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Effects of engine lugging

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What exactly does let's say severe lugging do to an engine? Clearly (maybe) you are not supposed to do it. But what damage does it actually do? And how long does it take to occur?

Has anyone ever seen any comparison of a never/rarely lugged engine and an always/often lugged identical engine under otherwise comparable circumstances?

Are we just talking bottom end? Top end? Weakest link (whatever that is)?

In other words, has anyone ever seen an engine and said “oh yeah this motor’s been lugged it’s whole, presumably short, life”?

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Too low an rpm under load can cause the piston to rock in the bore. The piston wears as does the bore. In some high performance engines, the piston can actually be flipped and destroy the engine. Too low an idle speed can also be detrimental. Lugging can cause excessive speed variations between power and exhaust strokes which puts unneeded stress on parts..

Think of riding a bicycle up a hill. Ride in too high a gear and you will barely be able to pedal, your motion terribly jerky. That is lugging. You want to pedal where your rotation is smooth.

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I think it’s like many things an issue which was far more an issue longer ago.

Since engine builders tuners etc don’t anticipate WOT at 1/4 RPM of redline to be a a common operating condition the fueling and ignition may not be acceptable for long term. I can easily see the timing being to aggressive and fueling ending up too lean and potentially detonation with very high cylinder pressure and low flow through the carb you’d get with WOT and low rpm.

 

Now with throttle by wire, complex fuel and ignition tables combined with knock sensors and fuel injection plus stronger better engine parts its probably not as detrimental as it once was but obviously still not good.

I think piston rocking and other balancing/dynamic issue still exist but I’d guess avoiding knock was a big part of it.

 

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I've seen top ends that were in engine's that were "lugged." Think filthy deposits on the crown. LOTS of deposits. 

I've heard small/big end bearings committing suicide in engines. Whether it was 'lugging" or insufficient oil quality, it's hard to make that determination, unless a complete tear-down is performed. 

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without detonation there is not much of a problem IMO.

with low port velocity and the intake valve closing quite late compared to what would be optimal for that rpm the cylinder pressure is much lower that you would see at peak torque.

I think a lot of injected stuff fuels the shit out of it to keep it from detonating and it just bogs like a bitch.

where if a something carbureted is lean in that region it could cause detonation and then we would have an issue.

On a plain bearing engine if oil pressure or more importantly flow is quite low and you heavily load an engine for an extended period of time it could promote bearing wear.

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increased bearing wear and valve seat recession in regular automotive engines.  

these bikes have super alloy valves and hardened seats etc, but Im sure a bike lugged all the time would show seats that were not happy and valve faces cupped to shit.   

2 strokes no problemo.  

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4 hours ago, bentvalves said:

increased bearing wear and valve seat recession in regular automotive engines.  

these bikes have super alloy valves and hardened seats etc, but Im sure a bike lugged all the time would show seats that were not happy and valve faces cupped to shit.   

2 strokes no problemo.  

How do you propose the valves will wear quicker from lugging?

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8 hours ago, EnglertRacing said:

How do you propose the valves will wear quicker from lugging?

It is well documented englert, lemme dig around in some books tonight and see if it is covered in depth or just mentioned.  

Believe it has something to do with high EGT.

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so, if the engine is going to detonate (i don't think these modern rippers do) it is most surely going to be doing it when its highly loaded at low RPM.  all you would hear is chain slap but its actually your piston dying and valves getting smoked and your timing chain being stretched.  

revs dont kill, lugging, and ultra high revs/limiter do.  

 

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On 9/5/2019 at 7:21 AM, bentvalves said:

It is well documented englert, lemme dig around in some books tonight and see if it is covered in depth or just mentioned.  

Believe it has something to do with high EGT.

Lugging create's high CHT which quickly turn into high coolant temps, eventually detonation.   EGT's can stay low because the throttle is plenty open for the rpms it's turning.   Usually see highest EGT at light loads and throttle settings in the midrange where the timing is most advanced. Lot of motors burn down in the midrange.

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The main problem that it causes is combustion knock and pre-ignition.  That causes very high cylinder pressures, which is hard on the entire rotating assembly, and especially pistons.  

Another problem aside from that is the fact that the engine will be under a lot of load to get the desired amount of work done.  So if you shift down a gear, the engine can operate under a lower load, and do more revolutions to do the desired amount of work.  High loads cause a lot of side-loading on the piston, compressive force in the connecting rod, torsional load on the crankshaft, and very high peak forces on the transmission gears. 

