How much HP do you lose for each 1000' in elevation change?

Being 5100' - 7000' at some spots here in AZ. How much HP do I lose?

:confused:

 

a bunch, around a 1/2 second .5 in the 1/4mile from us here at sea level...... :lol:

 

I live in Flagstaff, I'm sure you know how high that is.

Estimated horsepower loss is around 20% in Flagstaff. This is simply due to air pressure. Quarter mile times would be a lot more than .5 sec difference here, luckily there are no tracks at this altitute. Speedworld and Firebird are both about 1,200ft.


[Modified by 66ImpalaLT1, 11:38 PM 3/30/2004]


[Modified by 66ImpalaLT1, 11:39 PM 3/30/2004]

 

For every 1000 feet you loose 3% of power. So in you case its
between 15% and 21% loss.

Frederik

 

:cool: :cool: :thumbs:

 

Also, your profile says Scottsdale, thats only 1200' or so. Not a huge power loss there.

I can tell you where to go if you want to drive to 9,200' on paved roads. :eek: Its a few miles from my place.

 

i don't know exact #s, but i can tell you there is a major SOTP difference between Georgia and Colorado

 

Here, read this very recent thread and figure it out:
http://forums.corvetteforum.com/zero...140#1546742140

If you don't want to, according to the United States of America, it's 3.5% for every 1,000 feet for a normally aspirated engine.

 

You lose about 3% per thousand feet, depending on temp, humidity. The correction factor for our dyno (at 5.600') is usually 1.2, with good air 1.18, hot days 1.23. :eek:

 

Weather patterns can swing barometric pressure an inch or more over a day or two as well. An inch of mercury equates to about 1000 ft so you could have a 3% loss/gain just due to a low/high pressure system.

In our area, we have a phone number where you can call for aviation weather forecasts. I don't remember the phone number any more but part of the forecast was something like a "virtual elevation" where they take relative humidity, air temp, and barometric pressure, and give you a new "virtual elevation" based on some standard/average conditions. For example, they first give the real elevation of the weather station (say 800 ft) and then the "virtual elevation" (my term, maybe someone knows the real term) based on weather conditions. If the virtual elevation is 1100 ft, you'll know that your car will be performing slightly worse than average that day. If the VE is 300 ft, you'll know that your car has more air to breathe that day and will perform a little better than usual.

I used to call the number all the time before going to the track. :cool:

Mike

 

While the USAF may use 3.0%, the civilian portion of the United States of America, i.e., the FAA, has it at 3.5% per 1,000 of density altitude, as in:

"Reduction in engine horsepower due to DA increase:

As density altitude increases, engine horsepower decreases. If you know what the decrease is and how to apply that knowledge, you can determine roughly what your performance degradation will be and how much weight you may have to take out of the airplane to operate safely. The following is based on standard atmospheric pressure lapse rate and reciprocating engine efficiency.

Rule of thumb: A normally aspirated aircraft engine loses approximately

3.5% hp per 1000 feet increase in DA

Let's use a common airplane as an example, a Cessna 182.

Example: 230 hp airplane at the 5000 ft airport where the DA = 7600 feet

HP reduction.................= 3.5% x 7.6
....................................= 27% reduction (approximately 73% available)
230hp x (73%)...............= 168 hp available at 7600 ft DA

Another way to determine the amount of power reduction is to know what your engine will produce at full power on the MP gauge at sea level on a standard day. Then, determine the amount of power per inch it is producing, and calculate power reduction based on the fact the engine will lose one inch of manifold pressure per 1000ft DA.

Example: given a 230-hp engine that produces 28" MP at full power, sea level standard day

Horsepower per inch..................= 230hp divided by 28"

........................................ .......= 8.2 hp per inch

At 7600 ft DA the engine will produce approximately 20.4" (28" - 7.6") at full power and the horsepower will be:

8.2 hp per inch X 20.4"..............= 168 hp available at 7600 ft DA

Either of the above methods work in estimating your power reduction and give similar results. The amount of reduction may seem a shock at first, but it should alert you to the realities of density altitude related performance problems."


[Modified by EHS, 9:49 AM 3/31/2004]

 

I dont know the horsepower figures but I can give you a quarter mile ET sample to think about. Our race track was at 3200' above sea level. My 1969 Plymouth GTX 440 ran 15.10 at our track. I went to the Seattle track on a vacation and the car ran 13.90 at that track which must be close to sea level. :seeya

 

"Density altitude"... that's the word I was looking for with respect to the aviation forecasts. That's what I used to use to know how well my car was going to run at the track at any given day just by calling the aviation weather number.

