I posted that altitude information in the other thread. So, you want other people - or MAYBE other examples of how "true" this statement is?
Why do you think that passenger jet airplanes have to be pressurized in order to run at 35-50,000 feet? Jet airplanes run up there because there is less air molecules (more air molecules=more air pressure) that they have to push out of the way up at that rarefied altitude, so they use less fuel - and they can go faster. The cabin has to be pressurized to keep people from passing out at that altitude (pilots also have to use oxygen bottles if they fly unpressurized above about 10,000 ft).
Why do you think that water boils at a lower temperature the higher you go above sea level? Why do baking instructions vary by the altitude above sea level? Why do mountaineers have trouble climbing MT Everest (or any of the other peaks above about 14,000 ft for that matter)?
Why do you think that carb jetting is dependent on altitude above sea level? Why does a fuel injected motor REQUIRE the use of a Manifold Air Pressure (MAP) or Mass Air Flow (MAF) sensor, depending on which type of injection they use (speed/density vs. Mass Airflow)?
Different air pressures exist at different altitudes. The closer you are to sea level, the more air pressure there is - about 14.7 psi at sea level. The higher your get in the atmosphere, the less air pressure there is. For example, space craft have to be pressurized - that is because the further they get away from a planet, the less air (or more vacuum) they have. There isn't a sudden transition from atmosphere into the vacuum of space - the air pressure gradually gets less and less, until it is near vacuum.
This principal even extends into the submarine world - Deep Sea Divers have to use saturation diving techniques - basically pumping up the air pressure (well, a mixed gas that replaces the nitrogen gas in normal air) inside the containment vessel, in order for people to work on the sea floor - because of the crushing effect of the water pressure around them, they need every advantage to keep the pressure vessel on the sea floor from collapsing. The higher air pressure inside the pressure vessel allows them to use thinner metals on the pressure vessel - as well as the fact that that higher air pressure lets the saturation divers go to greater depths while wearing a deep sea diving suit that isn't so heavy that they can still walk in it.
They also have to put skin divers in a decompression chamber anytime divers go deeper than about 200-300 ft, to slowly bring the high air pressure down inside the pressure vessel to prevent the bends. It is basically the same principal - the deeper you go, the higher the pressure is.
So now - hopefully - you know that different air pressures exist at different altitudes. And yes, 6000 feet roughly translates into about 80 percent of cranking pressure.
Don't assume that my multiple examples in this thread are meant as a "stomp" on you on this board. They are not. I am giving multiple examples of things you already know, to convince yourself that this principle is actually true, and that you have know it all along.
