Loss of power at altitude

[rdkind62]

Here is a question for those of you that may have the knowledge to do proper calculations or maybe anecdotal evidence of power loss at a higher elevation such as Denver or Colorado Springs. I am trying to find the percentage of power loss I would have if I got the 128. Here is some info that I am sure someone would ask if they are trying to do calculations. If you need more than this then let me know and I will try to get it. This data is current as of the time I did this message.

Altitude 6700 ft. or 2042 m
current temp 31 F or -0.9 C
humidity 62%
dewpoint 19 F or -7 C
Pressure 30.09 in or 1018.8 hPa

I did these calculations myself and found a 15.5% loss in power from max rated for the engine. I could not believe this is correct, but if it is that may force me to get the 135. Not that that is a bad thing necessarily. OK I admit it. I'm trying to put together a good argument to convince myself I NEED the 135.

[atr_hugo]

I think those calculations may be pretty close. I covered the Pikes Peak hill-climb for a couple of years and those numbers sound about right based on discussions with some of the participants.

[Bizall]

Just do it. You would hate to regret getting a 128...just splurge and enjoy the 135i. Oh, I'm in the Springs also. :biggrin:

[jeffgingras]

Good rule of thumb is a 3% reduction in power for every 1,000 feet of altitude. There will be small variations due to temperature, pressure, humidity, etc., but those occur at any altitude.

[Nixon]

From the calculator below, it says that under those conditions, you would go from 108.7% of rated HP at sea-level, down to 81.3% of rated HP at 6,700 feet in altitude. Or from 250.01 down to 186.99 HP just for altitude changes. That's a 63.02 HP loss due to altitude under the atmospheric conditions you listed.

http://www.csgnetwork.com/relhumhpcalc.html

Set the calculator to SAE standardized conditions (77 degrees, 29.235 pressure, and 0 humidity) and the calculator will show that the engine goes from 100% of rated HP down to 75% of rated HP just due to the altitude. In other words, the rated HP of 230 under SAE standardized conditions drops to just 172.5 on altitude change alone. That's a 57.5 HP drop (25%) for altitude. I can't vouch for this calculator, so I don't know if it is 100% accurate. But it sure looks spiffy, so it must be right.

Theoretical power loss can be calculated, but there are also other considerations that will affect how NA and turbo cars actually perform at altitude.

First off, turbo engines can only give back the power lost to high altitude when the turbos are operating inside their optimal range of efficiency. Turbo engines have all the same power losses at very low engine rpm's as NA engines have, until the turbo spools up. It also takes slightly higher rpm for the turbos to spool up, because they need higher engine rpm's before the turbos can start to spin fast enough to make up for the altitude difference. This means you will feel more turbo lag at higher altitude. There is also an upper limit of the RPM your turbo can spin at to make up the difference in altitude. As a car pushes a turbo up to it's rpm limits at higher engine rpm's, you may see the power drop off earlier as the engine approaches redline at higher altitude. How much this will actually impact your driving experience will depend upon the size and pressures of whatever turbo system a car has. I also firmly believe that if you run 93 octane at sea-level, you will still need 93 octane at high altitude to get the same power in a turbo engine. Unfortunately, premium is mostly 91 octane up here. So if you can't get 93 octane, you won't be able to recoup all the power you had a sea level. On the other hand, NA engines can get away with less octane than at sea level.

The 135i should do better than most turbo cars, because it has two small turbos that don't have a problem with lag in the first place, and are said to charge straight to the redline without letting up. My APR chipped 1.8T A4 is a different story. The lag is obvious up to 2500-2700 RPM, and power starts to drop off at 5500-5700 rpm. I've done multiple back-to-back test drives against BMW 330i's, 330Ci's, and 330Xi's here at 5,000+ ft of altitude. The test drives always turn out the same. The NA BMW engine feels much better from 1000-2700 RPM, and from 5500 rpm to redline. But the 4-cyl chipped turbo feels stronger than the 6-cyl NA engine in the 2700-5500 rpm range. Because of that, I haven't been able to justify buying a 330 over my A4. Instead I've adjusted my driving style to keep the engine between 2500-5700 rpm.

The turbos on the 135i is the biggest reason I'm considering buying one. Then again, I firmly believe for the same reasons that a chipped 123d will out-perform a 128i (or my chipped A4) at high-altitude. I will just have to change my driving style to keep the engine between 1400-5400 rpm. The 123d would also get nearly double the gas mileage while out-performing a 128i or A4. So I still really still want the 123d over the 135i.

