Effect of cruising RPM on wear?

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6,336
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KY
My Club Sport has a close-ratio 5 speed; 5th gear is 1:1. At 80 mph it's turning 4,000 rpm. I've run it at that speed for hours at a time. Its closing in on 150k miles and it runs like new.
 
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704
Location
Alberta
I would think that the lower revving engine would eventually show less wear than the higher rev engine, but in today's engines the difference wouldn't start showing up until very high mileage.
 
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715
Location
FL
When I tow I run 3k rpm in 3rd to prevent going in and out of overdrive and lugging.
My 5.3 is ok running 3k plus on the interstste. Oil probob ley takes a little more of a betting.
 
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7,127
Location
Roanoke Virginia
I would say yes. I never cruise mine when it starts getting up to the end of the gear I shift it. Last time I let it go for awhile something in the transmission started to rattle and hasn’t stopped since so I don’t know what it is lol 😂 hope it’s nothing that will fall apart on me.
 
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728
Location
EU
Cruising at elevated rpm is certainly detrimental, fitting the small engines to heavier vehicles as in last century's european SUV or limousines of course is what really has one consume some volatile metals as they force into load & revs.
But downsped / downsized engines of today and sufficient overdrives make for a new era and high revving nonsense is mostly gone anyway. Underdrive is for blowers when there's a variator missing.
 
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72
Location
Europe
I'd stick with selecting the highest gear possible and the lowest gear necessary.

Meaning the highest gear in which the engine manages to supply the necessary torque to the wheels without lugging it.
I want to keep fuel consumption, noise and vibration as low as possible.

From a wear-perspective, I guess it does not really make a difference whether you cruise in 5th gear at 3000rpm @80mph or in 4th gear at [email protected]
In the first scenario, friction and centrifugal forces are lower.
In the second scenario, the oil (pressure) and coolant flow through the engine is higher (for traditional oil and waterpumps) while the peak cylinder pressures are lower.

If an engine is properly engineered and has reached operating temperature, wear is almost 0 if you stick within certain parameters (load / rpm).
This intended operating range is much further than + / - 500 rpm and will most likely not have a noteable impact on engine longivety whether a car cruises with 1000rpm more or less.

By the way, the old MB diesel engines which often rack up multiple million kms do have a very short gearing as well.
The W124 300D w/ MT turns with around 3000rpm at 80mph.
 
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1,965
Location
USA
More cycles creates more wear in a given time if load is identical, but thanks to Moft there isn't really a legitimate linear equation I've been able to find.

In general terms that's correct UD but remember that only the journals have MOFT, everything else is a mixture of mixed and boundary so we have to further qualify wear into which part or region are we referring to or a machine as a whole.

Even then, as stated up thread, its not so much the 'steady state' ( defined as everything equalizes) that is the wear generator but the period between "X" and "Y" ( commonly referred to as the "revving process") where mechanical contact ( and all that involves) exceeds the lubricants ability to counteract.

That's where finish, dimension and geometry change leading to getting on the failure curve.
 

4WD

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16,815
Location
Texas
Another part of this is what that engine is pushing … I see that in our boxy shaped SUV … at some point all the gears, AFM, etc just can’t deal with mediocre aerodynamics at high speed and the ECM has to increase engine RPM … I doubt at that moment wear significantly increased … One of the great things 6 speed to 10 speed AT’s bring us.
 
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2,291
Location
WA
I would guess the least engine wear is at the minimum RPM that would result in the optimum/max hydrodynamic lubrication. so it depends on many different factors and variables ...

Just a guess, I ain't no expert!
 
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1,965
Location
USA
I would guess the least engine wear is at the minimum RPM that would result in the optimum/max hydrodynamic lubrication. so it depends on many different factors and variables ...

Just a guess, I ain't no expert!

Extremely close but its the relative duality relationship along the curve between the RPM ( cycles) and the load ( required force resulting in stress to the component).

Even in the best cases, modeling and math "might" get you to 80% accuracy because of the virtually infinite different factors/variables affecting that relationship
 
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3,183
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Western S.C.
... remember that only the journals have MOFT, everything else is a mixture of mixed and boundary so we have to further qualify wear into which part or region are we referring to or a machine as a whole. ...
Yes, and stress on components that are subject to significant inertial loading goes up with the square of speed.
 

Avery4

Thread starter
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554
Location
USA
Thanks for all the responses everyone, I appreciate all the information. I have a couple things to add.

First, I'm not sure that slightly elevated RPM as described above does result in meaningfully higher oil temps. I have an oil temp gauge on my 05 Civic with a 1.7 VTEC engine (a JDM D17A) and I see no meaningful difference in oil temp between cruising in 3rd gear or 4th gear, oil temp always equals coolant temp give or take a few degrees regardless of RPM. This could be because I installed a large oil to coolant heat exchanger, I unfortunately didn't try that test before I installed the cooler.

Second, lower RPM doesn't always result in more MPG either. I consistently get about 2 more MPG if I leave my trans in D3 around town and only shift to D4 once I am running at 3K RPM or more (45+ MPH) compared to leaving it in D4 and letting it shift into overdrive as early as possible. Torque converter lockup isn't a factor, I have manual lockup and always cruise with the converter locked. Every time I have tried the traditional advice of running at the lowest RPM possible without lugging the engine, my MPG has been lower. You'd think that cruising at 1800 RPM in overdrive would be more efficient than cruising at 2600 RPM in 3rd gear, but not always. My theory is my Crower camshaft may not be efficient at less than 2K RPM, but I have no plans of reinstalling the stock cam to verify that theory.
 
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36,473
Location
ME
I've heard that an engine will run "forever" at 80% load at 80% of redline RPM.

Too little load, like revving in neutral, is said to be bad b/c the con-rods stretch out. They wouldn't if they had a beefy a/f mixture to compress.
 

Avery4

Thread starter
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554
Location
USA
I've heard that an engine will run "forever" at 80% load at 80% of redline RPM.

Too little load, like revving in neutral, is said to be bad b/c the con-rods stretch out. They wouldn't if they had a beefy a/f mixture to compress.
Very interesting, I have never heard that before. I wonder how true that is.
 
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2,046
Location
Minnesota
Both motors are producing the same horsepower, one sees higher rpm, the other sees higher torque. Don't know the answer but I would think it would depend alot on the design of the motor.
 

Avery4

Thread starter
Messages
554
Location
USA
Both motors are producing the same horsepower, one sees higher rpm, the other sees higher torque. Don't know the answer but I would think it would depend alot on the design of the motor.
I bet it would. It would also depend a lot on what the weak link on said engine is. For example, if an engine is known to wear out its timing chain early, a lower RPM would most likely be better since the chain is directly affected by RPM and not directly affected by load. However, if the engine in question was known for wearing out rod bearings early, I suppose a bit shorter gearing would be better (within reason obviously) to keep the load on the bearings down.
 
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