My old man taught me that you should never lug an engine because it could kill the rod bearings. You never seem to hear about lugging an engine any more. I can see lugging stalling an engine, but I can't see it killing rod bearings. Is it possible that my old man's advice dated back to the days when the connecting rod bearings relied on splash lubrication? I can see too-low rpm not providing much splash - under heavy load, this could be a problem What are your opinions? Is lugging still a no-no, and if so, why?
The old soft babbit would pound. One semi modern comparison about lugging damage: Aircooled VW's will pound out the center main bearing bore as well as the the front one at the flywheel, if lugged. Same motor was in the VW bus till 71 and those were pounded too, due to being so underpowered for the weight and the air drag.
Help me understand that VW example. The babbit is pounding because of - why? The amount of power that is being delivered? But at higher rpm more power is flowing, so why wouldn't the babbit pound out then? I wonder if the problem is that maybe at low rpm there is not enough lubrication. Low lube plus high load; that I can see pounding babbit. But not simply the load since the load is greater at higher rpm/throttle opening.
I've read old magazines that claimed lugging a cold engine was even worse. That would go hand-in-hand with your proposed answer that there's not enough lubricant to the bearings when you lug the engine, because the oil doesn't flow as well when cold.
It is difficult to lug an engine with an automatic transmission and that is probably why you don't hear it much anymore. Experienced drivers know you shouldn't do it but most people learn to drive with an automatic and never think about it. Why is lugging bad? It puts more load on the rods which can't be good and can lead to detonation (unless the engine is computer controlled). Charlie Stephens
I figure a lot more motors were lugged back in the day when non-synchro 3 speeds were common. I was driving my Dart in stop and go commute traffic for a couple months when I was between normal cars... I was a lugging mo-fo... it seems people behind you don't want you to stop to shift into first when traffic is moving 10-15 MPH
Let's say you take off in second gear - car kinda bounces trying to go right. that bouncing is being caused by the engine trying to stay running. The powerstroke of that particular piston is supposed to be turning the crank. Instead its just pushing hard on the crank in a vertical line because there's too much load and not enough momentum for that fraction of a second. Lets say your cruzing in 4 th when you should be in second. Rpms are very low relative to speed and the momentum comes into play again. The engine is sort of compression braking and trying to accelerate at the same time. The pistons aren't traveling at the right speed compared to the crank shaft eventually connected to the wheels. At top dead center that piston actually stops for a few degrees of crank rotation. Lugging slows it down just before the firing and wont let it move fast enough after the firing so it pounds the bearings.
That is a pretty accurate description.... Basically, at real low RPM's, the combustion gases fire before the rod has gone past TDC and piston has started it's trip down...this puts significantly more force on the rods. Here is a good ****ogy.... Your pushing your kids on the swing set.....if you push at the exact moment that stop on the back arch of the swing, you have a greater resistance...but if you wait just a moment then it's a lot easier....even if your applying the same amount of force.
In air racing, we ran a monster Merlin V12 engine that required the pilot to keep it out of cruise due to the weight of the heavier Allison connecting rods installed to handle the 180 inches of manifold pressure (full race P51 that went well over 500 at Reno). If he ran around in cruise (and at low boost), the engine would ready for an o'haul after a couple dozen hours. When he kept everything up, the bearings lasted a lot longer. It cost about 200k to go through the engine, so he stayed out of cruise.. Contrary to this, my old 53 Chevy didn't like high rpm. It would be time for a rebuild if it were driven at 70 mph for a few hours straight. -Andy
So then the motor would knock, maybe stall, maybe even kill a piston. But it would not kill a rod bearing. In my original post, I stated that it was the claim that lugging would kill rod bearings. I am not claiming that lugging is the desired state, only trying to understand the statement that ***ociated lugging with wiped rod bearings. I still can only see insufficient lubrication in splash engines (my old man had experience with bangers and early stovebolts, and these motors were basically splash) as the possible culprit.
Just like you have to push harder in a higher gear on the bicycle. The engine also has to push much harder in the higher gear and it's pushing on the rod bearings.
Your legs are pushing harder but at a lower rpm. To climb the same hill in a lower gear, you push less but spin your legs a whole lot faster. So the overall power is the same either way. The motor might stall. So what. (Power is work over time.) Look at it this way, you send way more power through those rod bearings accelerating away than you do puttering along at a couple hundred rpm in high gear. The rod bearings handle the higher power demand no problem -- because they are lubricated. I still figure that the difference was in the early splash engines not getting enough lube at low rpms. Of course, they also didn't get enough lube at high rpms, either!
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