So what exactly is 'lugging' the engine?
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Post Fiend
Joined: Apr 2004
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From: Lafayette, IN
So what exactly is 'lugging' the engine?
Since we read so much here about how bad it is to lug the engine, I was wondering what exactly is going on in the engine when it is being lugged? I don't want to hear about WHEN it happens, I know that--trying to accelerate from 25mph in top gear with a manual tranny will lug the engine. But what is happening internally, and why is it bad?
Jason
Jason
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From: Melbourne, Aus
It is a point where the combustion struggles to overcome the force required to move the vehicle.
What can happen is overheating within the cylinder, pitting on valves and cylinders, pinging, amongst other things - not to mention inefficient combustion.
What can happen is overheating within the cylinder, pitting on valves and cylinders, pinging, amongst other things - not to mention inefficient combustion.
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Post Fiend
Joined: Apr 2004
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From: Lafayette, IN
Thanks, BigF350, I had forgotten about the pinging--it's been a little while since I owned a manual vehicle, even though I prefer them.
One of them was a '87 Mercury Lynx (sister car to Escort), with the 1.9L TBI and a 5-speed. As long as I was cruising above say, 25mph, I would shift through all the gears, up to fifth. If I mashed the throttle at 25mph in fifth, it would lug, but if I accelerated gently, it wouldn't. I guess I'm wondering why the computers aren't programmed to overcome this and prevent it?
Jason
One of them was a '87 Mercury Lynx (sister car to Escort), with the 1.9L TBI and a 5-speed. As long as I was cruising above say, 25mph, I would shift through all the gears, up to fifth. If I mashed the throttle at 25mph in fifth, it would lug, but if I accelerated gently, it wouldn't. I guess I'm wondering why the computers aren't programmed to overcome this and prevent it?
Jason
FTE Leadership Emeritus
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From: Melbourne, Aus
Well, that would be a cable operated throttle, hard for a cable to to be controlled by a computer 
Even on vehicles equipped with throttle by wire, manufacturers prefer to give the person what they want usually (i.e. If someone floors the throttle at 25mph in top, it will go straight to WOT)
With things like fuel injection, and variable valve timing it is pretty hard to get an engine to "lug".
Even on vehicles equipped with throttle by wire, manufacturers prefer to give the person what they want usually (i.e. If someone floors the throttle at 25mph in top, it will go straight to WOT)
With things like fuel injection, and variable valve timing it is pretty hard to get an engine to "lug".
Fleet Owner
Joined: Aug 2001
Posts: 29,941
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From: Drummonds, TN USA
One of the main reasons "Lugging the Engine" is a bad idea is because it puts severe forces at work on the main bearings and other friction surfaces, and yet at the same time the low speed at which it happens means that oil pressure may not be enough to provide complete protection.
At low speeds with a maximum load the viscosity of the oil film on the bearings may be overcome, resulting in extreme wear and tear.
At higher RPMs these loads are offset by increased oil flow and sufficient pressure to maintain a protective film over friction surfaces even if the oil has begun to thin out from extended use.
At low speeds with a maximum load the viscosity of the oil film on the bearings may be overcome, resulting in extreme wear and tear.
At higher RPMs these loads are offset by increased oil flow and sufficient pressure to maintain a protective film over friction surfaces even if the oil has begun to thin out from extended use.
Last edited by Greywolf; Nov 13, 2005 at 06:44 PM.
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Post Fiend
Joined: Apr 2004
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From: Lafayette, IN
Ok, so how would this be different from taking off from a stop with a heavy load in a stick (like all those 300 I-6'ers like to brag about doing 
)? That is, the low oil pressure, high load, etc.... I mean, surely the torque(load on bottom end) can't exceed the peak torque that occurs at a higher RPM, right?
Maybe I was just always hard on my stickshifts. I would always take off right at idle RPM until the clutch was fully engaged, almost never revving it up first.
Jason
)? That is, the low oil pressure, high load, etc.... I mean, surely the torque(load on bottom end) can't exceed the peak torque that occurs at a higher RPM, right? Maybe I was just always hard on my stickshifts. I would always take off right at idle RPM until the clutch was fully engaged, almost never revving it up first.
