ft - 83.3%
6000 ft - 80.0%
7000 ft - 76.7%
8000 ft - 73.3%
9000 ft - 70.0%
10000ft - 66.6%

BigBore · Dec 30, 2001

Cool! Thanks again, you've been a big help. :)

WoodsRider · Jan 2, 2002

Originally posted by Tony Williams
My point exactly.... you factored in temperature, altitude and humidity from some standard.

Yes, I used the altitude from the local airport. Average temperature, humidity and air density were based on a ten-year history from the local weather station.

A very accurate measurement of air density can be calculated, and has a direct proportion to the horsepower produced. This, of course, is assuming an engine that is not turbo or supercharged. How 'bout that big boat? Is there any type of pressure augementation to the air intake?

The engines are two-cycle turbocharged diesels. The turbo is gear driven and incorporates a clutch that over-rides the gear drive at approximately 70% load. We modified the nozzle ring to produce more horsepower at the higher elevation. The real trick was simulating these conditions in the turbocharger test cell.

Secondly, it sounds like your engine example is "rated power", and not absolute available power.

You are correct.

I could pull up all the calculations except Okie would probably ban me for wasting that much bandwidth. My example is rather unique since it's not every day you have marine diesel engines operating at 12,000 feet. Interestingly enough had the turbos not been modified the horsepower loss would have been around 30%.

Tony Williams · Jan 3, 2002

Originally posted by WoodsRider
My example is rather unique since it's not every day you have marine diesel engines operating at 12,000 feet. Interestingly enough had the turbos not been modified the horsepower loss would have been around 30%.

Looks like without the turbos, you'd be closer to a 40% loss ;)

I had an idea that I could use a 10 second blast of nitrous oxide at 7000 foot Mammoth mountain. I'd be slingshotted out of the gate! (OK, at least compared to the 39 other bikes).

Thanks for the reply,

Elevation and horsepower loss

BigBore

yzeater

hoosierf

KYHU

yzeater

Tony Williams

Tony Williams

Tony Williams

weekendtoy

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