Minimum crawling RPM?

[Das_Cheese]

Not entirely sure where to put this, so general it is!

So there's a bobistheoilguy.com thread I ran across where an engineer that took part in designing the 3.6L Pentastar gave a whole bunch of wisdom (disable ESS, drive the motor hard, etc). He said, "the center timing chain idler doesn't go fully hydrodynamic till about 1650 RPM on 5w-20."

So this got me thinking when I had my new 6sp (manual) 3.6L JLUR out on the trails -- what is an accceptably low rpm to be wheeling at?

In my old 2.5L TJ, I run all the way down to 5-800rpm when it's not going to stall it (like flatter ground or slow rocks). With what OIL_UDDER said, is that not a great thing to do with the Pentastar? It's got plenty of torque to climb some fairly steep techy hills in 2hi at 1k RPM +/-200 or so. Or is it fine because that's super low load on the motor? And I guess does the fact that I have the PUG make any difference in this (with lighter oil and whatnot)?

Any insight is appreciated!

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[grimmjeeper]

tl;dr version: It depends.

The deeper the gearing, the more torque multiplication you get. So you can get moving even when the engine is well under it's point where the torque curve gets to an acceptable level. So the exact minimum RPM you need goes down. As tire size goes up, minimum RPM goes up. Deeper gearing cancels out bigger tires.

Ideally, you want to have a decent amount of control when crawling. And that differs depending on a bunch of factors. If you have to choose between slipping the clutch excessively or launching up the obstacle way too fast, your gearing isn't appropriate to the terrain. You need deeper gearing (or smaller tires) to get your RPMs up without going faster. But that point differs between Jeepers based on lots of factors.

If you have an automatic, you get to take advantage of a reasonably well tuned torque converter that has a high enough stall speed to let the engine get up to the RPMs it needs to run well, regardless of gearing.

Lots of different things affect the final number for each individual. So really, there's not a solid one size fits all answer.

 

[Das_Cheese]

It's late, I could be missing the point entirely, but you're talking about general stall point, right? 4.10 gears and the 4:1 tcase, this thing refuses to stall, lol. I'm more referring to how low of rpm becomes bad for the motor. If I'm below 1650 RPM on trail, is the timing chain idler (or any other internal components) causing unnecessary wear due to potentially not being hydrodynamic? I can imagine when you get to the point of the motor shuddering, that's probably bad. But what about just above that, 800+ rpm?

 

[azjl#3]

so, 30psi at idle and 70psi at 3000 isn't enough?

Maybe you heard wrong? You have 30psi, engine is within operating design specs for lower rpm. There's no more there there unless you trip it up to 70 psi, not sure you want to crawl at 3000 rpm.

 

[Ratbert]

4.10 gears and the 4:1 tcase, this thing refuses to stall

Man, I stalled mine lots when crawling over boulders.

I've owned a lot of manuals and that one ('20 3.6L JLUR) was the worst. Maybe they used mine to train their noobs on the assembly line?

 

[wrath]

I think what he's getting at is the amount of oil getting to the front of the poorly designed valvetrain is not sufficient for high load for extended periods of time. It can probably idle for a decade just fine in a clapped out extended interval police cruiser... but idling around up and down hills in a Jeep is probably marginal.

He's probably right. Blip the throttle once a minute and call it a day if you're loading it heavy (more than no throttle/riding on the ECM to do the throttle work) to get oil all the way out to the front of the motor.

If you've ever heard a manual vs automatic 150k pre-2015 JK during a cold start cycle you can literally hear the valvetrain hate life for the first 30-40 seconds on an auto where the manual quiets down within 10-15 seconds. The manual usually hasn't been run dry as much as the auto.

 

[NWJeepr]

When wheeling in the technical stuff, I've always tried to keep RPM north of 2k for both power, and cooling/circulation.

Sometimes it's not possible to do that because lower or absolute minimum crawl speed is needed for obstacles, and there's nothing you can do about that, except put in lower gearing. That's why guys who exclusively do low speed technical stuff will swap transfer cases and go 4.88's, 5.13's so they can inch along and have plenty of power to play with.

Some people like to run 2h or 4h for as long as they can. I'm of the mindset it's better to shift in early.

Don't overthink it. But, if you're lugging all the time on your favorite trails, it's probably time to swap some hardware.

 

[grimmjeeper]

It's late, I could be missing the point entirely, but you're talking about general stall point, right? 4.10 gears and the 4:1 tcase, this thing refuses to stall, lol. I'm more referring to how low of rpm becomes bad for the motor. If I'm below 1650 RPM on trail, is the timing chain idler (or any other internal components) causing unnecessary wear due to potentially not being hydrodynamic? I can imagine when you get to the point of the motor shuddering, that's probably bad. But what about just above that, 800+ rpm?

