SpeedGeek

Of course it is - that accounts for all the power loss.

The power loss / altitude ratio is non-linear because air is a gas and thus compresses under pressure. So the difference between sea level and 2k feet is greater than the difference between 6k and 8k feet . IOW, that 3% per 1k feet is not constant as elevation rises, but it's a good approximation at the altitudes we're talking about.

At 11k feet, any n/a engine will be wheezing for air. Even turbos will struggle unless specifically designed for that altitude. The perception may be different in a heavy truck with a big, torquey V8, but take a high-revving sports car to 11k feet and it will clearly be gasping for breath. Just like many people at that same altitude. By 25k feet, there is no longer enough oxygen to keep the human body alive for any extended period.

OldGuy

Speed Geek I am only in Los Angeles a few times a year in the GT3 but I go there. I havent
had this one there yet but I took my 996 GT3 to the Long Beach grand prix this year and it was a monster
no doubt about the power loss at 8000 its noticable -but not so bad your getting passed by Big diesels not even chipped diesels like mine.
I used to play a lot of tennis 10 years ago and I played in tourneys in LA and Mammoth as well as here
at 2000 feet. I noticed a slight difference at 8000 that the ball traveled farther BUT at Sea level I noticed
a huge difference in how far the ball DIDNT travel. I was a monster in Hunnington Beach. I could hammer the ball there and it wouldnt go long -All my serves stayed in and I could put the ball anywhere.
I couldnt do that at 2000 feet or 8000 feet but as sea level the air was denser and the humidity made the ball heavier.

GrantG

I've driven a Porsche up to 14,300 ft and that is tough on car & driver :-)

OldGuy

Grant Pikes Peak?? were you going real fast??

GrantG

Nope, Mt. Evans. Pikes Peak is not paved, but Mt. Evans is. Just West of Denver, but no, I was driving pretty slowly. Didn't want to hit a big-horn sheep nor take a plunge

OldGuy

I love Denver Been to the old Mile high 3 times and have friends in Arvada
If I had to live in a big city, Denver would be it.
I accepted a job with Lockheed Space on the west side of Denver but declined the salary offer.
I loved the campus like feel.

GrantG

Well, Mammoth is pretty special in my memory too. Skied there for the first time ever on my 10th Birthday (1975)

allegretto

Well, I hate to seem disagreeable here, but i will disagree. You see, we are at the "bottom of the ocean (of air, that is)" when we are at sea level. Much as a diver experiences the largest pressure change in the first 50 ft of a dive and little thereafter. So changes in altitude really don't make much difference in air pressure until we get considerably higher than your example. As always, I don't expect you take my word for this, please refer to the following table;

http://www.sablesys.com/baro-altitude.html

The real reason engine output changes non-linearly is two-fold;

1) Air compresses non-linearly as a spring. This is also why tennis ***** and golf ***** respond very differently even though pressure changes relatively little at normal habitable altitudes. So air density changes more but air "pressure" changes less as the table shows. (but perhaps I misunderstand you and you actually meant this).

2) The ECU resonds non-linearly as it tries to compensate

GrantG

Here's a horsepower calculator that works with altitude:

http://www.slowgt.com/Calc2.htm

allegretto

That is interesting, thanks

Of course this assumes no compensation from the ECU

SpeedGeek

Your analog of a diver is incorrect. Water is a liquid and thus does not compress. So the pressure of water increases linearly with depth because its density remains constant. This is totally different from our atmosphere, whose density and pressure approach zero at about 100 miles up. The density of water at the ocean's surface is the same as it is at the bottom of the deepest ocean. An ocean of gas is not comparable to an ocean of liquid.

I'd be fascinated to learn how a diver experiences the largest pressure change in the first 50 ft of a dive and little thereafter? Unless the laws of physics have changed since I was in school, I'm quite certain pressure increases linearly as you dive down into a liquid.

The reason air does not "spring" in a linear fashion is because its temperature rises significantly as it compresses.

Tennis ***** are affected as much by air flowing over their surface as by pressure. And modern golf *****, having no open space inside, are hardly affected by air pressure at all. They are affected only by air density flowing over them.

The table you have referenced is not accurate in this discussion because it shows how things change as you move up and down in altitude at a given location, on a given day (like in a baloon). Notice how, in the table, the temperatures drop constantly as altitude rises. The table includes an adiabatic lapse rate of temperature vs altitude. An average day here in Johannesburg may be 80 deg. But when I drop 6000 feet down to Cape Town, I find an average day there is also 80 deg. Thus your table is confusing you as it takes the adiabatic lapse rate into consideration, which should not be considered when discussing ground-level elevation changes where temperatures remain mostly steady.

You claim "pressure changes relatively little at normal habitable altitudes." So why do my ears pop when riding an elevator just 10 or 15 stories high? Imagine the pressure in your ears going from seal level to 8000 feet if you did not equalize.

You also claim "changes in altitude really don't make much difference in air pressure until we get considerably higher than your example". But in fact that is exactly the wrong way round. Pressure changes more with altitiude low down than it does high up.

In summary, you are confused in two ways: First, by comparing an ocean of compressable gas to an ocean of uncompressable liquid. And second, by referencing a table that includes the adiabatic lapse rate.

allegretto

Oh dear me. There is so much to discuss here. I will try to answer later. However, consider this; what is the water pressure at the surface of the ocean?

And isn't the tennis ball affected by density, which is a function of pressure and temp?

Finally, one should not consider physiologic compensation (ears popping) as an absolute or accurate measure of pressure change, it is sensitive but not specific in a mathmatic sense. I was simply stating that your comment that there is a much greater difference in pressure between sea level and 2000 ft than between 6000-8000 ft. is not terribly correct since both are quite a bit below the surface and the air column above is still quite high.

More anon

Nordschleife

to say nothing of the way all the people from down under don't know **** 'cos all the blood rushes to their head - stands to reason dunnit?

must be why the kiwis and wallabies and boks and pumas are playing such an outstanding game of football.

people are confusing what happens with the water and what happens with the gas dissolved in the body

I'd been hoping the ABs would meet the frogs in the final, but it might be you guys now

R+C

Bill_C4S

Speedgeek - believe the point about pressure change for a diver was from the vantage of relative % change per unit foot

case a: 33fsw to surface: 2 bara to 1 bara, 50% change in pressure,
case b: 99fsw to 66fsw : 4 bara to 3 bara, 33% change in pressure.

a diver experiences the greatest change in relative pressure/unit foot in the first foot of descent...


and from the pedants vantage all fluids(gas, liquids) are compressible - c.f equations of state - its an engineering approximation that water can be treated as incompressible.

SpeedGeek

It's one atmosphere. And at depth, it may rise to hundreds of atmospheres. Yet its density remains constant at 1000kg per cubic meter regardless of the depth or pressure. This is totally different from a gas in which density varies significantly with depth.


Yes, you are correct. My original statement is correct in theory, but in reality, there's not a meaningful change over just a few thoudsand feet in an atmosphere tens of miles deep. So for practical purposes, pressure changes linearly between sea level and Mt. Evans. But mathematically, it's not linear.


BTW, Grant's calculator shows a 400 hp car losing 81 hp between sea level and 6k feet. I want a discount on my next n/a car purchase.