Originally Posted by M&M
Technically speaking, that is not true. When density changes, the volume remains the same.
But you are also compressing air that is less dense. As air gets thinner the molecules are further apart. So there is less oxygen per square inch at 5000ft than there is at oxygen rich sea-level. Ever wonder why climbers use oxygen masks at the top of Mount Everest? Each breath you take, you ingest the same amount of air you ingest at sea-level. But the air is pretty much useless as the 02 molecules are so sparse it doesn't feed you.
Ok on that point I agree with you. When density changes, pressure varies by:
P2 = P1 (D2/D1)
So a 5.5psi charger at 5000ft will produce = 5.5psi(850mbar/1000mbar)
= 4.675psi or 15% less boost.
So you would need a 15% smaller pulley to make the same boost.
But even if you get the same boost, the quality of air is not the same. So actually you will need an EVEN smaller pulley to compensate quantity for quality.
But hey, that's just my opinion. It's a great discussion with lots of grey areas si I am open for correction.
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. That is, the partial pressure of O2 is about 1/5 that of the ambient air pressure. Period.
Without going too much into physics, it's the amount of the O2 that's in the cylinder that decides how much fuel the engine can take since we need to control mixture (A/F ratio). More fuel, more power. Since the volume inside the engine's cylinders is constant (at TDC) the amount of oxygen in the cylinder is then proportional to the density. The density depends on the pressure (altitude) and temperature. Higher temperature or higher altitude means less oxygen in a normally aspirated engine. There's less pressure up there but also less temperature. Pressure of the air is not determining quantity; it is density that is the key factor. This, of course, excludes talk of detrimental effects of too high a pressure or too hot the air.
For example, the density of air (standard atmosphere) at sea level is 1.225 kg/m^3. The density at 5000 feet is 1.056 kg/m^3, i.e. you've lost about 14% of the O2 (normally aspirated). Pressure-wise, it's 14.696 psi (1.013 bar) at sea level and 12.228 (0.843 bar) at 5000 feet, or about 17% loss of pressure, but not O2. And that is a loss of 0.17 bar.
With forced induction (super charger or turbo charger) you are forcing more air into the cylinders, increasing the pressure, but hopefully not the temperature, so the density of air is higher and hence more oxygen gets into the cylinders so that you can burn more fuel per stroke. To make up for loss of density from 0 to 5000 feet you only need to produce 0.17 bar of boost, assuming temperature is constant.
You'll notice a bit more power at sea level than at 5000 feet in an FI engine, but you'll need a dyno or accelerometer rather than your butt dyno to realize it.
I hope I got the point across. Sorry for being long-winded.