Does the "Red Line" Shift with Mileage?

[SSTJ]

Rookie question, which I suppose applies to any vehicle really: does the RPM "red line" change with high mileage? Is there any mileage milestone at which one should consider a lower RPM to be the effective "red line"? I have the four-cylinder engine, if that matters.

 

[707kevin]

No.

 

[707kevin]

Ok more.
If you don't think the engine can handle touching redline, then you need to fix your motor. Otherwise...why wouldnt it be ok?

 

[sab]

No.

I disagree to a degree. Plain bearings rely on precise clearance to work. Too much, and oil pressure can’t develop adequate film thickness, resulting in metal to metal contact. Not enough, and the same thing happens, but for a different reason - there’s not physically enough room to develop film thickness. In an old engine, plain bearings are generally worn so that the clearance is getting to the maximum limit. Open throttle, high RPM (very rarely are you not at WOT when you approach redline) is when the loads on those bearings are the highest, and significantly so. If your engine has lead a hard life, limiting RPM can prolong it’s life some. How much? Maybe not enough to matter. I’ve never seen data on that, but in theory, it could matter.

 

[707kevin]

I disagree to a degree. Plain bearings rely on precise clearance to work. Too much, and oil pressure can’t develop adequate film thickness, resulting in metal to metal contact. Not enough, and the same thing happens, but for a different reason - there’s not physically enough room to develop film thickness. In an old engine, plain bearings are generally worn so that the clearance is getting to the maximum limit. Open throttle, high RPM (very rarely are you not at WOT when you approach redline) is when the loads on those bearings are the highest, and significantly so. If your engine has lead a hard life, limiting RPM can prolong it’s life some. How much? Maybe not enough to matter. I’ve never seen data on that, but in theory, it could matter.

Then see my follow up comment.

 

[Apparition]

Drive it like you stole it until you kill it or sell it. No regerts.

 

[JPHikr]

It depends on the ridge on the cylinder wall.

 

[707kevin]

It depends on the ridge on the cylinder wall.

If you hit redline more often, the ridge would not be an issue, right??

 

[jjvw]

The automatics shift when they want to. Oftentimes well above 4500rpm. What do the old delicate engine pearl clutchers do about that?

 

[BlueC]

Hold it down until you hear a knock, then back off a couple of hundred RPM.

 

[sab]

The automatics shift when they want to. Oftentimes well above 4500rpm. What do the old delicate engine pearl clutchers do about that?

We drive manuals, although, admittedly, the extra hand- and footwork required does make it hard to clutch pearls. You have to choose your poison...

 

[jjvw]

We drive manuals, although, admittedly, the extra hand- and footwork required does make it hard to clutch pearls. You have to choose your poison...

Same engines. That's the point. There is a long history of manual drivers scared to wring out their Jeep engines between shifts. The automatic crowd can't do anything about it. Yet we rarely, if ever, see any concerns about it.

 

[SSTJ]

@jjvw

We are in the middle of a move. So assembling furniture has been how we spend a lot of time lately. Your profile pic made me laugh so hard I took a screenshot of it and shared it with my wife.

 

[SSTJ]

Thanks for the replies so far. Eager to hear any other thoughts on this.

 

[BlueC]

Eager to hear any other thoughts on this.

So to answer your original question of does the red line change with time/mileage, simple answer, no. Theoretically, it is likely to drop a little due to weaker springs and/or mass buildup on the valves over time. I would expect any change to be minimal and not significant in any way. The factory computer is set to "rev limit" several hundred RPM before any mechanical issues.

 

[hear]

So to answer your original question of does the red line change with time/mileage, simple answer, no. Theoretically, it is likely to drop a little due to weaker springs and/or mass buildup on the valves over time. I would expect any change to be minimal and not significant in any way. The factory computer is set to "rev limit" several hundred RPM before any mechanical issues.

I think the right question to ask is "what determines what the red line is for a brand new motor?" And then determine if time/wear/etc degrade any of the components or clearances that are used to answer that question.

 

[BlueC]

"what determines what the red line is for a brand new motor?"

I'm not an engineer or engine builder, just a gear head, but typically the limiting factor is valve train control. I would think that's the case for 95% of engines. When control is lost, pistons and valves start kissing. I would think bearing surface speeds would be next in line, then piston speed.

 

[hear]

I'm not an engineer or engine builder, just a gear head, but typically the limiting factor is valve train control. I would think that's the case for 95% of engines. When control is lost, pistons and valves start kissing. I would think bearing surface speeds would be next in line, then piston speed.

I thought the 4.0L was a non-interference engine, so valves & pistons shouldn't ever kiss (or at least that was my understanding)....but I'm not really even a gear head so out of my depth here. Just trying to take a socratic approach to get to the answer rather than just saying "No."

Of course OP has a 2.5L which may or may not make my point irrelevant, IDK.

 

[Dr. Internet]

I regularly drive my wife's 2000 VW Golf TDI, 4 cylinder Diesel engine. After letting it warm up and driving normally for a few miles, I open it up and run at redline for 10 minutes, or so. It has 111,000 miles and a new timing belt. I wish every vehicle ran as good as this one does. I do the same thing to my '06 LJR, but it won't wind as high as the Golf. In my experience, unless there is something broken, you should red line early and often!
BTW, we had Diesel (Detroit Diesel) V-6 engines on the ship that we used to generate AC power. They ran at red line 24x7 for at least two years. Regular oil and filter changes was it. No problems.

 

[TJ Jim]

So when I was younger and dumber I needed to get to Nashville for a plane flight and was late leaving. 80 miles away. I drove my MGB which had over 100,000 miles on it. Like I said being young and dumb I redlined it all the way which was close to 110 mph and got there in time. Along the way a big blue puff was in the rear view mirror and then not too much farther a second big blue puff. When I tore the engine down two of the pistons had broken rings. So from this my opinion is if you run it hard you’re more likely to brake something on our old engines. But, maybe it was just its time to go. Cylinder ridge broke them maybe?