Originally Posted by ersin
Quality? Erm, no. If what you mean is O2 fraction of the air, it is the same at sea level or 40,000 feet, viz. about 20%. As you go higher than this the composition of the atmosphere changes but down here, whether sea level or in the mountains, the proportion of O2 in the air is the same.
No I did not refer to the proportion. That will, of course, always be the same.
Maybe you misunderstood me.
In one cubic foot foot of air at sea-level. How much oxygen is there? Not what proportion of oxygen, what absolute amount of oxygen is there?
So now. Go to 5000ft. I know the proportion is the same. What absolute amount of oxygen is there in one cubic foot of air? Is it the same as at sea-level?
Ever wonder why an NA doesn't require as much octane at altitude as it does at sea-level? I think someone has done some dyno testing at altitude and proven it.
On each stroke the air that is ingested into the cylinder has less oxygen than the equivalent sea-level car.
Hence my point that yes you and Bob are right you need a smaller pulley to make the same boost. We are all in agreement with that. But that "quality" of the extra air is poorer, so you need to add even more boost to get the same oxygen content in.
Make sense? Maybe I should have googled it instead of shooting from the hip with my personal experience of forced induction cars at altitude. I must say it's a great discussion as no-one really knows the real truth up here as most research is done at sea-level. So if we can learn, it will be for the benefit of all us mile-highers on this forum.