Originally Posted by thecatsandi
I am towing...with a ...5.4L gas...A V10 would be better. A diesel would be have been over kill
I think this shows a common misunderstanding when it comes to truck motors. Clearly, the hot-rod towing motor is the V10, not the diesel, which has advantages in other respects. Here are the horsepower and torque numbers for some Ford truck motors. I've converted the horsepower to ft-lbs for comparison:
5.4L V-8 300HP/315FT-LBS @ 5,000 RPM - 365 FT-LB @ 3,750 RPM
6.8L V10 362HP/400FT-LBS @ 4,750 RPM - 457 FT-LB @ 3,250 RPM
6.0L PSD 325HP/517FT-LBS @ 3,300 RPM - 570 FT-LB @ 2,000 RPM
7.3L PSD 250HP/505FT-LBS @ 2,600 RPM - 525 FT-LB @ 1,600 RPM
While torque actually does the work, horsepower shows what can be done with the engine torque by multiplication of gearing by the time it hits the ground.
Here are some rpm at speed observations I've made with our truck. I use the terms Overdrive, Drive, and Drive minus 1 to encompass both 4 speed and 5 speed automatic transmissions.
RPM at 65 MPH in OD (0.71:1) w/3.73:1 gearing and P265/75R16 or equivalent diameter tires: 1,850 RPM
RPM at 65 MPH in D (1.0:1) w/3.73:1 gearing and P265/75R16 or equivalent diameter tires: 2,600 RPM
RPM at 65 MPH in D -1 (1.53:1) w/3.73:1 gearing and P265/75R16 or equivalent diameter tires: 4,000 RPM
Looking at the 5.4L specifications, we can estimate that it puts out about 340 ft-lbs around 4,000 rpm. I'm assuming its Drive minus 1 gearing is about the same as that in the transmission of the larger motors, 1.53:1 so to estimate the torque being fed at the transmission output, we can multiply 340 pounds by 1.53:1 to find 520 ft-lbs, about the same as what the diesel outputs in 1:1 Drive.
5.4L @ 4,000 rpm: approx 340 ft-lbs x 1.53:1 (D -1) = 520 FT-LB
In theory, the 5.4L in D -1 should have the same pulling torque at 65 mph as the 7.3L PSD does in Drive, even with the same axle ratio and tire diameter. The difference is the 5.4L is screaming along at 4,000+ rpm, in a gear where the torque converter is unlocked and generating transmission heat, while the diesel is chugging along at 2,600 rpm, in a gear where the torque converter is locked. I use the + on 4,000 rpm because with the torque converter unlocked and slipping, rpm for the given speed will actually be higher than the gearing indicates.
In fact, if the 5.4L's axle ratio was lower, allowing it to get to the rpm where it develops 300 HP vs the 7.3L PSD's 250, the axle shaft torque of the 5.4L should be even greater than the 7.3L's at 1:1, due to the additional torque multiplication. This is basic physics, and when it comes to doing work over time, horsepower rules.
If we look at the 6.8L with the same 3.73:1 gearing and tires as the diesels, here's its transmission output torque in D -1:
6.8L @ 4,000 rpm: approx 425 ft-lbs x 1.53:1 (D -1) = 650 FT-LB
Clearly that's a LOT more than the either of the diesel's maximums in 1:1 Drive.
What's missing in this simple analysis is that a significant portion of the gas engines' greater outputs can be lost in a slipping torque converter, and dissapated as heat from the transmission cooler. That wouldn't be the case with a manual transmission. OTOH, a manual transmission let's a driver hang onto a higher gear when a lower one is called for to get the engine in its powerband and generate the torque required.
The major advantage here is that the less powerful 7.3L and 6.0L diesel motors get their horsepower with greater torque at lower rpm, the rpm it needs to be at to run in Drive or OverDrive, where the torque converter is locked and not converting power to heat instead of pulling power.
The other major advantage is fuel economy, and that's partially because at less than full throttle, gas engines have to be strangled by a throttle plate in a carburetor or fuel injection system. This is because the fuel only burns properly within a narrow range of air/fuel mixtures. And it means that the cylinder fill on each intake stroke is at less than ambient pressure, so on the compression stroke, what's being compressed is a partial vacuum. That gets even worse at high altitudes, where ambient pressure is less.
Because it burns well over a wide range of air/fuel ratios, diesels operate at efficient full-throttle all the time, even at light-throttle cruise without a towing load. Even without turbocharging the diesel is compressing near-ambient pressure at light throttle, and with a higher compression ratio, it's compressing it even more than the gas engine. This gets even better with a turbocharger, especially at higher altitudes where the air is thinner. As the air gets thinner, its drag on the turbo goes down, meaning the turbo runs faster and achieves nearly the same manifold pressure (this same advantage also applies to turbocharged gas engines).
I won't repeat what Canoe stream said about the cost issue.
So to say that a diesel is "overkill," espeically for towing a 31' Airstream, simply doesn't reflect the facts.
Removed some incorrect comments about the 5.4L vs 6.0L diesel I caught as soon as I posted