Originally Posted by M&M
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.
I'm still not sure I understand what you mean by "quality". On each stroke the cylinders have less O2 at higher altitudes than at sea-level. But it also has less "other stuff", mainly nitrogen. If the air is 20% less pressure than at sea-level, then the O2 is 20% less than sea-level too. Also, there is 20% less O2 in the cylinders by mass (which is the important thing) and therefore 20% less fuel can be used (to keep the same A/F ratio).
I don't really follow why you need to add even more boost over that of overcoming the pressure loss due to higher altitude.