Mike

 

EHS---since I guess humidity does not play a big part in things at 7000 ft does the 3.5% consider this, the formula you supplied me did not seem to take humidity in account--in FL that is a major factor---

I have a good example for you on the correction factor for my car recently on a dynojet at Mike Norris--we tuned it to 519rwhp (avg) with 390 gears, the adj factor on the Dynojet was a --3%--the DA at the time was 630ft which would be a conservative +++2% adj--that is a diff of 26rwhp so corrected properly it would be 545rwhp plus a minimum of 7rwhp for the gears which would have been 552rwhp and 538tq--MTI recently post a 427 with the FAST 90 setup and they got 548/530 very close to these numbers--my car has a few more cubes a little more cam and a higher flowing exhaust so you would expect all things equal to have a few more hp--these numbers make since--I am ordering a booklet I found with the SAE formulas for corrected HP in them--

What does SAE consider the perfect everything by which they run their scale--is it 0 humidity, 0 dewpoint, 0 altitude, 70 degrees or what--Orlando is basically 96 ft above a typical good time of the year 49 dewpoint, hg's in the good time are high 29's low 30's temps in the 70 range in cooler part of day--

It is just a rule of thumb like so much loss at the rear wheels, I am trying to establish a rule of thumb we all use for gears--whether the pilot 3.5% considers humidity or not it is an acceptable rule of thumb for running our on DA--the above example of the incorrect dynojet corrections is why when people come to FL they dyno 5 to 10% less--do your own DA here on uncorrected numbers and you will see the they coincide with other parts of the country--I recommend the guys in CO and at altitude take a look at their stuff as well, and right or wrong we will have a rule of thumb that is at least much more consistent than the way it is on the dynojet--

Just my suggestion but I say uncorrected number, check current DA, times 3.5% per 1000' of DA (plus or minus) and that is a fair comparison and you will find it much more consistent than the corrected numbers--


[Modified by NEPTUNEBILL, 9:43 AM 3/31/2004]

 

I was posting on density altitude, and you are correct, humidity will affect that considerably. It isn't necessarily realted to true altitude as much as to ambient temperature. In other words:

Pressure Altitude + Temperature = Density Altitude

Density Altitude + Humidity Correction = Corrected Density Altitude

Below [40 degrees F the correction is small. Even at 100% relative humidity, the correction is on the order of 250 feet.

Above 50 degrees F the exponential correction takes off and may become very significant. The term is not as dominant as temperature but, with high humidity, the cumulative effects produce a correction in the 3,000+ foot range.

A couple of tables, straight from the United States of America Federal Aviation Administration:

First, the old "rule of thumb:"



Then the "real deal:"

 

i dyno'd my stock z06 this past fall and here were the numbers on their dynojet:

361 sae horsepower / 350 sae torque ft-lbs

295 actual horsepower / 287 actual torque ft-lbs

it was 74 degrees, 30% humidity and the altitude in their parking lot is 5,340.

so that is about 19% loss in hp and torque and that is with good summer gas.

:cheers:

 

EHS--based on your great charts I built some formulas into my spreadsheet on a 2000' adj being represented by 100 degree temp and 100% humidity and made a sliding scale to compute how many feet to add to the current DA this will compensate the 3.5% per 1000' of DA not considering humidity--everyone should be able to use these formulas with their uncorrected numbers and have uniform dyno results anywhere in the country and rely on the dynojet to use their correction factors which are apparently not very consistent or reliable the way they are set up--I just realized on the lower graph at 100 degrees it appears to be at 1500' so to be more conservative I will change that in the spreadsheet--


[Modified by NEPTUNEBILL, 4:49 PM 3/31/2004]

 

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So cruising around Sedona (4800) or Prescott (5200), my motor is only making 280-290 HP, that S*CKS! :cry

So cruising around in Jerome, AZ the car only makes 263HP (7000').

The Arizona roads are scenic and windy but the HP loss stinks! :(

Hopefully the Vararam install can bring it back close to stock levels.

 

The Vararam wont even make it come close. My car is still making less power than stock up here, I have most bolt-ons and a cam with stock heads.

The problem is that the barometric pressure is 76-80kpa all year. The only thing that will compensate for that is a supercharger or turbo.

Highway 189 north west of the peaks is almost 8,300ft elevation, this isnt the highest sign I passed, just the most convenient to stop at.




[Modified by 66ImpalaLT1, 10:49 PM 3/31/2004]