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How does the BMW E39 adapt to high altitudes when you go from sea level to 6000 feet?

10K views 31 replies 19 participants last post by  bluebee 
#1 ·
This thread today, where a user went from a low altitude warm area to a high altitude cold area, got me wondering how the E39 adapts to altitude when it comes to engine control adjustments:
I recently moved from San Diego to Breckenridge, CO and I have been having some trouble with how my car runs!
I've noticed in the past when I drove from San Jose (about 100' ASL) to Tahoe (around 6,000' ASL), my car used to stumble more so than I thought it should (but I've since replaced all the hoses due to a vacuum leak in the throttle boot and CCV lower vent hose and air pump vent hose).

To better understand how the bimmer adapts to altitude, I ask the general question:
Q: What does our E39 do differently at altitude than at sea level?
 
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#2 ·
In a nut shell, thats what your O2 sensors and MAF sensors are for. They are constantly measuring the air intake and sniffing the exhaust mixture for the proper fuel air ratio which will properly compensate for temperature and pressure. In the old days with carburetors, if you were going on a trip to the mountains, you would turn the mixture screws in a half turn to lean the carbs out a bit. Then undo do it when you got back.
 
#3 · (Edited)
Bee-when warm, any car is in closed loop mode so the maf is measuring air passing maf and sending an anticipated fuel injector spray duration period to the ECU. Then, the pre-cat O2 sensor has the final word and determines how well the ECU did based upon Maf signal. The leaner air at higher altitudes will tell the O2 that car is running rich cause there isn't enough air to support your adaptation values for normal operation. ECU will start to change the adaptation values to take fuel injector pulse duration down to make the O2 sensor happier with less gas to match the lessor air. The car does not know how long you are going to be in the higher altitude, so it may not make a full adaptation for the height above sea level in one trip. If you moved to higher altitude permanently, it would change your whole table and then your car would run lean at sea level until adaptation values got used to being at sea level and getting more air and lengthen the fuel injector pulse time. Make sense?
 
#6 ·
The key thing is that the MAF (Mass Air Flow Sensor) reads the Mass of air entering the engine, so the signal it gives to the ECU already account of change in air density due to the altitude. The higher up you go the less air mass the MAF will signal to the ECU, so the less fuel the ECU will inject. The O2 sensors of course send fine tune to the ECU to adjust the air/fuel mixture which adjusts the short term fuel mixture adjustment, which over time adjusts the long term fuel adjusts.
 
#8 ·
Any Naturally Aspirated engine will lose noticeable power as you increase altitude (in my experience). Air gets thinner, oxygen is more scarce. Same effect as with temperature, the hotter the weather, the more power you lose due to thinner air. Although you may accelerate easier at really high speeds due to less resistance (not sure on that one).

According to information online, at 6000feet, air pressure is at 11.1 PSI, which is is a 25% drop. I'd say that you would experience pretty significant and noticeable power losses.
 
#23 ·
This information, added today, is apropos to this thread:
I think EDJACK and FUDMAN have it right regards engine performance. Engine sensors, particularly the MAF (mass air flow) and O2 sensors, compensate for changes in atmospheric parameters. FWIW, carbureted engines were/are an entirely different matter - 5,000 of altitude would definitely require re-jetting to maintain best performance. And depending on engine and carb, best at altitude may be a far cry from sea level.

Temperature ranges are primarily an oil issue, i.e. is the oil in the engine suited to the ambient? If you use BMW LL-01 certified oil you will be fine. The owners manual says approved oils in 5W-40 or 5W-30 viscosities "may be used for driving in all ambient temperatures."
If you are in below freezing temps for a prolonged period beware of potential CCV problems. Check Best Links for the story. Bottom line: avoid a steady diet of short trips that can build up sludge in the CCV or install the cold weather version of the CCV.
My 2003 530 takes ~10 minutes driving (not idling) to reach normal coolant temp in -25C, -13F. I've no indication of engine problems using LL-01 synthetic.

Maximum power depends on the mass of air that can be drawn into the cylinders. At 5,000 ft air density is ~83% of sea level. So maximum power would be about 83% of sea level. This corresponds pretty well with the rule of thumb that max power is reduced 3% for every 1,000 of altitude on normally asperated engines, which ours are. So, WOT throttle acceleration will be a little less than at sea level.

Highway gas mileage should improve. Aerodynamic drag accounts for ~80% of the power requirement at highway speeds. Drag is proportional to density. So a 15% reduction in density should result in a noticeable improvement in MPG. City driving MPG is primarily a function of tire rolling resistance and energy wasted on stops. Neither of these vary with altitude; city MPG should be the same as sea level.

Note though, as discussed in other threads, MPG depends on so many factors that any change compared to your current driving patterns and routes may be difficult to detect.

Your car, like mine, has an open (not limited slip) differential. This is good while moving; loss of traction on one driven tire does not transfer all torque to the other tire and jerk the car sideways - especially if accelerating at the instant. Not so good if stopped and one tire is spinning on slippery stuff, in which case you get none or almost no torque on the other tire. This is called being stuck. :rofl:
But your chains will solve that problem, if anything can.

