Towing MPG

Atlee wrote:
If you don't own a 3.5L Ecoboost, how do you know that engine is worked to death, and it's insanity to try to do so?

I also have a F150 HDPP and 3.5 Ecoboost. This past summer, we traveled from Virginia to New Mexico, Colorado, Utah, Idaho, Wyoming, Montana, and South Dakota before coming back home.

We were in the Rockies a lot. At no time was my 3.5L Eco every over worked going up the steep grades. And when I had to slow way down until a slower vehicle got out of my way while going up a steep grade, I had zero problem getting from the low speed back up to the speed limit. And no, the engine was not screaming while doing so. Usually, 3500 rpm was all I needed to get back to speed.

The only problem was going down steep grades. I really had to manually down shift sometimes to 2nd gear to keep my speed down and not have to use too much braking.

Now, I would not begin to tow 10k GVWR trailers with my 3.5L F150.
My trailer is 6500# GVWR and it worked fantastic behind my F150 3.5L Eco w/ HDPP.

I know the engine is worked to death because I have worked on and built engines for over 20 years. Why is your engine 3.5 liters? Answer: because it has fewer cylinders, and less bore and stroke than a larger engine. What does that indicate to me? That the parts that are taking the load inside that engine (pistons, rings, crank/rods, bearings) are fewer and smaller. While there have been marginal gains in efficiency and boost certainly produces a significant jump, torque still comes from burning fuel and applying the force of combustion to the top of the piston and transferring it into the remainder of the rotating assembly. Boost, in raising volumetric efficiency, also raises cylinder pressures. So not only are you applying the force over a smaller area bore, ring surface area), you are also doing so with more force (call it higher cylinder pressure if you wish, pressure = force/area). I don't need to own one to know those to be facts based on the published data--similar horsepower to a larger engine in a 213 cubic inch displacement. No amount of technology is going to change that. While year after year, engineers try, there is still no replacement for displacement.

I guess it depends on what you consider "overworked," because I live out here in the west and I know how hard a truck has to work to get up those mountains. Even with only my truck camper on board (3,000 pounds), I have watched the boost gauge on my Ram head toward the top of the range on mountains in Utah, Colorado, New Mexico, and Arizona to name a few. Could it be that you just don't realize how hard your truck was working to push the weight up those grades? Even the semi truck tractors that are used out here in the west have larger radiators and sometimes higher rated engines than a company would use out in the flats of the midwest.

It is only by virtue of modern technology that I'm even willing to allow my truck to exert that kind of effort. Modern oils are protecting the internals and a computer is keeping the engine from getting hot enough to melt itself. In your case, a sophisticated knock sensor system is preventing a boosted, 10:1 compression engine from knocking itself apart, especially if you have 87 in the tank. A similarly complex fuel injection system is likely enrichening the mixture to ward off that possibility, and the knock sensor pulling timing if needed. Both are needed to protect the equipment, but neither are good for fuel economy. A larger displacement engine with a lower compression ratio could make the same torque without being on the ragged edge of detonation or having to enrichen the fuel mixture to starve it off. I've seen modern vehicles that run so much boost under load that they have to pull almost all of the ignition advance out of the engine under certain conditions. The return is better fuel economy on flat ground with no load, but the heavy load/high boost parameters are, as you've probably experienced, not going to produce better mpg than a small V8.

If the engine wasn't above 3,500 RPM, you weren't even really using it. The horsepower peak is at 5,000, so you weren't getting max acceleration or anything close there. If you were happy with the acceleration for the conditions, that's fine, but there's really nothing wrong with "screaming" a modern gas engine.

The reason you're not experiencing the engine braking you're probably used to is that while boost can raise the cylinder pressures under load, it can't do that when you're going down the mountain. Again, no replacement for displacement here as well. The amount of "braking" horsepower your engine can make is lesser than a naturally aspirated engine of similar output would make. I don't know that I'd really be too concerned about it (just might have to slow down more and use the brakes more than otherwise), but it's a factor.

Of course there's no free lunch. I'm sure the Ecoboost has the potential to get great fuel economy when cruising down the highway at 16+:1 AFR with nothing in the truck but the driver. The tradeoff is the things you observed. Like all things in life, it's a compromise. I'd rather have an economical car to drive when I don't need my truck, so I have no use for a few more empty/unloaded MPG in my truck. If I did, I might consider something like the Ecoboost.

FWIW I don't consider 6,500 pounds to be too much weight for a 1/2 ton truck with the right gear ratio and hitch setup. I do think the people pulling 10k or more with them are absolutely nuts. Personally, I prefer not to own pickups with semi floating rear axles; I think they are dangerous for any kind of serious use and I would feel very differently about towing with 1/2 ton trucks were they equipped with a full floating axle. Even just the same axle assemblies they are using, converted to full floating design, would be far safer and make much more sense to me (Toyota Landcruisers had a smaller, 8" or so ring gear full floating axle in some models). The rear axle is really my biggest concern and the primary reason why I don't own any 1/2 ton trucks anymore.