AWDTerror

iTrader: (2)

Still doesn't answer anything. And I highly doubt that it is clear to you. It seems that when ever this question is brought up, all that is ever given is the same answers, covered up like some mythical black magic, and a person at one time used an analogy of a bicycle rider in high gear trying to go up a hill, which was the best explanation that I have heard thus far, a bit incorrect.

To interject your "hint": Explain how a high load situation is created in the low RPM range within the tallest gear say 1500 to 2000rpms?

The turbo doesn't magically spool where you have 300 load count and witchcraft starts spewing out of the block. Nor do you generate enough torque to break rods, so what happens then?

Plus at that low you would be hard pressed to even maintain momentum to even crest the hill.

If we use the reasoning just in this thread alone, then every Evo X that has gone up a hill on the interstate should have blown up in fire and brimstone. Hell can you imagine the repercussions if we use cruise control!!! Oh the humanity!

Iowa999

iTrader: (1)

Well, this is actually more interesting, then, because it's clear that we're all (now) on the same page as to what the issue is, but disagree on whether the damage would actually occur. It is, of course, only probabilistic, but I'm still not flooring it at low RPMs in 5th any time soon.

AWDTerror

iTrader: (2)

For as long as I can remember, "lugging the engine" was always bad, but whenever anyone asked why, they more often than no, looked at the person like some lost village idiot, although never offering any explanation.

By why is it probabilistic? Is there any incidence where flooring it in 5th has directly attributed to the failure of a motor? Now we can say yes, but to what ends? If we attribute it to knock, then it is more of a tuning issue, than mechanical, but shouldn't a tuner tune for off boost conditions as well?

My opinion:
Catastrophic engine failure is not a direct result of flooring a car in 5th gear, regardless of how low the RPMs may be. Engine failures of that magnitude is a combination of "off boost tuning perimeters" causing excessive AND repeated knocking, which begins to tear apart the motor. The feeling of pulled timing and other things the car tries to do to save itself is masked by the "lugging" condition, and the driver is unable to feel timing being pulled or anything out of the norm like we would during a regular WOT pull and we would let off the throttle.

Now, what is the problems that can be cause by knocking. CTS-V owners, who say have no intention of modifying their car, or say decided on a whim to upgrade their DeVille, to a CTS-V want to still drive it like one. I have seen more failed bearings on CTS-Vs than any other car. Why? Old man river wants to putt around town at 45mph in 6th gear barely pushing the RPMs past idle, and the oil pump cannot simply flow enough volume to lubricate the bearings, say when Mr. Wrinkly ***** decides to pass someone without dropping a gear, or going up a hill.

Now can lack of lubrication and high knock attribute to a catastrophic failure, yes it most certainly can, but looking at the aftermath, can we attribute one thing to another....probably not. To conclude, Iowa, yes you are correct, regardless it is POSSIBLY problematic, in terms of bearing damage and failure, but not at 3,000RPMs. Lower than that, you would be hard pressed to even manage enough power to climb a hill or pass someone.

Iowa999

iTrader: (1)

Here is the explanation, as I understand it.

First, note that engines do not turn at constant speeds, the flywheel and mass of the car not-with-standing. The engine turns faster during each power stroke.

Second, note that the power strokes are more spread out (in time) when the engine is turning slower.

Third, note that it is more difficult for the engine to increase speed in 5th than any other gear, because it requires a greater change in the speed of the car for the same change in speed of the engine.

Finally, note that when RPMs rise slower, it is easier for the car to achieve peak torque at a lower RPM.

Combine all this and you get a car at peak torque at a low RPM when it's harder for the engine to change speeds. So what happens instead? A rod bends.

AWDTerror

iTrader: (2)

If this was the case then load should increase dramatically, but it doesn't, because a 300wtq car can't magically produce 400wtq, even with the long gear ratio.

A lower gear would make peak torque happen faster at a lower RPM, not the other way around, especially with a laggy stock turbo. If that were the case, then drivers would be up-shifting as they come into a corner, and lugging it as they go around.

Even in 4th gear, peak torque is coming in around between 3,500 and 3,750RPMs. 5th gear is going to be higher.

What happens...knock happens.

AWDTerror

iTrader: (2)

To add, what is the most often ignored area for tuning? Part-throttle, because many people don't got automatically to WOT if they are lugging around. You will find that a majority of shops tune for WOT and leave the rest stock.

Iowa999

iTrader: (1)

Two things. First, what we're talking about is people being silly enough to go to WOT at a low RPM in 5th. That you shouldn't do it begs the question.

Second, the issue with 5th compared to the lower gears is that it is harder for the pressure in the cylinder to rotate the crank when it presses down on the piston. What the rating of wheel torque is isn't the issue. The problem is that the force must go somewhere, so if the crank is harder to turn, the rod bends. In the lower gears, the crank is easier to turn, so that's what happens, instead of the rod bending.

AWDTerror

iTrader: (2)

This is true but it is fun to debate mechanical theory, and theory in general. And yes it is silly, and most with common sense should never have an issue arise, because they should never do this, but it is fun to talk about.

Now to add:

I see what you are saying, but the loads still have to be great enough to bend the rods, and at that low of an RPM, your not generating enough force for that to break the rods, your definitely not even close to exceeding what forces you see at WOT. Now I can see a knock issue exacerbating this issue. Coupled with the knock and the extra force, I could see potential for catastrophic failure.

Iowa999

iTrader: (1)

Yes, it might take something else, such as knock, for a rod to be bent by going to WOT at a low RPM. What I've been trying to explain is why damage is much more likely in a higher gear.