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Power Loss at High Altitudes


Any engine will produce a given amount of power based upon how much fuel and air it consumes. High power loads and RPMs consume more fuel. Because fuel and air are in a somewhat constant ratio, the engine will require more air at higher power outputs. Unfortunately, as we drive our motorhomes up into higher elevations, the air gets thinner and the engine loses it's ability to produce it's maximum rated power. This results in a corresponding drop in generator output wattage.

Different engines will react in varying amounts. For instance, the output wattage of the Onan Marquis Gold series of gasoline powered generators will lose 3.5% for every 1,000' in altitude gain over 3,000'. The Onan QuietPower Diesel 10,000 watt will derate 3.5% for every 1,000' over 500' and it further derates by 1% for every 10 F increase in temperature over 85 F. A good rule of thumb for most gensets is that you will lose 10% of the rated output every time you gain 3,000' in altitude. This means that a 7,500 watt generator running at 9,000' of altitude will only be capable of producing a bit over 5,000 watts at that altitude.

Diesel engines are pretty well able to handle the fuel injection requirements at higher altitudes but carbureted gasoline powered engines do not have that luxury. The Onan Marquis Gold series of generators uses carburetors. These gensets do have an altitude compensation adjustment on the carburetor. By dialing in a higher altitude you will lean out the mixture to prevent spark plug fouling and excessive smoke. It is very important that you also dial it back when descending to lower altitudes or else the engine will run lean and you will burn the engine's valves.


Submitted by Mark Quasius - 3/15/06

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