From that point of view, it really doesn't matter too much. Unless you only crawl the Jeep and never drive it at speed. Spending time at low RPMs won't hurt anything assuming you otherwise drive the Jeep normally.

 

[jadmt]

I could no more tell you what rpm I am at when wheeling than I could tell you who was last years WNBA coach of the year........I just use whatever throttle feels appropriate...automatic but when offroading always in manual mode and when crawling in 1st..

 

[Das_Cheese]

Yeah, maybe I'm overthinking it, but this thing was so expensive, I want to do as much as I can to extend the lifespan, lol.

@azjl#3 - From reading the bobistheoilguy thread, it sounds like less about the oil pressure and more about the properties of the oil itself. The word "hydrodynamic" was thrown around a bit.

@Ratbert - You don't know easy-to-stall until you wheel a gutless 4 cyl TJ with larger tires, stock weight flywheel, and 4.10 gears, haha. The crawl ratio in this JLUR is at least double. (I've since gone to 4.88 and installed a weighted flywheel, it drives like a tractor now, but still not as slow as the JL)

@wrath - Yeah, I'm not sure what the motor internals are like to know the implications of prolonged 800RPM driving (no lugging). The thread I linked sounded like the timing chain idler was the main one, but valvetrain wouldn't surprise me. Will probably take your advice and make sure to give it some skinny every so often to keep things lubed sufficiently.

@NWJeepr - I've always been of the opinion that if you don't need the power, don't run the RPMs up. It feels more efficient and calm to run down easy trails with the rpms as low a I can get them without lugging. Especially with our fans being electric, higher RPM doesn't necessarily result in that blowing more air through the radiator. But yeah, I probably need to stop overthinking it, haha.

@grimmjeeper - Yeah, that's kind of what I figured, just wanted to be sure I wasn't doing something stupid with the motor

@jadmt - I would normally say the same, but I can't hear the motor with this stock muffler. A little more rumble is something I intend to do in the future.

 

[NWJeepr]

@NWJeepr - I've always been of the opinion that if you don't need the power, don't run the RPMs up. It feels more efficient and calm to run down easy trails with the rpms as low a I can get them without lugging. Especially with our fans being electric, higher RPM doesn't necessarily result in that blowing more air through the radiator. But yeah, I probably need to stop overthinking it, haha.

Fans have been electric since at least the beginning of JK's. Cooling isn't just airflow, it's also circulation: Water through the radiator, engine oil through the lubricating circuit, power steering fluid through the cooler, auto trans pump sending fluid through its cooling loops, etc. For all the things which depend on RPM for circulation, a little more engine speed can be better.

 

[Ratbert]

@NWJeepr - I've always been of the opinion that if you don't need the power, don't run the RPMs up. It feels more efficient and calm to run down easy trails with the rpms as low a I can get them without lugging. Especially with our fans being electric, higher RPM doesn't necessarily result in that blowing more air through the radiator. But yeah, I probably need to stop overthinking it, haha.

Try Off-Road+ in 4lo sometime. It keeps the revs way up, implying that the engineers at Jeep aren't concerned about higher revs while crawling.

 

[Das_Cheese]

@NWJeepr - Yeah, that is a good point (though aren't our steering boxes purely electric these days? And ain't got an auto trans pump in a manual, lol). You're saying all those things start happening as you get closer to 2k rpm? I think that answers pretty close to my original question. Running too low rpm prevents some of the essential motor processes from happening.

@Ratbert - Yeah, higher revs are better for torque on the rocks, but what if I don't need the torque in the first place? All the higher revs are going to to once you get to a certain point is just burn more gas and make more noise.

 

[roaniecowpony]

It's late, I could be missing the point entirely, but you're talking about general stall point, right? 4.10 gears and the 4:1 tcase, this thing refuses to stall, lol. I'm more referring to how low of rpm becomes bad for the motor. If I'm below 1650 RPM on trail, is the timing chain idler (or any other internal components) causing unnecessary wear due to potentially not being hydrodynamic? I can imagine when you get to the point of the motor shuddering, that's probably bad. But what about just above that, 800+ rpm?

The last thing you need to worry about in a Pentastar is the timing chain idler. Trust me. But if you are concerned about that particular situation of when the timing chain transitions from boundary lubrication to hydrodynamic lubrication, putting thicker oil in it will lower that transition point.

 

[Das_Cheese]

@roaniecowpony - Good to know, thanks for the reassurance! I'll likely do thicker oil sooner or later for that case. Though seeing that thread raised the question of if there's anything else that I should be concerned about at super low rpm. And I think NWJeepr more or less answered that. It sounds like oil and cooling circuits are probably the main ones.

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