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Author Topic:   Towing vehicle horsepower
Dr T posted 07-02-2004 11:59 PM ET (US)   Profile for Dr T   Send Email to Dr T  
I recently picked up a copy of "MotorHome" magazine and came across a couple of interesting formulae (See "MotorHome", July 2004, page 90 in a column by Ken Freund). These formulae were proposed as rules of thumb for sizing the power requirements for motor coaches, so they should work well for those who want to tow their 25 ft. Whaler across the Rockies.

The first formula is a conversion from Torque (in lb-ft) to horsepower at a given engine speed in RPM:

Torque*RPM/5252 = Horsepower.

The second formula gives the horsepower demand to pull a grade of a given percent at a given speed in miles per hour for a GCVW (gross combined vehicle weight--this includes the weight of the trailer) in pounds:

Horsepower Demand = (GCVW/37500)*MPH*(% Grade).

So, suppose you have a big trailer and you want to pull Loveland Pass out west of Denver at 65 MPH. You want your engine to be turning about 3000 rpm at that time. Suppose your combined rig weighs 18,750 pounds.

The maximum grade of Loveland Pass (along about Silver Plume) is 8%. This means that your horsepower demand will be (IF I haven't made an error in calculation) 260 horsepower. So, you will need an engine in your rig that will develop 260 horsepower at 3000 RPM to pull the grade (at that engine speed). At that engine speed, your wonder wagon will be cranking out about 455 lb-ft of Torque. This makes a real good arguement for a diesel engine.

But suppose that is not the case, and you can grab a lower gear and wind up the rig to 4000 rpm to get you over the steep places. In that case, your Torque requirement will be 341 lb-ft at 4000 rpm--a bit more realistic for a gasoline engine.

Clearly the Horsepower Demand rule of thumb breaks down for very slight grades. However, if you are going to pull your Whaler over some serious mountains, these formulae may be useful to keep in mind.


gunny posted 07-03-2004 01:26 PM ET (US)     Profile for gunny    
I worry more about whether my transmission can take the load going over mountains rather than the horsepower of my engine (350 C.I.).
It is something to consider though.
Dr T posted 07-03-2004 05:48 PM ET (US)     Profile for Dr T  Send Email to Dr T     
Our experience on the ranch has been that we lose most transmissions to heat. If towing in the mountains, especially in the west where the elevations are high and the air is thin, you have to have adequate cooling.

I have seen more than one passenger car (with no trailer!) sitting next to the road near Eisenhower Tunnel. In all fairness, most of these were Chrysler Corp. minivans--not your typical tow vehicle.


Jerry Townsend posted 07-03-2004 07:04 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
tds - I will have to get a copy of that "MotorHome" and read that article. Aside from those questions - recall that the conversion from torque to horsepower is also dependent on engine rpm. So the author of that article had to assume some engine rpm. And I see no evidence of consideration of rolling friction or wind resistance.

In short - the calculation is a bit more complex than that given in your referenced magazine.

Now, according to manufacturer's data as of perhaps 4 years ago, there are two gasoline powered engines that would work, right out of the box, in your example of Loveland pass - the GMC 454 and the Dodge V-10. I have the 454 and my gross vehicle weight is a less than that used in your example - and I would not bet anything on my topping the pass at 65 mph - it might be close. But it has been a long time since I was over Loveland pass. And then too - the manufacturer's published horsepower/torque data are at sea level and Loveland pass is up in the air. ---- Jerry/Idaho

Dr T posted 07-03-2004 11:33 PM ET (US)     Profile for Dr T  Send Email to Dr T     
"Up in the air." VGP (very good pun, and much appreciated).

On you note concerning lack of exactness: I quite agreee. Engineering "Rules of Thumb" are just that. A first order approximation that can be used to decide the "Does it have a chance of working" criterion. (Where "First Order" refers to the first two term in the Fourier series.")

Loveland Pass is sort of an "Extreme Point" (in the sense of Convex Analysis {see Rockefeller's book on the topic}--plus I happpened to know the parameters off the top of my head).

But, Jerry, my gut instinct is agree with your conclusion: The Dodge V-10, the GMC 454, and (possibly) the Ford BIG Diesel, The Ford V-10, and the Cummings Diesel will probably work.

But you would have to want to make that long grade faster than I would to take the whole thing at 65 MPH.

Plus, what goes up must come down. But the issue of engine braking is another topic.

Thanks for your feedback, Jerry. I was hoping to get it it.


Chuck Tribolet posted 07-04-2004 09:29 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
A typical steep hill is 6%. That might make a better planning
point than 8%, and just plan on downshifting for the 8% and
tucking in with the 18 wheelers crawling up.


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