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45 minutes ago, dirtjumpordie said:

The main problem that it causes is combustion knock and pre-ignition.  That causes very high cylinder pressures, which is hard on the entire rotating assembly, and especially pistons.  

Another problem aside from that is the fact that the engine will be under a lot of load to get the desired amount of work done.  So if you shift down a gear, the engine can operate under a lower load, and do more revolutions to do the desired amount of work.  High loads cause a lot of side-loading on the piston, compressive force in the connecting rod, torsional load on the crankshaft, and very high peak forces on the transmission gears. 

I agree. Lets say the culprit would be the valves dont get adjusted. Causing carbon buildup from incomplete burn. Plug would show it. Mabey it needs to be leaned. Compression pressure is lower then firing pressure. Exhaust valve will leak escalating cylinder head temps from firing pressure. Rings wil stick then causing blowby. If crackcase does not have negative pressure beside fuel and ignition tuning is off, oiling pressure  has a problem fighting other vrackcase pressure. Bearings fail and thats that. Piston rock on two stroke are from NOT having iron sleeve. Aluminum distorts differently when heated. Electro plating does notta. CAST PISTON or no piston. .0015 for cast piston or .0030 to .0040 for forged piston cylinder to wall clearance. Most temps are NOT in the operating range before given load. Piston rock will happen especially with these water cooled bikes. Cast rings are good with steel liner compare steel rings on a electro plated aluminum bore that swells around the exhaust port. Catching a ring on the port with piston rock is normally how it ends. 

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On 10/23/2019 at 12:11 AM, Aaron Larson said:

I agree. Lets say the culprit would be the valves dont get adjusted. Causing carbon buildup from incomplete burn. Plug would show it. Mabey it needs to be leaned. Compression pressure is lower then firing pressure. Exhaust valve will leak escalating cylinder head temps from firing pressure. Rings wil stick then causing blowby. If crackcase does not have negative pressure beside fuel and ignition tuning is off, oiling pressure  has a problem fighting other vrackcase pressure. Bearings fail and thats that. Piston rock on two stroke are from NOT having iron sleeve. Aluminum distorts differently when heated. Electro plating does notta. CAST PISTON or no piston. .0015 for cast piston or .0030 to .0040 for forged piston cylinder to wall clearance. Most temps are NOT in the operating range before given load. Piston rock will happen especially with these water cooled bikes. Cast rings are good with steel liner compare steel rings on a electro plated aluminum bore that swells around the exhaust port. Catching a ring on the port with piston rock is normally how it ends. 

The high pressure from combustion knock really beats the shit out of 4 stroke valves, valve seats, pistons, and rings.  Not good for bearings, either.  Maintenance aside, combustion knock will shorten the life of a top end drastically. 

Iron sleeves do not expand as much as plated aluminum cylinders, that is true.  

However, the reason that cast and hypereutectic pistons have lower thermal expansion coefficients is because they have more silicone in them.  That same silicone makes the pistons more brittle.  Because they are more brittle, those pistons cannot handle things like piston slap or the shock loads that come from combustion knock.  Combustion knock will destroy a cast or hypereutectic piston very very quickly.  

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On 8/24/2019 at 11:02 AM, William1 said:

Too low an rpm under load can cause the piston to rock in the bore. The piston wears as does the bore. In some high performance engines, the piston can actually be flipped and destroy the engine. Too low an idle speed can also be detrimental. Lugging can cause excessive speed variations between power and exhaust strokes which puts unneeded stress on parts..

Think of riding a bicycle up a hill. Ride in too high a gear and you will barely be able to pedal, your motion terribly jerky. That is lugging. You want to pedal where your rotation is smooth.

Motor Authority Mag:

"Picture yourself on your old 10-speed road bike. You're cruising along, having shifted up into the higher gears. Suddenly you need to accelerate and when you try to turn those pedals, you find it takes a lot more effort than it would if you were in a lower gear. You are working far harder than necessary just to get that bike moving again.

In this example, you've lugged your body because you're the engine of the bike."

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On 6/2/2020 at 10:22 PM, Stef Delarge said:

Motor Authority Mag:

"Picture yourself on your old 10-speed road bike. You're cruising along, having shifted up into the higher gears. Suddenly you need to accelerate and when you try to turn those pedals, you find it takes a lot more effort than it would if you were in a lower gear. You are working far harder than necessary just to get that bike moving again.

In this example, you've lugged your body because you're the engine of the bike."

Not exactly, the engine produces peak forces at peak torque.

Lugging will only generate higher forces if detonation happens

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