Jason
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Post Fiend
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From: Iowa
Greywolf makes good points, but there's more. There is bearing velocity that needs to be figured in. The faster the two surfaces (crank and bearings) are sliding against each other, the stronger the oil film. Basically, there's not enough time for the oil to get out of it's own way. So at lower engine speeds, with higher cylinder pressures, the oil film can be overcome. Metal to metal contact, and we have wear. Same kind of things happens on the piston skirts, also.
Interestingly, this is also why larger bearings typically show better wear characteristics.
Interestingly, this is also why larger bearings typically show better wear characteristics.
Posting Guru
Joined: Jul 2003
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From: Alvin, Texas.
The difference: Any truck, I6 or anything else, if driven properly will build up revs before taking off with a heavy load.
Oil pressure has built up, higher torque output resulting from higher revs (to a point) allows for easy take off. The I6 guys get it rough. That 300 peaks at something like 2000rpms and falls on its face at 2100rpms. (Okay, that's a joke, though some agree in a sense...)
I take off around 3000 revs and occasionally stay in 1st gear past 4000 revs--pulling a load, I would do the same as it is easier to lighten up on the revs once I am moving rather (momentum has already kicked in) than build them back up in that same instance. Engine does fine at 253,500ish miles on the ticker.
Oil pressure has built up, higher torque output resulting from higher revs (to a point) allows for easy take off. The I6 guys get it rough. That 300 peaks at something like 2000rpms and falls on its face at 2100rpms. (Okay, that's a joke, though some agree in a sense...)
I take off around 3000 revs and occasionally stay in 1st gear past 4000 revs--pulling a load, I would do the same as it is easier to lighten up on the revs once I am moving rather (momentum has already kicked in) than build them back up in that same instance. Engine does fine at 253,500ish miles on the ticker.
Last edited by AlfredB1979; Nov 13, 2005 at 08:29 PM.
Post Fiend
Joined: Mar 2004
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From: San Francisco, Ca.
There's always constant load on a running engine.
It's just more of a load when you lug the engine.
Taking off with a heavy load and with a manual trans, you have the means to control the amount of lug, with the accelarator and/or clutch.
Yea, there's probably a bit of lugging when taking off, but it's short term; not like lugging your engine up a grade or something.
That's one of the reasons they have gears in transmissions.
Another reason to lesson the strain of lugging the engine when taking off from a dead stop and with a heavy load was the introduction of compound low in manual truck transmissions, like the T-18 for an example.
Automatics have fluid coupling between the engine and auto trans to lesson some of the strain of lugging the engine. It's called a torque converter.
Just my personal opinion on this.
It's just more of a load when you lug the engine.
Taking off with a heavy load and with a manual trans, you have the means to control the amount of lug, with the accelarator and/or clutch.
Yea, there's probably a bit of lugging when taking off, but it's short term; not like lugging your engine up a grade or something.
That's one of the reasons they have gears in transmissions.
Another reason to lesson the strain of lugging the engine when taking off from a dead stop and with a heavy load was the introduction of compound low in manual truck transmissions, like the T-18 for an example.
Automatics have fluid coupling between the engine and auto trans to lesson some of the strain of lugging the engine. It's called a torque converter.
Just my personal opinion on this.
Postmaster
Joined: Feb 2003
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See if this also makes sense. By definition, lugging is high load, low rpm and probably open throttle (maybe even wide open). The whole dynamics of this situation are in motion but in this case the rpm is so low that even the base ignition advance setting is too high. Therefore ignition starts too early and pressure starts building before the piston reaches TDC. This tries to force the piston backwards putting counter forces on the crank and bearings. At one point the force vector may be at its greatest when the piston is at TDC creating a linear force straight through a line drawn from the center of the piston through the main bearings of the crank.
At a higher rpm, the combustion event is timed so peak cylnder pressure is at or just after TDC and the forces don't line up against the main bearings so bad. The force is used to turn the crank rather than trying to push it through the side of the block.
Also, since the throttle is open, more air and fuel are getting into the cylinders at what would otherwise be an idle rpm. So the engine is actually making more power than it would be at no load idle.
At a higher rpm, the combustion event is timed so peak cylnder pressure is at or just after TDC and the forces don't line up against the main bearings so bad. The force is used to turn the crank rather than trying to push it through the side of the block.
Also, since the throttle is open, more air and fuel are getting into the cylinders at what would otherwise be an idle rpm. So the engine is actually making more power than it would be at no load idle.
