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Originally Posted by larsdennert
Horsepower is a bad indicator for towing capacity. I have 300hp Lexus IS and wouldn't tow anything with it. A Mercedes Unimog has an 80hp diesel and hauls all sorts of farm and military hardware.
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Lars is quite right about this, but didn't expand on why this is true. If you don't mind, Lars, I'll add a bit that I learned from RockyMtnRay, our resident towing expert, now departed.
Horsepower varies with engine RPM, and of course, the advertising always tells you the maximum horsepower. For gasoline engines, the peak horsepower occurs at a very high RPM. For example, the Tribeca is advertised as having 250 hp at 6600 RPM. I would bet that 6600 RPM is very near "red line" (max RPM) for the Tribeca engine. Unfortunately, you can't run an engine near red line for very long - it will self-destruct. You can zoom up to that RPM for a quick burst of acceleration into the passing lane, but for long-term jobs like trailer hauling, you have to run the engine at lower RPM.
If you are an automotive engineer, you will look for a graph of horsepower vs RPM. And what you will find is that, especially for "performance" cars or "sporty" cars, the graph has a rather narrow peak, meaning that the peak horsepower is high, but drops off rapidly at lower RPM. So in the case of the Tribeca, your question might be, how much horsepower does the engine have at (say) 4000 RPM? And that information is hard to find.
However, Ray used to say, "Horsepower doesn't tow trailers. Torque is what tows trailers. And torque is what moves you ahead from a stoplight, especially since the engine RPM is low at that moment."
An automotive engineer will also look for a graph of torque vs RPM. Peak torque generally occurs at a lower RPM than peak horsepower. For various technical reasons, diesel engines tend to be much torqu-ier than gas engines (they have a higher torque for a given horsepower spec), and the peak torque of a diesel engine tends to occur at a much lower RPM. More importantly, diesel engines tend to have a torque curve that is much flatter, so you can operate the engine over a wide range of RPM and still have good torque output.
That's why the Unimog (or a diesel pickup, or the diesel in an 18-wheeler) can haul immense loads, even though they don't have very impressive horsepower numbers.
I don't know if this helps anyone, but there it is as I understand it. I'd welcome more informed information.
Bill