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Old 02-17-2016, 06:36 PM   #1
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Altitude towing capacity discount factor

What is tow rating discount factor per 1000' above sea level?

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Old 02-17-2016, 06:50 PM   #2
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I believe HP for naturally aspirated engines drops 3% for every 1000 ft above the sea level. Turbo engines, have a lesser drop.

This is from a Porsche Cayenne Manual regarding towing in the mountains:

"When the vehicle is driven in the mountains, the engine output decreases as altitude increases. The maximum weights stated are the values at sea level. The total permissable weight of the car-trailer combination must therefore be reduced by 10% for each increment of 3,280 ft. (1,000 meters) altitude. A fractional increment counts as the full 3,280 ft. (1,000 meters). Please take this into consideration when planning your route"

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Old 02-17-2016, 08:37 PM   #3
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Other ways to manage higher altitude (if it becomes an issue) that do not include taking a different route are lowering speed to reduce wind drag and selecting a lower gear, selecting a lower gear with higher rpm to get into the best power band of the engine, and towing a trailer with a lower, rounded shape for less wind drag.

A more important consideration than power uphill is braking downhill, best controlled with a combination of engine braking and truck and trailer brakes. Speed matters.

Tow ratings are but a starting point for a good towing combination, with little to no consideration for good equipment, good technique and experience.
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Old 02-17-2016, 09:36 PM   #4
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From one who lived at 7,000 feet and has traveled over passes at 11,000 feet: the 3% per 1,000 feet is a pretty good rule of thumb for fuel injected motors. Carburated engines will do worse than fuel injected, turbocharged or supercharged engines will do significantly better.

You may not feel much below 5,000 feet, but above that it will be increasingly noticeable. I hot-rodded my VW bus motor to get me around the Rockies, and supercharged my Yukon for towing the trailers in the mountains with performance that meets my personal standards for performance.
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Old 02-17-2016, 11:18 PM   #5
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I don't know how many of the pickup or automobile manufacturers derate their towing recommendations for altitude, as the Porsche manual quoted above implies. If they do so, it implies that their tow ratings are limited by hp, not by chassis or other factors. I wonder whether they are managing towing safety, or customer expectations on cruising speed. Ford's towing guide suggests a 2% derate on GVW or GCW per 1000 feet in order to maintain similar performance, for gasoline engines (not considering the Ecoboost).

The hp developed will drop at higher elevations, whether for naturally aspirated engines (a lot) or turbocharged engines (less, but there is a turbo limit) but that just means you go up the hill somewhat slower. The transmission and cooling systems will thank you, it is easier on them since they are dealing with less power.

As a data point, I worked in the supply of large mining equipment, including haul trucks (mechanical drive, turbo diesels, up to 400 tons). It depended on the specific engine in each model, but we derated hp per the manufacturer, at 8,000 to 10,000 feet, unless we spec'd a high altitude arrangement (larger turbochargers). Those high altitude arrangements took us to the 16,000 foot range. All this just meant less hp, and so potentially slower up the hill. There was no load capacity derate at any of these altitudes, for that equipment.

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