To lug or not to lug?

[280Z Turbo]

What are the effects of lugging an engine with extreme load/low RPM?

I know a guy with an '05 Silverado 2500HD 5 speed manual that lugs the shit out of the motor. He shifts into 5th gear at 35 mph while pulling a 3 axle trailer with two cars on it. He also sometimes starts out in 3rd gear and never uses 1st. He smoked the shit out of the clutch while attempting to pull said 3 axle trailer up a slow, steep hill in 2nd.

Also, at what point does lugging the engine hurt gas mileage? Is it better to hold on to the revs or bog it by shifting early?

[Secret Chimp]

The main problem with lugging is lower RPMs=lower oil pressure, which when you really LUG an engine means bad news bears for all things oiled. Whenever I run my engine in a gear lower than what would have it at around 1800 or more I always stay light on the gas and downshift one or two if I want to scoot instead of flooring it.

[Eye of the Tiger]

I believe lugging the motor increases load on the motor, which is good for seating piston rings in a new engine, but I don't think it hurts an old engine - except perhaps increasing the chances of blow-by. I think it does use less gas, though I may be wrong because it may use more gas relative to the same amount of power produced higher in the power bad with less throttle.

[Eye of the Tiger]

The main problem with lugging is lower RPMs=lower oil pressure, which when you really LUG an engine means bad news bears for all things oiled. Whenever I run my engine in a gear lower than what would have it at around 1800 or more I always stay light on the gas and downshift one or two if I want to scoot instead of flooring it.

Oil pressure has nothing to do with throttle position. If it has good oil pressure at idle, it doesn't matter if you are light on the gas or not.

[3.0L V6]

What are the effects of lugging an engine with extreme load/low RPM?

I know a guy with an '05 Silverado 2500HD 5 speed manual that lugs the shit out of the motor. He shifts into 5th gear at 35 mph while pulling a 3 axle trailer with two cars on it. He also sometimes starts out in 3rd gear and never uses 1st. He smoked the shit out of the clutch while attempting to pull said 3 axle trailer up a slow, steep hill in 2nd.

Also, at what point does lugging the engine hurt gas mileage? Is it better to hold on to the revs or bog it by shifting early?

The gas is cheap compared to the clutch job.

Also, low engine speed = low oil pressure. With that amount of loading on the bearings and almost no oil pressure, well, yeah.

[Eye of the Tiger]

The gas is cheap compared to the clutch job.

Also, low engine speed = low oil pressure. With that amount of loading on the bearings and almost no oil pressure, well, yeah.

The load on the bearings vs. oil pressure is a good point, but doesn't the oil pressure increase suffeciently just off-idle in most engines?

[Secret Chimp]

Oil pressure has nothing to do with throttle position. If it has good oil pressure at idle, it doesn't matter if you are light on the gas or not.

That's not what I'm saying. If you're pooping along at 1200rpm, that's fine. But if you ask the engine to PULL PULL PULL PULL by running it with more throttle at the same RPM, you're asking it to try to work harder with the same oil pressure.

[Eye of the Tiger]

That's not what I'm saying. If you're pooping along at 1200rpm, that's fine. But if you ask the engine to PULL PULL PULL PULL by running it with more throttle at the same RPM, you're asking it to try to work harder with the same oil pressure.

I think most engine make plenty of oil pressure at low rpm for this to be a non-issue, but I could be wrong.


I also suppose extreme lugging while towing a 3-axle trailer could also cause the engine to overheat if the waterpump is a POS and the cooling fan is belt-driven.

[3.0L V6]

The load on the bearings vs. oil pressure is a good point, but doesn't the oil pressure increase suffeciently just off-idle in most engines?

A rough estimate of oil pressure vs. rpm is approximately 10psi oil pressure per 1000 rpm.

Opening the throttle wide 'tells' the computer to dump in more fuel to accelerate the engine. Since the engine can't accelerate quickly (due to load and the high gearing) it chugs.  Hence, you're wasting fuel and damaging the engine trying to accelerate in high gear.

[280Z Turbo]

I've seen him try to accelerate the truck below 1000 rpm. I can hear it knocking and grunting. The redline is aroun 5200 rpm, but I've never seen it above 2500.

It is a 6.0L Vortec, but it's not THAT torquey.

Doesn't this put stress on the rods, bearings, rings, etc.?

[Eye of the Tiger]

A rough estimate of oil pressure vs. rpm is approximately 10psi oil pressure per 1000 rpm.

Opening the throttle wide 'tells' the computer to dump in more fuel to accelerate the engine. Since the engine can't accelerate quickly (due to load and the high gearing) it chugs.  Hence, you're wasting fuel and damaging the engine trying to accelerate in high gear.

It's probably worse on old carbureted motors, but fuel injected engines with computer-controlled ignition shouldn't suffer too much.

