Is this true, the part in bold. I saw it on another forum I came across. I was searching about engine power at elevation.

Your engine will lose rougly 3% of your total engine output per every 1000' of elevation. This is due to loss of air density. For example, you will lose 21% of the HP you had at sea level at 7000'. Turbo engines won't lose as much because the turbo can be overspeeded to equal the sea level boost. For a normally asperated engine you can gain some performance back by rasing cylinder pressure. You can advance the timing (not on most late FI engines). Or, you can raise the compression ratio ( 3-4% HP gain per point of compression (again not easy). Your vehicle needs less octane at altitude, because the low cylinder pressure the low density air causes. I have been in some high altitude towns that had 89 as the highest octane available.

Temperature has the same effect though not to the same degree. Hot damp air is the least dense and will make less HP than cold dry air.

 

You are correct on the octane question, I live in CO and the maximum octane at 5,000 feet is 91, and it is due to the lower air pressure (less dense air is less prone to premature detonation as it needs to be compressed more to detonate). But I wanted to correct one thing:

The turbo is not "ovserspeeding" in the truest sense of the word. Yes, it is true that less air density per cubic foot means less resistance on the vanes. People use boost controllers at high altitude to keep the boost pressure at the same PSI as sea level. By using a boost controller to keep the boost up, you're really just closing the wastegate more, and doing it sooner than normal. Because it relies on the wastegate to control the boost, even turbos have a "critical altitude" beyond which the wastegate is completely closed, and no more pressure can be made by the spinning vanes. At that point, the turbo starts to lose boost pressure and the engine power drops the higher it goes. Go to Pikes Peak if you want to see what I mean. 14,000 feet will ensure you feel the power loss, most engines lose 45% of their power at that altitude. However, the critical altitude does vary with the size of the turbo.

Without a boost controller, I've seen turbo engines lose the same kind of power as supercharged or N/A engines (A stock WRX, for example, is still slower than my Stage 2 SS S/C at 5,000 feet, but a boost controller is one way of fixing that problem for the WRX driver).

Hope that helps

 

Thansk for the answer

 

Colorado is the only place I've ever seen 85 octane. It was way up in the mountains.

 

We have 85 octane in Denver too, it's all over the state, our standard pumps are:

Regular: 85 Octane
Plus: 87 Octane
Premium: 91 Octane

This doesn't change if you're next to the base of the Rocky Mountains like I am, or out on the plains, the average elevation of CO is 6,800 feet though, so that's part of why we can use 85 safely everywhere.