Second off, the engine ECU has to be able to make the needed adjustments for altitude for the engine output to match the theoretical. High altitude requires changes in fuel flow and timing in an NA engine to correct for the change in airflow. And it requires changes in turbo RPM's on turbo cars to get back to sea-level pressures. Some engine ECU's do a much better job making these adjustments then others. An extreme example is a Volvo 123 GT I had, that I actually had to pop the hood and ear-tune the timing just to keep the engine from stalling whenever I went over 9K+ ft of altitude. I'm going to assume that the 128i would do a good job of adjusting automatically -- but the point is that calculating the theoretical doesn't always translate directly to the real world.

Heck, BMW might even de-tune their 128i engine at sea-level, and then put some tuning back in at high altitude to make actual altitude losses less then the theoretical. We don't know. BMW has so much control over the performance of their engines through the ECU, that the only way to actually tell how much performance difference you will feel at the seat of your pants will be to drive them back to back (or dyno them back to back).

[rdkind62]

Thanks for your thoughts everyone.

[mikeo]

Good piece, Nixon.

I'm at 7000ft. in Santa Fe, and I think the 135i is the way to go vs. the 128i for this altitude. The N/A 128i at flatland altitudes would be a sweet drive though. I would also have to say that the 135i is not making rated HP at this elevation, and I also notice a power improvement in our Cooper S when it's down in low country.

I'm with you on the 123d all the way. My concern is it will be a least a couple years before BMW brings it to the U.S. The sooner the better.

[john970]

I would love to see an uncorrected dyno run here in denver (and might pay for one...)

what do dyno runs cost?

[F0Bman]

Low density in air?

[kiddomoney]

I have yet to find out... Santa fe is a bit higher up than albuquerque, but we shall see the difference.

[mikeo]

I think the altitude at 7000 ft. in Santa Fe definitely prevents rated horsepower output even in the 135. One hint, after 5200 miles my average fuel economy is 24.2 mpg. Less air pumping through the engine means less fuel needed and therefore, less horsepower. My N/A M3 was down about 25% at this altitude. I'm too cheap for a dyno to prove my theory though.

[john970]

Originally Posted by mikeo

I think the altitude at 7000 ft. in Santa Fe definitely prevents rated horsepower output even in the 135. One hint, after 5200 miles my average fuel economy is 24.2 mpg. Less air pumping through the engine means less fuel needed and therefore, less horsepower. My N/A M3 was down about 25% at this altitude. I'm too cheap for a dyno to prove my theory though.

Thinking about taking my 1er to santa fe, much to do there? recommend any hotels/activities?

[jbenington86]

how about getting some high octane gas to compensate

[john970]

Originally Posted by jbenington86

how about getting some high octane gas to compensate

high octane gas needs high air density to work...

[808@702]

So your saying when I ship my car from Vegas (2500 ft) to Hawaii (sea level) I can expect a 8 hp gain? Yeehaa! Why live in the mountains.

[Yuzi]

high altitude = turbo/supercharge world

[vachss]

Originally Posted by Yuzi

high altitude = turbo/supercharge world

Hate to say it cause I prefer the feel of NA motors, but I totally agree. If I lived at high altitude a motor with some kind of forced induction would be an absolute necessity. That or just give up and drive an electric car - they work well enough in a vacuum.

[Zinister]

Originally Posted by mikeo

I think the altitude at 7000 ft. in Santa Fe definitely prevents rated horsepower output even in the 135. One hint, after 5200 miles my average fuel economy is 24.2 mpg. Less air pumping through the engine means less fuel needed and therefore, less horsepower. My N/A M3 was down about 25% at this altitude. I'm too cheap for a dyno to prove my theory though.

Mike, Its not a huge difference on FI (force inducted) cars at all. While yes, the air outside going into the TC/SC is less dense than sea level, the pressure at the manifold is well below sea level.
Im in ABQ (5,700 ft). N/A cars can gain .9-1.0 full second at sea level, where as the FI cars only gain maybe a couple tenths, If that. Its all subjective though. Results will vary.

[new2the3]

I drove my old E90 330i from Florida out to Vail, Colorado and took the car up to elevations of 13,000 ft. In Denver the power was ok, definitly not a total dog but quite a bit slower then sea level. When I took the car into 7,000+ the car just fell on it's face. 11,000+ altitude felt like 80hp.