What causes an engine to lug?

[mechman600]

You could probably lug any modern engine all day long above 1000 RPM and not hurt it.
Bucking and jerking happens at extremely low RPM, like after nearly stalling when dumping the clutch at <500 RPM. If you are constantly running in the <500 RPM range, something is wrong with you.

[XYZ]

Originally Posted by Daox

So, not that I have this problem, but if you did have lugging issues, how could you fix them? Increase oil pressure? Increase flywheel mass?

By the time damage is done it might be too late to correct it. As has been mentioned, there are potential detrimental effects that can take many forms. It's hard to predict which symptom might arise.

Ecomodders tent to want to be in the highest gear and at the lowest RPM's, and that can lead toward lugging the engine. But you can feel the effects of the engine lugging when it is happening, so it is avoidable.

An ounce of prevention is worth as pound of cure... so IMHO it is best to avoid doing it.

[user removed]

Originally Posted by Frank Lee

I'd say you not only don't want the damaging loads on the crank and rod bearings and cylinder walls, you don't want the bucking and jerking. That's tough on the motor mounts too.

Simplest thing is to operate above the lugging threshold. If you are gentle on the gas pedal you can even operate down in that lugging rpm range if you only lightly load the engine. So if you're in 5th gear at 20 mph and you step on it to accelerate and it starts bucking and protesting, back off on the gas. You may still be able to accelerate but at a very gentle rate until it reaches a higher rpm.

If I was certain I wanted to operate in the lugging rpm range and fix it, the first thing I'd do is add flywheel mass. I don't think adding oil pressure would help. And if this low rpm operation is happening because of a tall re-gearing, I'd look at whether a new cam could be ground to optimize the engine for this new low rpm range.

Precisely stated and I agree 100%

regards
Mech

[user removed]

Lugging was much worse in carbs with an accelerator pump, which if you floored it, put the same amount of fuel in regardless of the engine speed. When done at too low a speed it was way too much gas and usually atrocious spark knock, which fuel injection does not really allow all the way down to idle speed.

My 37 Ford idle was 350 RPM. To check it you drove the car in top gear with your foot off the gas and it read 7 MPH, you were good. The engine was not "lugging" on flat ground at 7 MPH, but if you touched the gas pedal any more than just a smidgen, you were lugging. It was possible to pull away in 3rd from an idle but it took like 5% throttle to avoid lugging, which also restricted the accelerator pumps injection of fuel.

Redline on that flathead was 3800 RPM or 82 MPH in top gear depending on the rear axle ratio. That 37 weighed the same as my Fiesta, around 2500#, and the old flathead was maybe 217 cubes.
Good old torque monster.
No problem driving in modern traffic until you had to stop really fast!

regards
Mech

[Cobb]

If you can tell us the make and model we can do wonders for you. In general either reduce load or increase rpms. Maybe inflate the tires another ten psi? Under drive pulley for the accessories? Turn off the AC? Check for vacuum leaks, do a general tune up.

My example with my sidekick seems to like to be around 3 thousand rpms. Since going up 3 tire sizes I find situations where I need to go from 5th to 4th where as I use to leave it in 5th. The benefits of the bigger tires well out weigh that issue.

Originally Posted by Daox

So, not that I have this problem, but if you did have lugging issues, how could you fix them? Increase oil pressure? Increase flywheel mass?

[Daox]

Its all theoretical discussion. No specific model in mind.

[darcane]

Originally Posted by Daox

So, not that I have this problem, but if you did have lugging issues, how could you fix them? Increase oil pressure? Increase flywheel mass?

Downshift...

[RustyLugNut]

Originally Posted by Daox

Its all theoretical discussion. No specific model in mind.

Seat of the pants definitions where vibration and noise are the measure do not adequately tell you if you are "lugging" an engine. Many engines are fine being run in low, heavy loads while others fail very quickly.

I won't go into engine design theory of peak pressure to brake-mean-effective pressure, but, Old Tele Man's description pretty much sums things up. Assuming your engine holds together, the point at which the cylinder pressure causes displacement of your lubricating film is when things go south. Low speed, high power demands result in increased cylinder pressures. Coupled with most engines which use mechanical lubrication pumps who's output follows engine speed, there is a point at low revolutions where the oil film is displaced and damage occurs.

Of course, even if an engine does not incur damage at high loads and low speed, it can feel awful. The bucking and vibration can damage motor mounts and other tertiary components. The configuration and design of an engine has much to do with how violent "lugging" can feel. The 3 cylinder Suzuki/Geo Metro engines are inherently rough at lugging speeds whereas the Daihatsu CB90 engine of the same time period is much happier being lugged with its balanced shafts.

For heavy duty towing, I have always preferred the inherent smoothness of the straight 6 configuration if the engine has 7 large bearings. I worked with an old 70's Dodge pickup with the venerable slant 6. We lost first gear, and for about half a year, we would start out in second gear using the the column shifter. The engine seemed happy to pull our 3500 pound trailer, though the second gear starts wore out the clutch pack. Even after such abuse, the engine ran for a couple more decades.

I still have straight sixes in my current Dodge trucks, but of course, they are both diesels. They are designed to be "lugged" from the get go.

[Cobb]

You can try reclutching it. This is where you press the clutch and rev higher and try engaging it again.

For off road vehicles you have a low range and most of those can be modded for even more reduction. In the event you stall on a sticky situation many 4wd have a clutch over ride switch so you can start in gear and roll off.

Originally Posted by darcane

Downshift...

[Cobb]

Automatics "reclutch' by unlocking the torque converter and using the 2 to 1 ratio of the converter on top of selecting a lower ratio from the gearing. Many cases they use the knock sensor on top of the MAP to do this in relation to speed.

When I would use cheap gas in my supercharged tacoma it loved to change gears. When I ran 93 octane with a bottle of 104 on top of it once it got to over drive it stayed there and mashing the throttle just added BOOST.