What is so bad about "lugging" your engine? - Harley Davidson Forums: Harley Davidson Motorcycle Forum
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Old 04-02-2008, 07:45 PM   #1 (permalink)
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What is so bad about "lugging" your engine?

I've heard it said before and am not rejecting the idea. I simply do not know what it means and therefore do not understand.
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Old 04-02-2008, 08:13 PM   #2 (permalink)
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Did you ever ride a bicycle up an incline in too high of a gear? It puts tremendous strain on your legs and if you downshift it's much easier for you to make it up. It's less strain on your legs. Think of the pistons as upside down legs.
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Old 04-02-2008, 08:15 PM   #3 (permalink)
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They are upside down legs!!!! That my dear man is ingenious! Thank you
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Old 04-02-2008, 09:59 PM   #4 (permalink)
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Very nice analogy there Ed!
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Old 04-03-2008, 12:20 AM   #5 (permalink)
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good information. i'm guilty of doing this to my truck: i try to go uphill alot in overdrive gear with the rpms below 2500
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Old 04-03-2008, 06:23 AM   #6 (permalink)
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Lugging the engine, as well as putting more mechanical strain on, also causes pinging, or detonation (pre-ignition), and does it when the oil pressure is at its lowest due to lack of revs - the worst possible combination.

If you look at a typical torque curve, you'll see on the left side the torque is relatively flat until revs increase and torque starts to improve. This flat area is where lugging occurs, as the engine is not getting enough fuel/air to make the power being demanded of it to accelerate the weight of the machine and rider.

It is not harmful to be in this area as such - for instance when decelerating, cruising downhill, or stopped at idle - it's wide throttle openings in the lugging zone that put demand on the engine to generate power which it is not getting fuel/air to be able to do.

Generally, it's best to be firmly on the rising torque area before applying heavy acceleration loads. You will likely get away with light acceleration on the flat or downhill, but it's bad practice anyway.

Personally, my bike is supposed to peak torque at around 3,000 rpm. I like to keep it in the 2,000 area to accelerate moderately, a bit more if I want to accelerate hard.
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Old 04-03-2008, 06:36 AM   #7 (permalink)
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Ok Mr. Logan, that expands my understanding, but due to my ignorance leaves me with another question: Is the lack of fuel simply because of lack of throttle being applied?
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Old 04-03-2008, 07:43 AM   #8 (permalink)
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Quote:
Originally Posted by DiamondLil View Post
Ok Mr. Logan, that expands my understanding, but due to my ignorance leaves me with another question: Is the lack of fuel simply because of lack of throttle being applied?
Too much throttle, not enough revs. On a carb, lack of revs = lack of vacuum, so you are effectively dumping a load of fuel in while not having enough vacuum to pull in sufficient air. You're not getting enough mix, but also you are getting too much fuel for the air you can pull in.

As the pistons are moving more slowly, the resulting rich (denser) mixture is compressed for longer, creating higher chamber pressures, which in turn creates more heat, which in turn causes the mixture to go bang when it shouldn't (detonation).

So with mild lugging, you just feel the bike bog down which is a signal to downshift.

If you persist on keeping the throttle wide open without downshifting, you get the more extreme symptoms like detonation which put a lot of extra stress on the engine.

EFI is supposed to compensate by not allowing excess fuel when it can't get enough air, regardless of the nominal throttle opening. Some systems are more successful at this than others, depending on how they monitor, measure and calculate.
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Old 04-03-2008, 02:53 PM   #9 (permalink)
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Lugging your engine is tough on the main bearings. The connecting rod rides on a thin layer of oil in your main bearings. You can damage the bearings by too much RPM or too low RPM under load. Low RPM under load can cause the film to break down and you will get metal-to-metal contact at the main bearings. This leads to the dreaded "rod knock". Sometimes known as the "death rattle".
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Old 04-03-2008, 04:40 PM   #10 (permalink)
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And load is combustion in the chamber?
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