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Having lived in the Denver area with a Saab turbo...
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Posted by REM in Seattle (more from REM in Seattle) on Fri, 23 Apr 2004 20:05:30 Share Post by Email
In Reply to: Re: CFishgo is totally right...., CFishgo, Fri, 23 Apr 2004 10:43:44
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the short answer is use premium gas regardless of the altitude.

Let me see if I can answer the "why".

The earth's atmosphere or "air" is primarily nitrogen (N2) comprising 78 percent, with oxygen (O2) comprising about 21 percent, carbon dioxide (CO2) and other gases making up the remaining 1 percent. The percentage of oxygen in the atmosphere remains fairly constant up to around 70,000 feet. However, the pressure and density of the atmosphere changes from 14.7 PSI at sea level to 12.2 PSI at 5,000 feet and only 10.2 PSI at 10,000 feet.

In an internal combustion engine, the valves open for a set period of time during each piston stroke. The amount of air that is drawn into combustion chamber while the intake valve is open on the intake stroke is a function of the duration or time the intake valve is open and the amount of pressure pushing the air through the intake. The oxygen in this air charge is combined with fuel and burned to produce power. As long as there is sufficient intake pressure to fill the cylinder before the intake valve closes the engine will have a complete air charge.

Since the duration or time that the intake valve is open is a fixed amount (even on variable valve timing engines the duration is a fixed value at a given RPM), and the amount of vacuum created by the descending piston is a fixed value, then the main variable becomes the pressure of the intake air. That is, the amount of charge air that can be drawn into the cylinder is directly proportional to the intake air pressure.

A normally aspirated engine must rely on atmospheric pressure to push the intake air. Even if the engine has more than enough time and intake pressure at sea level to get a complete charge of air, there are two factors working against the engine at increasing altitude.

Factor One is the decreased oxygen content per charge volume (air being less dense at altitude) being drawn into the cylinder. Less dense air = less oxygen = less power.

Factor Two is the decreased air pressure pushing the air through the intake as altitude increases. Less intake pressure = less air charge before the intake valve closes = less oxygen = less power.

Obviously this is a doubly bad scenario for the normally aspirated engine.

A turbocharged (or supercharged) engine still has to deal with ‘factor one’, that there is decreased oxygen content (air being less dense at altitude) being drawn into the cylinder. But the turbocharged (or supercharged) engine creates its own intake pressure, so ‘factor two’ is less dramatic. Starting from a stoplight at higher altitudes, there will be more power lag time compared to sea level as the intake pressure takes more time to build up. But once the intake pressure has built up, the engine gets a complete air charge (i.e the cylinder is stuffed full before the intake valve closes) and therefore more oxygen that a normally aspirated engine. Obviously, even a turbocharged (or supercharged) engine will produce less maximum horsepower at altitude as the intake boost pressure is regulated to some maximum pressure. The newer T7 & T8 systems will do better as they are designed to compensate for the decreased air density to a greater degree by being allowed to have higher boost pressures at higher altitudes. Of course, higher boost pressures mean more intake air heating which will cause pre-detonaton problems at some point, so new system is perfect.

I found that at higher altitudes, I simply tried to keep my Saab turbo running in the 3,000-5,000 RPM region and had plenty of power. Not as much as I had at sea level, but still a lot more than a normally aspirated engine.

REM in Seattle
1998 Saab 900SET; 5-door; Automatic; 68,000 miles
1991 Alfa Romeo Spider; 5-speed; 95,000 miles
1984 Saab 900T; 3-door; 5-speed; 242,000 miles (RIP)




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