[Allan Gibbs]

For example, Los Angeles has an altitude of 325 feet above sea level. Phoenix, has an altitude of 1125*.

Would the 800 feet difference result in a noticable difference in acceleration? Or, how much of a difference would be noticeable?

* http://www.calle.com/world/

 

[offroadr35]

i believe the number is usually somewhere around -4% horsepower for every 1000ft of elevation. The loss of throttle response is even worse.

 

[khamai]

While we didn't take any performance measurements at the WCLM the elevation of Lake Tahoe is just shy of 6300 ft. I didn't detect any driveability issues, no hesitation in acceleration, and seemingly no lack of power.

Having said that I'm sure at sea level we'd experience better acceleration, but the car was quite quick even up in thin air.

Kiyoshi

 

[Randy Chase]

Yeah, no comparison, but the car felt good at Lake Tahoe. The driver did not. I have a real problem with altitude and forgot about it until the next morning at Lake Tahoe. Sucks for me because I used to snowski a lot.

Some people advocate resetting the ECU if you have a large change in elevation.

 

[420HP]

More info then is needed, but .....

the below table is the NHRA approved altitude correction factors. Simply multiply recorded time/MPH by adjustments in list. Anything below 1200 feet is not modified.

For your particular case, there would be no adjustment so in NHRA terms that means "no difference". At 1200 ft, the Elise is ~2/10ths slower in the whole 1/4 mile (based on low to mid 13 second times). Not noticeable until you read the timeslip....

This is acceleration only of course. Can't say much about other aspects........

ALT ET MPH
1200 .9874 1.0129
1300 .9861 1.0143
1400 .9848 1.0157
1500 .9835 1.0171
1600 .9822 1.0185
1700 .9809 1.0199
1800 .9796 1.0213
1900 .9783 1.0227
2000 .9770 1.0241
2100 .9757 1.0255
2200 .9744 1.0269
2300 .9731 1.0283
2400 .9718 1.0297
2500 .9705 1.0311
2600 .9692 1.0325
2700 .9679 1.0339
2800 .9666 1.0353
2900 .9653 1.0367
3000 .9640 1.0381
3100 .9627 1.0395
3200 .9614 1.0409
3300 .9601 1.0423
3400 .9588 1.0437
3500 .9575 1.0451
3600 .9562 1.0465
3700 .9549 1.0479
3800 .9536 1.0493
3900 .9523 1.0507
4000 .9510 1.0521
4100 .9497 1.0535
4200 .9484 1.0549
4300 .9471 1.0563
4400 .9458 1.0577
4500 .9445 1.0591
4600 .9432 1.0605
4700 .9419 1.0619
4800 .9406 1.0633
4900 .9393 1.0647
5000 .9380 1.0661
5100 .9367 1.0675
5200 .9354 1.0689
5300 .9341 1.0703
5400 .9328 1.0717
5500 .9315 1.0731

 

[Rich]

Most online calculators put my altitude of 6500 feet at ~23-25% HP loss. I'll be driving a ~145 HP Elise...

I guess it's better than my current 105-110 HP Miata (140s at sea level).


The difference is noticeable. At the Evolution autocross school I did, one thing I had to work on was being easy with the throttle, since I was near sea level. At home I can mash it any time, because there's so little power.

 

[khamai]

23-25% loss is about what I was thinking. That may have masked the non-existance of inside wheel spin on the tight 180 on the autoX. That means the jury is still out on the need for LSD. At sea level the addition of 23-25% more power could be enough to get the inside wheel to spin in similar circumstances.

 

[Need4Speed]

One thought - don't modern ECUs take into account data from the O2 sensor and make mix adjustments accordingly?

 

[khamai]

One thought - don't modern ECUs take into account data from the O2 sensor and make mix adjustments accordingly?

Yes. That's why my tow vehicle, while down on power didn't have any driveability issues. Whereas my 1964 Elan with Webers didn't like the thin air, let alone the lack of power. I considered re-jetting to compensate for lack of O2, but I didn't bother.

 

[ConeFusion]

One thought - don't modern ECUs take into account data from the O2 sensor and make mix adjustments accordingly?

It will still loose power. The ECU will adjust to keep the air/fuel ratio at the correct level. But since you get less air, the total amount of mixture (and fuel burnt) is smaller, so you produce less power.