Regards tires.
There is so much difference in wet, snow and ice performace between tires, even within the same performace group, that your experience will depend entirely on what you are now running. Search for tests, surveys and reviews at Tire Rack
http://www.tirerack.com/index.jsp
for an estimate of how your tires will perform.

My personal opinion is that driver experience and skill with ice and snow are as important as the tires. Some drivers can't stay in their lane with snow tires in 2 inches of the white stuff. Others manage just fine with all season or even summer tires in 10 inches. Of course if you want maximum performance and safety in snowy conditions, dedicated snow tires are the way to go.

Best of luck in Colorado. Your biggest challenge may be keeping your eyes on the road - the scenery is stunning.
See also:
- How does the BMW E39 fuel trim behave at high elevation (1) (2) (3) & where is the BMW E39 altitude pressure sensor (APS) (1)
 
#24 · (Edited)
Altitude alone doesn't determine engine performance... nor does pressure or temperature.

The term is "DENSITY ALTITUDE" ... for all you aviation fans. It is possible to be at 6'000' and still perform as if you were at a lower altitude. Same holds true for the opposite.

Here is a calculator in case anyone is interested. Pretty straight forward. "Altimeter Setting" is your local atmospheric pressure (29.92 in. Hg is standard at sea level). You can get the required information from weather sources such as The Weather Channel.

http://wahiduddin.net/calc/calc_da.htm
 
#25 · (Edited)
The "LOAD" value is used by the DME (it shows up in INPA in the ANALOG 1 screen for the MS41.1). It is a calculated value that adjusts the air mass measure from the MAF (as a ratio of the table-stored MAXIUMUM air mass [at least I think it's table stored]. Here is the quote. It is taken from a Baum Tools document written with input from BMW. The ratio implies that if either the MAF or altitude sensor readings are wrong, then the DME will not adjust the fuel injectors properly - leading to running rich or running lean until it gets feedback from the O2 sensors and can adapt. However, if the error is big enough, the DME may not be able to adjust enough and the engine will mis-behave, IMHO. The ratio may not display correctly. It is [MAF / Max air mass] x [sea level pressure / current barometric pressure]

"When the engine operates at normal or higher load or at higher engine speeds, larger volumes of fuel and air are needed. In order to maintain a Lambda = 1 in these conditions, the ECU monitors the O2 sensor and calculated load (see figure 2) and compares the values against the optimal value for the fuel injection pulse width stored in the drive map. If this base fuel injection pulse width value does not yield a Lambda = 1 at the O2 sensor for the measured air mass, the computer increases or decreases the pulse width by a percentage (%) determined by the difference in Lambda from optimal. These percentages have been computed by the engineers at the factory from extensive dynamometer testing and are stored in a "weighted STFT value array1" in the drive maps.

Calculated Load = Current Air Mass X Atmospheric Pressure @ sea level
-------------------- ---------------------------------------
Maximum Air Mass Current Barometric Pressure
Figure. 2

When the STFT reaches the limit of its adjustment it will cause corresponding decrease or increase to the Long Term Fuel Trim. If the correction to the base value exceeds +25% or -25% for longer than 10 seconds a DTC is set for rich or lean stop for STFT."
 
#27 · (Edited)
Interesting read. I live in the high desert (above 4k feet) and my 97 328i has pretty good power at lower speeds (up to 50 mph).

I recently did a trip to the California bay area.
Below 2k feet elevation it felt like it added another 40 HP especially on the crazy freeways down there where 80+ mph speeds are common.
The car pulled hard all the way past the triple digits mph range. I was surprised. Once I got back home I was disappointed to lose so much performance due to the elevation increase.

Is there any work around to have the ECU/DME reprogrammed/mapped for better performance at higher elevations?
Or am I pretty much stuck with the dismal performance because of the thinner air?

From reading, unfortunately the later seems the case

 
#31 ·
OK
Yes I've considered going with a SC. Just have to see what deals are out there.

I'm working on a Porsche 968 for a customer, brakes, HU, dynamat, etc..
when I'm done with it he's getting a SC installed in southern Cal'

It's a 3.0 liter 4 banger with a 6 speed manual
236Hp rated
Not bad the way it is now, but with a SC that would be great :thumbup:
 
#32 ·
A question was asked about learning today ...
> E39 (1997 - 2003) > E39 ECU "Learning"??
Here's one for the BMW experts! I've been told that the 97/98 540i ECU has some kind of learning function in it to develop trim values for fuel. Over time, it will continue to refine these values. A reset can be done to clear them and start the process over. This was referred to as "adaptation" by someone on an older post on this forum, as well.

So, what's the deal? Does this ECU actually do that? And if it does, why? Any closed loop system with O2 sensors will continuously adjust the air-fuel ratio so I'm not sure i see the need for trim values.

That said, do all BMW models have this style of fuel control, or only certain models? I noticed on the later E39 models they went to one O2 (either before of after the Cat) instead on 2 (one before, one after the Cat). I wonder if that signifies a different control strategy.

Thanks for any intel!
 
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