Towing and Altitude Adjustment

[Pete Dumbleton]

Here's an interesting tidbit from another RV and a discussion about
towing at altitude and the Ford Owner's Manual and Trailer Towing
Guides stating that 'GVWR/GCWR/towing capacity should be reduced 2%
for every 1,000' of altitude' that I just posted to Yahoo Scampers.

quote
We read that in a friends manual and were amused. A turbo-charged
diesel is effectively altitude corrected to most elevations that have
paved roads.

We called the guy that wrote most of that stuff and he told us something
interesting. It turned out in this particular Ford pickup's case that
they had found cooling issues that were related to the low density air.
The ram air effect on the radiator is much less and that effects both
engine and transmission (automatic) cooling. Even low speeds are
effected because the cooling fan is way less effective even with the fan
speed gets higher as the fan clutch reduces slip.
end quote

[Brian B-P]

The idea that loss of engine power would reduce GCWR never made sense to me, since GCWR is generally based on reliability rather than power. A reduction based on cooling does make sense, although I doubt it is related to "ram effect" or fan effectiveness; less dense air simply won't accept as high a rate of heat transfer.

[Pete Dumbleton]

Brian, I usually agree with what you say, but I can't agree that GCWR is based on reliability over power because I truly don't know what criteria ANY of the manfs use, much less all of them... Seems to me that whatever they use, if something changes enough that it can become a weak link, then the rating is likely to need adjustment.

Certainly, Ford generally rates the same vehicle with a larger powertrain as having more towing capability. According to Ford's -2%/1000', someone towing in the Silverton area of Colorado is expected to reduce engine power by almost 20% and I rate that as significant.

I do agree that it's the heat transfer of the less dense air that is the root of the cooling problem. I would guess that the ram statement is linked to a lower air volume flowing thru the cooling at lower speeds (which is where one might be expected to find oneself if climbing the passes with a 20% penalty on a non-turbo engine).

However, the main point of this thread is really that there a lots of factors in towing and we users of the machines aren't going to be able to guess them all

[Brian B-P]

...I can't agree that GCWR is based on reliability over power because I truly don't know what criteria ANY of the manfs use, much less all of them... Seems to me that whatever they use, if something changes enough that it can become a weak link, then the rating is likely to need adjustment.

Certainly, Ford generally rates the same vehicle with a larger powertrain as having more towing capability.

Yes, because the larger powertrain can do more work (haul more) before reaching its limits of reliable operation. If Ford was dumb enough to sell the Powerstroke V8 diesel and 5.4 L Triton (gas) engines with the same radiator and other coolers, same transmission, etc., then the diesel would not have a much higher (and perhaps even lower) GCWR than the gas engine.

A 5.4L Mustang has much more power - from a variant of the same basic engine - as a 5.4L F-150 pickup truck. The truck is limited in GCWR by the engine (not the chassis or structure); we know this because trucks in the same line with different engines have different GCWR ratings. For the few minutes that it would last, the 'Stang would haul a four-ton trailer with greater performance than the truck, because that's what power is all about. The "few minutes" is what reliability is all about. And that's why horsepower is not correlated to tow ratings, but engine size is.

Pete's right that we don't know all of the factors which determine the GCWR (or any other limit), which is why we can't change one component and safely declare that we have increased the rating. The pattern is quite apparent in most vehicle specification listings, which help guide the search for appropriate configurations, but we need to be aware that ratings will be based on factors - like the altitude effect - which we hadn't thought of.

I agree with the "weakest link" assessment... and "weakest" in this case applies in the sense of least strong, as in most easily broken, rather than least powerful. I sincerely do not believe that the "weakest link" is ever power in a non-commercial vehicle, since big rigs tow with only ten horsepower per ton of GCW, and that isn't very much compared to any of our tugs.

[Pete Dumbleton]

I agree with the "weakest link" assessment... and "weakest" in this case applies in the sense of least strong, as in most easily broken, rather than least powerful. I sincerely do not believe that the "weakest link" is ever power in a non-commercial vehicle, since big rigs tow with only ten horsepower per ton of GCW, and that isn't very much compared to any of our tugs.

Theoretically true, but the reality is that our tugs do not have the range of gearing available to the engines that the big rigs do, nor would we accept the fuel penalties. Loss of engine power may be critical to a vehicle that doesn't have the gearing or rear end ratio (or wrong tire diameter) because the power is used so inefficiently (from a load point of view) in even normal application. At any rate, at least one manf (Ford) who makes a wide range of towing vehicles, has declared that altitude reduces towing capacity.