The case is somewhat different for turbo cars. Since the ECU normally limits boost pressure to a certain absolute level, you can still pump the same amount of air into the combustion chamber at higher altitude. I believe you typicially still loose some amount of power, but not nearly as much as a normally aspirated engine.

 

[993]

The case is somewhat different for turbo cars. Since the ECU normally limits boost pressure to a certain absolute level, you can still pump the same amount of air into the combustion chamber at higher altitude. I believe you typicially still loose some amount of power, but not nearly as much as a normally aspirated engine.

This is correct...at high altitude in Colorado, my 944 Turbo would blast up the mountain passes with ease while n/a cars struggled mightily.

 

[Surferjer]

That's a funny chart. I moved to Philly from Denver. In Denver the bottom of your chart is the top! We would start at 5280, and drive into the high country which usually involved a mountain pass over 10,000 feet! I suppose it's all relative-all cars would lose power.

Now I AM wondering how the Elise would pull up I-70 just west of Denver, where 4 bangers usually die in the slow lane, crawling to 30-35 MPH often. The rule of thumb in my mind was ALWAYS take a V-8 up there. Of course we have a poster on the board from Denver-Derek? Any thoughts?? The lack of weight would probably help significantly, of course...

 

[Ruediger]

Somebody knows which ECUs used in combination with a booster can adjust the boost level to the altitude in order to avoid such power losses up there in the mountains?

So far I only know that some - not all- aftermarket ECUs are able to avoid mixtures which are too rich up there but this does not compensate the power loss caused by the thin air.

 

[MattG]

Now I AM wondering how the Elise would pull up I-70 just west of Denver, where 4 bangers usually die in the slow lane, crawling to 30-35 MPH often. The rule of thumb in my mind was ALWAYS take a V-8 up there.

A V8 will suffer just as much as 4-cyl, in terms of percentage power loss. There's really no substitute for a turbo. My wife and I took our Jetta 1.8T on a road trip through Colorado, traveling over countless mountain passes higher than 10k feet. The car showed no power loss; its small four cylinder was totally adequate, even for high speed uphill passing. The only difference I noticed was standing starts were a little more difficult, as the car had less torque and no boost.

On another trip to CO, we rented a Mazda Protoege5. Fantastic car, but I quickly nicknamed it 'the gutless wonder', as it suffered extraordinarily over altitude as we traveled to Rocky Mountain National Park. Though to be fair, I never drove one at sea level, so maybe they all suck (this was the previous generation, 1.8l I think).

 

[Randy Chase]

In many trips to Colorado, including racing at Denver, I could not notice any power loss in my naturally aspirated MR2.

 

[MattG]

In many trips to Colorado, including racing at Denver, I could not notice any power loss in my naturally aspirated MR2.

MKII or Spyder? I've had my MKII NA up over 6k feet before and noticed a difference.

 

[Randy Chase]

MKII or Spyder? I've had my MKII NA up over 6k feet before and noticed a difference.

Actually both. With the MKII I expected a performance hit and was surprised to not notice one. As you know, we did ProSolo races and it seemed to pull just as well. I never raced the Spyder in Colorado, but have driven it, while towing a trailer) across Colorado and it seemed to feel similar to at sea level.

 

[993]

I remember driving an old Celica up thru the Eisenhower tunnel...power was totally gone. (This one used carbs, it was a 75 model.) My 87 944 Turbo, incomparison, just flew up the mountain and over the pass.

 

[Rich]

Now I AM wondering how the Elise would pull up I-70 just west of Denver, where 4 bangers usually die in the slow lane, crawling to 30-35 MPH often. The rule of thumb in my mind was ALWAYS take a V-8 up there. Of course we have a poster on the board from Denver-Derek? Any thoughts?? The lack of weight would probably help significantly, of course...

I wouldn't worry about it being able to keep up with traffic over I-70 or any other mountain passes. While I have to work my Miata up there, 80 mph isn't a problem.
The Elise is not quite double the power to weight ratio of my car, but it's close.

 

[Randy Chase]

I have driven the MR2 Spyder on I-70 without resetting a sea level ECU. This is the 1ZZ engine with a lot less horsepower and a couple hundred pounds heavier. I was pulling a trailer with racing tires and a bunch of tools, jack, etc.

I was able to do 90mph most of the way.