Chicken $*** hahaha.

What engines really really hate is lugging. This is where the engine is very low on RPM and fuel delivery is high under load. The engine is trying to increase engine rpm to an acceptable level so there is actually power and or avoid stalling.

You find that after ignition the forces on the piston are extremely high because it's [the piston] not moving at the speed the engine is designed to run at. The 2ZZ makes a particular knocking noise when it's lugged, some but not all engines do.

In addition it's harder on engine and drivetrain components on (lets say) a SUV with a trailer heading up a steeply (20%) graded hill highway on a high cruising gear. If the engine does manage (by some miracle) to keep a stable low rpm then you may face cooling issues with a long enough hill because the water-pump isn't going to be rotating at an acceptable rate to provide the quantity of coolant that will needed to be circulated (same with the automatic transmission fluid). So in this circumstance it's actually advisable to drop down a gear or two and make some noise because it will cause less wear on the drivetrain. Most people are too scared of destroying their engines to do this, instead the auto tranny will overheat and possibly pop... their loss.

In addition you have something designed like the 2ZZ that makes it's maximum 131 lb-ft torque at 6700rpm (127@4400 is stated), it's "not happy" below 4000rpm (especially at WOT) under load. It's funny because it's an NA engine which means it responds instantly but is lazy in the low rpm range.

I hear this "lugging" thing a lot, but I can't decide whether it actually makes sense or not. The friction in an engine doesn't start going up until well below idle speed, and at higher speeds the components are definitely under a lot of stress. Efficiency does go down near 1000-1500rpm typically but I think this is probably because the burning charge has too much time to cool down. The only thing I can see is that the rods are under max compression for a longer period of time but I don't think this is much of an issue especially for naturally aspirated engines.

old greg do you mind sharing your wisdom? :P

It has more to do with oil delivery to the journal bearings. As engine speed lowers it's harder for the hot oil to develop a hydrodynamic wedge. This leads to metal on metal contact which as Im sure you know is bad. There are some things that can be done to extend the low rpm performance of the bearing but all to my knowledge negatively effect the high rpm performance, in one case even causing the bearing to pump oil against the oil pump.

FYI, that dyno chart I posted is comparing stock headers to some JDM equal length headers that I installed between runs.

The headers/exhaust don't get rid of the dip.

That's a good point, usually oil pressures are decent at idle rotation. I'm a little skeptical on the metal on metal contact point. It could be possible with the exerted force, especially with the oil viscosity at temperature.

I've never seen a document on any engine lug tests. So this will interest me.

At low engine speeds, the hydrodynamic wedge that supports the conrod bearings isn't as strong. Consider the film strength of the oil in the bearing gap with no relative motion. Weak. Applying full torque is a bad idea at low engine rpm...

At high rpm when inertial loads dominate, the high speed gives a MUCH stronger hydrodynamic wedge to support the conrod.

How relevant is this? If BMW feels safe giving their engines full boost at like 1200 rpm or something then it's probably safe to say these effects only exist around 1000 or below right?

I guess it makes sense that the oil would get squished out of the bearings more easily at low speeds, thanks for the info. Didn't think of that.

I understand what he is saying. However you used "isn't as strong". This begs the following question: At which RPM point is the hydrodynamic wedge not able to support the forces?

Obviously this depends on many factors of the engine during lugging (including the engine itself). RPM conversely affects both oil pressure and forces on the bearings and respective journals.

I would be interested to see at what rpm and parameters the hydrodynamic film breaks down, throw in some different oils, different temperatures...

I think by "not as strong" he isn't actually saying it's less strong, but when the rotational speed is slower the bearing is under high stress for longer periods at a time and it gives the oil more time to be "squeezed" out. Or at least that's how I am understanding it.

At high rpm the forces on the bearings are greater but the oil film is being compressed for a smaller amount of time each cycle so the film can be maintained more easily.

In my head this is the picture: You have say 2 pieces of glass with water between, if you squeeze really hard and hold, the water will come out the edges and drip off. If you squeeze hard only momentarily the water doesn't have time to be displaced as much, so even if you are spending the same total amount of timing squeezing, if you "give it a rest" then the water will stay.

Hydrodynamic wedge is created by oil pressure among other things. The Cummins I rebuilt had 30psi oil pressure just after start-up at low idle. Oil warmed it went to 15psi at low idle as it thinned out. It reached 40psi at high idle (diesel redline) when we caned it.

