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Old 05-11-2016, 09:55 AM   #1
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HPs x Wind drag

Ok, I have seeing several posts about what TV would be best to tow a hybrid but usually the conversation is around weight carrying capacity, etc.
But what about engine Horsepower?

Since I could not find one about HP's I did the calculation for how much HP's are needed to win over wind resistance at 63 miles per hour towing a trailer with an 11'x8' frontal area.
@ 63 miles an hour you need 151 HPs only to win over wind resistance.
If you add 10 miles per hour of contrary wind, you need 235 horses....
And that only to win over the wind resistance !!!

There are some leeway on the calculation for the drag coefficient used is the one of a plate and does not consider the aerodynamics of the vehicle in front of the trailer, which should reduce that a little bit...

Does this match your experience?
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Old 05-11-2016, 10:29 AM   #2
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It's just not practical to consider horsepower alone. Gear ratio plays as much, if not more, of a role here. Payload capacity is right up there with that. I've got a 630 HP Corvette...theoretically, I can pull 10,000 lbs in a straight line..won't happen though.

I see what you're doing and it's interesting, but the reason you won't see much discussion about towing and HP is that there are too many other factors that influence it. Torque vs HP vs wind drag vs weight, on the other hand, could be an interesting graph.
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Old 05-11-2016, 10:40 AM   #3
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Quote:
Originally Posted by raspivey View Post
It's just not practical to consider horsepower alone. Gear ratio plays as much, if not more, of a role here. Payload capacity is right up there with that. I've got a 630 HP Corvette...theoretically, I can pull 10,000 lbs in a straight line..won't happen though.

I see what you're doing and it's interesting, but the reason you won't see much discussion about towing and HP is that there are too many other factors that influence it. Torque vs HP vs wind drag vs weight, on the other hand, could be an interesting graph.
Yes, torque and gear ratio helps when accelerating and going up hill but to keep speed you need HPs...
My curiosity is this:
Per my calculations, if you are traveling at 65mph and have a headwind of 10mph you need 254HPs... That is pretty close to ~ 280Hps that some trucks have so I'd like to know if people have faced this kind of situation... ie.: how close the numbers are from reality.
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Old 05-11-2016, 10:50 AM   #4
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I've towed with about 270 HP into a 25 mph headwind at around 60 mph. It didn't like it but, if I had to guess, I'd say that I'd probably have run out of power at around 80 mph. I was at about 60% throttle at the time.
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Old 05-11-2016, 12:16 PM   #5
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the only thing I can add is that my 5.2 liter Dodge magnum engine peaks at 240Hp at 4400 RPM... I typically run at 3100 RPM about 62-65 MPH and the Hp curve goes down to 210 Hp at that RPM... I have 4.10 gears though that makes up for that low of horsepower. I also have 195K miles on the engine so it is probably not quite up to those numbers.

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Per my calculations, if you are traveling at 65mph and have a headwind of 10mph you need 254HPs... That is pretty close to ~ 280Hps that some trucks have
not sure how your calculations jive with my reality
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Old 05-11-2016, 12:34 PM   #6
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In your calculations you probably noticed that air drag goes up with the square of speed. So, if you know what it takes to tow at 30 mph, you know that power required doesn't just double at 60 mph, but is 4 times as great. Increase the speed to 90 mph instead of 60, and it goes up by 9 times.
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Old 05-11-2016, 12:51 PM   #7
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Originally Posted by rsdata View Post
the only thing I can add is that my 5.2 liter Dodge magnum engine peaks at 240Hp at 4400 RPM... I typically run at 3100 RPM about 62-65 MPH and the Hp curve goes down to 210 Hp at that RPM... I have 4.10 gears though that makes up for that low of horsepower. I also have 195K miles on the engine so it is probably not quite up to those numbers.



not sure how your calculations jive with my reality
yes, based on Raspivey and your feedback C (drag coefficient) in my formula needs to be reduced.
As I stated before I used the "flat sign" coefficient (1.075)...
What is your speed and what % of throttle you are when towing at 3100 rpm?

Thank you,
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Old 05-11-2016, 12:52 PM   #8
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Originally Posted by schrederman View Post
In your calculations you probably noticed that air drag goes up with the square of speed. So, if you know what it takes to tow at 30 mph, you know that power required doesn't just double at 60 mph, but is 4 times as great. Increase the speed to 90 mph instead of 60, and it goes up by 9 times.
Yes, the formula I'm using is this:
F = 0.5*C*p*A*V≤
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Old 05-11-2016, 12:56 PM   #9
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No trailer has an eleven foot high frontal area. Measure the actual box, not from the ground. Considering the truck is going to displace a lot of the air out of the way first, this would also change the calculations. The top of my F250 is almost 7 feet high and it has a large, tall front end as well. According to the torque curve for my truck I am running at about 100 HP at 65 mph and 1800 RPM's in overdrive. I can do this all day long on a non windy day. Add wind and I am at 160 HP at 60 mph and 2500 RPM's. Once heading into a 30-40 mph head wind in Texas (and slightly uphill the whole way) I was at 260 HP at 60 mph and 3700 RPM's. My truck and trailer weights together total 98% of my GCWR.


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Old 05-11-2016, 01:12 PM   #10
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I pull a Roo 183 with a 2014 ford explorer sport (rated 365 HP, haven't looked at the torque curve). I'm at around 98-99% of GCWR when fully loaded. I've never felt under powered, but my mpg really suffers when driving over 65mph (<8 mpg). I'm sure alot of that is due to wind resistance.
I've thought about getting a wind deflector for my roof, but nothing I've read has convinced me that they make much of a difference.
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