No, air is only so dense. At higher altitude, the air is less dense. Lets say 10psi instead of the usual 14psi (1bar).
You can still only fit .5liters of air into a cylinder (in a 2.0l 4cylinder). And at 10psi instead of 14psi, you're at less pressure. Engine works harder (more compression and explosion cycles per minute) to generate the same amount of power at higher altitude than it would at lower altitude. It's generating less energy per compression though, you're just compensating with more revolutions.
I can't give you the math for this as I don't know it well enough, but you need more throttle not to get more air into the cylinder (that never changes, you always get the same amount of air into a cylinder regardless of throttle position) but to have more energy generation events in a time span.
Eh? Air is measured by mass, not volume, specifically because the volume of a gas can be compressed. The displacement of a cylinder is measured as if it were filled with an incompressible liquid, as are the combustion chamber and swept volume. The ensuing compression ratio dictates what the quantity of air sucked into the cylinder will become when compressed.
Less air in means less fuel in, which means the combined mixture is less dense under compression and less power is produced. This is all fact, unless I have had a severe brainfart somewhere. Now, though, onto speculation:
Less power produced in an idling engine is going to mean a lower idle speed, isn't it? Since you can't magically create more vacuum at the same RPM and throttle position, atmospheric pressure has no way to cram additional air into the engine. So you'd have to open the throttle more, I would think.
WOT is easier to think about, since it is, ideally, zero vacuum or close to it. If zero, MAP becomes atmospheric pressure, and the engine at altitude will always ingest less air and therefore make less power.
But to lower octane requirement, would you not still have to lower compression ratio? The lesser amount of air being brought into the cylinder would still correlate to a sea level engine that is simply running at a different speed/load, right? I can't see this any other way in my head, but I have absolutely no idea if I'm actually right.
What twists my brain around is thinking about what would have to happen with vacuum numbers.
Assuming an NA engine...let's say:
Engine A is in Denver...atmospheric pressure of somewhere around 12psi (~0.8atm).
Engine B is...let's go with 'somewhere else'
...standard sea level pressure of 14.7psi (1atm).
We know that an average engine idling at sea level will have about 18-20inHg of vacuum (20inHg = 9.8psi). That makes the absolute pressure inside the intake 4.9psi. So that's baseline.
The big question is what changes when you go up a mile with other engine? You've got thinner air, AND less atmospheric pressure to push it into the intake...so one would think you'd have to see a lower MAP value inside the intake so that a larger pressure differential is created...ugh, my brain.
GOOGLE, HELP ME.
http://www.motor.com/article.asp?article_ID=1354
Shit, what?
Wow...I briefly thought about exhaust backpressure in regards to lower atmospheric pressure, but dismissed it at trivial. Apparently, that's the key. It equals higher volumetric efficiency which means the engine can run on the less-dense air.
But wait...that still doesn't answer the octane issue. Fuck me.D:
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