[3.0L V6]

I've seen him try to accelerate the truck below 1000 rpm. I can hear it knocking and grunting. The redline is aroun 5200 rpm, but I've never seen it above 2500.

It is a 6.0L Vortec, but it's not THAT torquey.

Doesn't this put stress on the rods, bearings, rings, etc.?

Yup. Low oil pressure + high load + crummy combustion cause you're in too high a gear and with too much throttle will quickly trash the engine.

However, I'd think that the clutch would give first, eliminating the 'load' problem.

[280Z Turbo]

Is it bad for the driveline too?

[Eye of the Tiger]

Is it bad for the driveline too?

Umm... yeah, the clutch slipping.

[3.0L V6]

Is it bad for the driveline too?

Not so much. In high gear, the torque multiplication is low, so the trans and rear-end should be fine.

That's small consolation though.

[Eye of the Tiger]

What about diesel engines? Ignition timing problems aren't an issue, and they are supposedly built for high-load, low-rpm operation anyway.

[3.0L V6]

What about diesel engines? Ignition timing problems aren't an issue, and they are supposedly built for high-load, low-rpm operation anyway.

To an extent, you'd have less of an issue if the engine were right sized, but (assuming a pickup engine) they still have an optimal working rev range.

Get out of this range on the low side and you still have the same issue.

[Eye of the Tiger]

I am going to build a gasoline engine that idles at 1rpm. Now, I just have to figure out the optimal bore/stroke to attain a sustainable piston velocity.

[280Z Turbo]

Not so much. In high gear, the torque multiplication is low, so the trans and rear-end should be fine.

That's small consolation though.

I've had my chain come off my bike before from starting off in top gear. It seems to me that starting off in 3rd gear could be bad for the trans.

[GoCougs]

Engine nowadays are built much stronger and to much tighter tolerances, so it's not near the concern that it used to be, but extended lugging under a big load as described, and I would not be surprised at premature engine failure.

[Secret Chimp]

What about diesel engines? Ignition timing problems aren't an issue, and they are supposedly built for high-load, low-rpm operation anyway.

They also run on 40-60 psi of oil pressure. Even then nobody tries to set off in a loaded semi just barely off idle.

[Secret Chimp]

Engine nowadays are built much stronger and to much tighter tolerances, so it's not near the concern that it used to be, but extended lugging under a big load as described, and I would not be surprised at premature engine failure.

Tighter tolerances also means less "wiggle room" for areas being lubricated below optimal pressure for the load the oil has to bear, I would think.

[S204STi]

Oil pressure has nothing to do with throttle position. If it has good oil pressure at idle, it doesn't matter if you are light on the gas or not.

It has a lot to do with engine RPMs however.

[S204STi]

I've had my chain come off my bike before from starting off in top gear. It seems to me that starting off in 3rd gear could be bad for the trans.

You mentioned how he smoked his clutch a lot...well, that is basically the weak link in the drive line giving up the effort.  Lucky for him that he doesn't have a more grippy clutch.

[Eye of the Tiger]

It has a lot to do with engine RPMs however.

That was my point. And then Mr. 3.0 pointed out the issue of engine load vs. oil pressure on the bearings, which is the only thing that throttle position has to do with oil pressure.

I've had my chain come off my bike before from starting off in top gear. It seems to me that starting off in 3rd gear could be bad for the trans.

Have you dynoed your legs? The torque curve is completely different than any internal combustion engine.

They also run on 40-60 psi of oil pressure. Even then nobody tries to set off in a loaded semi just barely off idle.

yes they do

[r0tor]

lugging increases gas mileage and increase bearing wear because of the shit load of vibration you induce when you start bucking the powertrain.

[Soup DeVille]

They also run on 40-60 psi of oil pressure. Even then nobody tries to set off in a loaded semi just barely off idle.

Ummm, its the only way to do it and not buck around like a noob.

[GoCougs]

Tighter tolerances also means less "wiggle room" for areas being lubricated below optimal pressure for the load the oil has to bear, I would think.

Depends I guess how you define "tighter." I was thinking more of precision; i.e., the ability to more consistently machine bearing surfaces, as uniformity is a critical factor in maintaining the thin, high-pressure lubrication layer.

[JWC]

RPM has a direct connection to oil pressure. Just watch an oil pressure gauge sometimes while revving the engine. My VW produces 25psi at idle. Take it up to 2,000 rpm and the oil pressure increases to 40psi.

Lugging the engine is asking it to pull the vehicle and/or a trailer with minimum lubrication to the main bearings.  You're also asking it to do its job at a point where it isn't producing as much torque.  The strain is on the main bearings and that is where you're going to do the damage.

[S204STi]

What about diesel engines? Ignition timing problems aren't an issue, and they are supposedly built for high-load, low-rpm operation anyway.

They also use a very heavy oil with tons of antiscuffing additives and higher tolerances.

Apples and Oranges.