Hence I am wondering at what oil pressure is the hydrodynamic wedge broken? It has to due with the oil, oil pressure, and the forces exerted on the journals and bearings. Lack of centrifugal speed/time isn't as much of a concern as low oil pressure in this case.

Is oil pressure measured at the oil pump?

If it is the oil pressure is more of an indication of flow. Bearings aren't sealed I think so oil pressure probably doesn't have to do with that. I read on bobistheoilguy that the oil pressure being high when the engine is cold means the oil is actually not flowing enough and hence the engine experiences the greatest wear.

The hydrodynamic film is held by surface tension or whatever it's called lol. Oil sticking to the surface of the metal. Don't remember what that's called.

We had a pressure gauge on the main oil rifle that supplied the cam, crank, bottom end, gudgeon. It's a shop engine...

This is from the Cummins Manual, it shows the oil circuit of the engine. I hope this explains a bit because until I actually rebuilt an engine I had no idea what the oil circuit looked like or how oil got where...

http://i679.photobucket.com/albums/v...Oildiagram.jpg

Actually that definition is partially correct. The engine experiences bearing wear because while the engine is not rotating (stopped) there is little oil between the bearing and journal. Why? No oil pressure!

If the engine oil is not flowing enough after cold engine start up then someones made an error with the oil viscosity or the oil pump is kaputski (among other mishaps). It should reach an acceptable hydrodynamic film pretty much right after start. The oil should be flowing because the oil pump is positive displacement (typically a simple gear type), if not, the oil is extremely cold/wrong viscosity for conditions.

Sorry for going Way OT

On bobistheoilguy it said that ALL oils are too thick at startup, the thinnest synthetic bases are like 70cS at room temperature or something, while at operating temperature it goes down to single digits cS (the correct viscosity), which is why you don't rev a cold engine, because the cold oil doesn't flow well. Since the oil pump is positive displacement I take this to mean that the oil doesn't penetrate the bearings or something like that.

Thanks for the picture though.

EDIT: Oh HomemadeWRX I just noticed your comment...gee don't even get me started on that, I'm in math for god's sake :P old greg if it seems like I think I know everything, sorry for being such an eyesore.

It was meant lightheartedly. :happy0180:
You're obviously a very bright guy, you just have the confidence of youth. :)


Well, since you asked nicely... :D

Honestly, I'm really not an engine guy. Most of the stuff you guys talk about in that area goes right over my head, but I do know a bit about bearings.

Main and rod bearings are hydrodynamic bearings, they operate due to relative motion, not oil pressure. Oil pressure is required only because, as you mentioned earlier, the bearings are not sealed. The basic theory is that when the crank is not quite concentric with the bearing (due to load) the relative motion between the crank and bearings is trying to drag the oil through too small a gap for it all to fit. This creates a pressure differential and a force that counteracts the load without the crankshaft actually touching the bearings. The closer the crank gets to the bearing (higher load) the more force is generated, and the higher the speed the more force is generated for a given crank/bearing gap.

Lugging occurs when the load on the crankshaft (from cylinder pressure) is high enough that oil alone can not provide an equal reaction force and the crankshaft contacts the bearings. It happens because there is not enough relative motion to support the load, rather than not enough oil pressure.

hahaha thanks...
That makes sense. I read somewhere that if you're in high gear going up a steep slope and you're flooring it but the engine is decelerating anyways that's called "lugging". But the forces on the the bearings wouldn't change so this shouldn't damage the engine right?

Ahh yeah i see that.

You know i was looking at some different brands for headers and dyno runs on legacy and 2.5rs. So far i really don't see that big of a dip for TWE, OBX and Cobb headers. they look smoothened out, at the very least. I still see hints of that dip, and im looking at other threads with AFR readings and it gets pretty rich around 3-4k for the N/A motors. Lots of people have better looking curves with that ranged leaned out. But borla and some other knock offs with the same sort of collector design still show that dip regardless of increased in power.

When you keep saying relative motion, are you meaning bearing surface speed...which is the driving force of the boundary layers?

Yes. Like I said, I'm not an engine guy. :)

Think of it like riding on skis. If you slow down too much you'll start to sink.

Ok, for all you smarty pants, if torque is what moves you and is also what you ''feel'' why do F1 cars have basically no torque and super high Hp? I still argue that, in the end, HP is more important. That's why we see power to weight, not torque to weight metrics.