Towing with an SUV

MargaretB wrote:
We don't have an Expedition, but I noticed that some people on the TT board seem to be towing with over-sized SUVs, and we have a friend who tows with a diesel Mercedes. That's why I was wondering if they are appropriate, and what role the torque plays in the decision.

I think engine torque plays a bigger role than most suppose. Low end torque means that an engine delivers it's highest torque at low rpm.

Take the old tugboat example. Most older tugboats had very large low HP diesels that had a very long stroke and turned very large in diameter but low pitch propellers. The high torque of these engines allowed the operator to shift from forward to reverse without stalling the engine. The massive props had considerable momentum that had to be over come when shifting gears.

The same holds for your tow vehicle. When you initially start towing from a standing start, the torque is what gets things rolling. Take two engines both 300 hp but one has torque of 375 lb/ft@ 4000 rpm and the other 375 lb/ft@ 1800 rpm. The engine with the torque peak at 1800 rpm will have an easier time starting from a dead stop and will do so at lower rpm.

Consider also how torque helps manage your towing at highway speeds. Take the same two engines cruising at 65mph on the interstate. Both Tow Vehicles are shifted into 5th gear and torque converters locked on the flat and level. When these vehicles climb a hill (say an overpass) the vehicle with the torque peak at lower rpm will be able to hold 5th gear quite a bit longer than the the one whose torque peaks at 4000rpm. meanwhile on the flat, the TV with the low end torque could be geared higher and thus cruise at lower rpm resulting in better fuel economy.

That's at least partly why diesel tow vehicles are so popular. Most diesels produce more torque at lower rpm than their equivalent sized gas brothers. Even among gas engines there can be quite a bit of variance in torque specs. I once owned a VW Rabbit Diesel all 49 hp of it. You could time it in the quarter mile with a calendar. It was slow. But man it had low end torque. It would pull my 18 foot boat, engine, and trailer loaded with gear up a boat ramp no sweat. Now I wouldn't drive it down the road with the VW but it served as a tug motor for launching and retrieving in the boat yard.

The reason most tow with bigger engines is not just more HP but the bigger the engine, the more torque.

Choose the higher torque at lower rpm for towing purposes.

FWIW check out a VW Touareg for example. Only 225hp but look at the torque! 400+ lb/ft at 1750rpm and a 7000lb towing capacity. That's better than most 1/2 ton gas pick-ups.

The drawback to this car as a TV is the fairly short wheelbase but if you want fuel economy maybe you can tolerate a somewhat rougher ride.

This is the second reason most people tow with over-sized SUVs and trucks. The longer the wheelbase the smoother the ride. Trucks and 3rd seat SUVs are generally up in the 130 inch plus wheelbase range where the Touareg is only 112 or so. The shorter the wheelbase the more the trailer will waggle the TV especially when getting passed on the interstate by a big semi.

Fuel economy is not great with the big TVs though 10-14 mpg is common. My I6 Trailblazer gets around 11mpg if I keep the speed down under 65. A gas V-8 1/2ton Pick-up might do 12 or 13 loaded the same and it might ride better than the TB due to the longer wheelbase.