Is it possible to calculate payload?

[Dalko43]

I can totally see how the 5.7 Hemi can cause a reduction in towing capacity - it generates less tractive effort than the 6.4 at any given road speed.

But how does that limit payload!? The engine and transmission itself can't tell if they're towing or hauling a load. If a 5.7 can tow 10k lbs, how does it struggle at 4k lbs payload? The resistance offered by 4k lbs in the bed is far less than 10k lbs behind the hitch.

The diesel's reduction in payload I get - it's much heavier than the smallblocks. But a 5.7 weighs less than a 6.4...

More weight in the pickup bed means more work for the engine to do. I've no idea what the exact limits of the 5.7l Hemi are, but I'm not at all surprised to see that the 6.4l has a higher payload rating by comparison.

A lot of people report that the 5.7l is barely adequate for the 2500/3500 platform...I can't imagine it's a fun engine for hauling around extra weight.

 

[Buliwyf]

Well, since the OEM engineers are the ones speccing it, it's hard to prove they "falsify" something.

However, many OEM ratings are strictly marketing driven. Case in point: how does a Ram HD with 6.4 have a much higher payload (and GVW) than a 5.7 variant? The frame, axles, brakes, are identical. Engine, transmission, and diff ratio limits TOWING, not payload capacity.

As an engineer (ME) myself, I'd have zero issues acting as subject matter expert witness in court, if someone crashed a 5.7 Ram HD, while under the GVW of a 6.4. I'd challenge any OEM engineer to show me a vehicle dynamics model, proving that an engine with 0.7L extra displacement could somehow prevent such an accident.

Axle, suspension, brakes, and frame, sure, these are safety components that dictate a certain GVW. Not engine or transmission.

The GVWR isn't always safety driven. Sometimes a certain component just doesn't have the balls to carry weight and survive. Possibly the 5.7, or something that comes with it, is just a dog. The 5.7 and, I think it was a 5.8, were real dogs in our service trucks. We only had a few.

Kinda like how a F350 with a 4r100 had a much higher tow rating than the ZF 6 speeds. The Germans only guaranteed their transmission for so much, so the truck and trans ended up under rated, despite being known for equal or better strength.

 

[RoyJ]

More weight in the pickup bed means more work for the engine to do.

Yes, but not nearly as much work as towing at max GCWR.

I've no idea what the exact limits of the 5.7l Hemi are

The 5.7's GCWR is the max mass the 5.7 powertrain is rated to move. Which is lower than the 6.4's or Cummins' GCWR.

but I'm not at all surprised to see that the 6.4l has a higher payload rating by comparison.

See above - no surprise the 6.4 is rated to TOW more. By that line of reasoning - shouldn't the Cummins have the highest payload of them all?

A lot of people report that the 5.7l is barely adequate for the 2500/3500 platform...I can't imagine it's a fun engine for hauling around extra weight.

It's all relative, a 5 ton truck has MUCH worse power to weight ratio than a 5.7L dually. At max GVW (not GCWR), a 5.7 dually can easily climb a 6% grade at 65 mph. Try that with a fully loaded 5 ton.

My point? Engine power limits TOWING, not payload. Unless in extreme cases where the towing is actually less than the pyaload.

 

[RoyJ]

The GVWR isn't always safety driven. Sometimes a certain component just doesn't have the balls to carry weight and survive. Possibly the 5.7, or something that comes with it, is just a dog. The 5.7 and, I think it was a 5.8, were real dogs in our service trucks. We only had a few.

Kinda like how a F350 with a 4r100 had a much higher tow rating than the ZF 6 speeds. The Germans only guaranteed their transmission for so much, so the truck and trans ended up under rated, despite being known for equal or better strength.

See my replies above to Dolko.

I'm talking about payload, not towing. If a 5.7 Hemi has enough power to motivate a 12000 lbs load behind the hitch, why should payload be a factor?

 

[Buliwyf]

Your chance of wheelspin drops to zero with that weight on the back. The tires no longer slip like a fuse.

Ok, so now with that established, that the tires hook up so well, that it could be a problem, imagine this. Crossing a wet steel plate with all that weight. Tire spins, hits dry pavement, tire stops hard, tons of instantaneous torque surge right up the driveline like a hand grenade. Maybe the 5.7 has a weaker input shaft combo to the trans, flex plate, motor mounts, block thickness, anything. There could be a weak link anywhere.

It could even be as simple as the extra chassis flex rips the bolts out of the trans. Weird freak things.

It's kinda like the ''why can't we put an Ecoboost in a Superduty'' topic. ''It has as much power as a 6.2l.'' Then again, maybe dodge just plain has a spec typo, or wants to sell the other engine. That's not exactly rare with big three.