Lowest RPM it's safe to max load engine? (not lugging)
 

Self explanatory because it taps into other questions elsewhere/I realize I might not have thought through something...

Whether stock or modified, at what rpm would a typical LS engine be considered to be lugging and how far above that speed is probably safe to say, pull super heavy loads (like a hotshot rig up a hill) or maintain a constant power (like a marine engine at a cruise rpm)?

I'm hoping it's more objective than subjective but i'm sure things like size of the bearings are relevant no matter HOW strong the rods and other parts.

Depends on the engines tuning..
Your limited by the pressure that the gasoline + 02 mixture can tolerate without detonation..
Diesels operate under detonation, hence the massive crank shafts and block design.
6.6 Duramax = 3.1459"
6.2 L92 Gas V8 = 2.559"
8.1L Gm gas V8 = Darned if I can find it.. LOL


Fun comparison:
8.1L Vortec Horsepower & Torque 340 hp @ 4,200 rpm 455 lb-ft @ 3,200 rpm
6.6L Duramax Horsepower & Torque 2001 235 hp @ 2,700 rpm 500 lb-ft @ 1,600 rpm
6.2 L92 HP & Torque 2019 420hp @ 5600 460lb-ft @ 4100
5.3 L83 HP & Torque 2019 355hp @ 5600 383 lb-ft @ 4100

Seems like that to get diesel style power you need 20% more air and fuel and you are limited by knock. May need E85 to hit your number..

I would web search around about "Turbo Normalized" engines in light aircraft. Some of that theory may cross over.

When you say "hot shot" in my head I"m thinking oil field. On the steepest of lease roads i picked a gear i thought i could pull in without down shifting under full load and held it there reving anywhere between 3-5000 rpm staying around 3500 most of the time. Worry more about your tranny than the motor once you hit the lease road. If this is your personal rig/company I"d be getting a bigger aux trans cooler on there.

There's so many variables that must be considered. What gear the vehicle is in. Is the vehicle under a load. Camshaft. Just a few that come to mind. To broad of a question. You can cam a engine according to where you want and need the power.

Take a look at what a stock a4 2.73 locks the converter at in third and fourth, I see mine locking as low as 1,100 RPM in both gears, able to tolerate a great deal of throttle input as well, I can also pull from second gear with the stock converter at a dead stop that's around 1800rpm fully loaded and It doesn't mind at all and with the higher dynamic compression it's incredibly responsive at those speeds, I think modern EFI setups are able to nail down air fuel and spark much more accurately at lower RPMs than the older stuff could and therefore can be loaded at lower RPMs more efficiently or effectively what have you, it's still not as efficient for MPG or stuff like that probably as you see the OEM's going to bigger and bigger valves and ports with more and more gears getting the engine up to an RPM where all the harmonics can be balanced better together blah blah blah I have to go pick up an order at Target

Hotshotting is when guys tow loads that seem more worthy for a semi truck - like 20,000 to 30,000lbs - behind newer diesel pickups on 40 foot trailers instead of using a Class A/semi truck. I'm using it to represent outside the normal uses of a personal use vehicle since no RV fifth wheel camper weighs that much for instance.

I'm looking more for insight on the physical engine design I guess... the speed of combustion of gasoline vs diesel, the size of the bearings spreading out a given level of load (you can upgrade rods but bearings aren't really much 'stronger' from my understanding, actually the stock bearings are supposed to be harder if i'm not mistaken), etc.

"Lugging" has always been defined as the engine turning too slow for the intended load/ie bad for it however just because something can thump along at 50hp in overdrive at a slow speed may not mean that it can do so at 500hp and have the bearings live a decently long life. Where the torque falls off steep on the bottom end is often one clue.

I used to deliver materials to construction sites,, a 2 ton 74 ford flatbed with a 30' implement trailer,, got scaled once at 38K,
Luckily for me at the time Montana only had max weights for diesel powered trucks..

I burned 50 gallons of fuel to get over a pass to the next gas station .. was on empty.. about 30 miles..
pulled the whole pass in second gear foot on the floor, the 390 only had a 2 barrel carb.

You can make any vehicle you dont care about do stuff it isn't supposed to.. :)

I would have picked 3000 if offered. Any rpm away from pk tq is not utilizing the engines ability. Pk tq isn't going to happen at 1600. So... if rpm is below pk tq by > 300 it is going to lug engine when under heavy load. Timing around pk tq is most sensitive, especially with elevated engine and intake air temps.

Rod Legnth vs Crankshaft Stroke
 

The greatest factor about engine "lugging" is how fast the piston moves away from the combustion chamber.
A "short rod" engine will have more position change as the crankshaft rotates.
A "long rod" engine will have less position change as the crankshaft rotates.

Study cylinder peak pressure AND a high R/L, then one with a low R/L.
You will have the answer.

Cool, a specific insight yielding understanding. :) Though... what rpm would you choose then or how would one calculate the designed/intended/best one for the 4.8L, 5.3L and 6.0L for instance?

R/L Effect on the Torque Curve
 

The case, two engines, a 4.8 (R/L 1.91) and a 6.0 (R/L 1.68)
The SAME camshaft and compression ratio.
The 4.8 would "dwell" the piston at TDC much longer with respect to crankshaft rotation.
Combustion gases expand at the same rate.
Piston speed at a higher RPM is greater than at low RPM.
The "work" on the piston would be greater with the 4.8.

The SAME effect could be seen by using "offset" piston pins.

ANOTHER method of predicting "lugging RPM" would be pin offset AND the direction, the side, of the pin offset.

Thus another method of a "work around" as you asked

Good thinking Lance!

Interesting! That finally makes some sense. Are offset piston pins something available outside pricey custom whatever? Obviously by the time that's relevant it's cheaper to get a complete 6.0 instead of a smaller engine.