Effect of cruising RPM on wear?

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554
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USA
Hello everyone, I am wondering what effect cruising RPM has on engine wear. Obviously extremely high RPM (say cruising in 2nd gear near redline) is going to cause premature failure, no need to discuss that. What I am asking about is more whether the transmission's gearing has an appreciable effect on engine life. For the purpose of this question, let's assume that we have 2 different cars with the same engine that are used in the same way and both maintained correctly, only difference is one has a 3 speed trans and cruises at 3K RPM at 60 MPH and the other has a 4 speed trans and only runs at 2K RPM at 60 MPH. Let's also say that both cars are driven mostly on the highway. With all else being equal, would the engine attached to the the overdrive transmission last significantly longer than the engine attached to the 3 speed transmission?

Does anyone have any experience that suggests that a lower cruising RPM appreciably extends an engine's life? Something like "I worked at a car dealer and saw that model X with a 3 speed transmission had a worn out engine at 100K miles, but model Y with the same engine and an overdrive transmission made it to 150K miles without problems".
 
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1,965
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USA
With all else being equal, would the engine attached to the the overdrive transmission last significantly longer than the engine attached to the 3 speed transmission?

Difficult to say but with all things equal and assuming properly designed/maintained for the application.....

ALL machinery prefers to run "steady state" and will achieve the longest lifecycle in that mode. ( provided that "steady state" is within the guidelines established by your and my conditions)
 
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798
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98245
Intuition says the lower revs promote longer engine life. But, my experience with motorbikes says no. Typical 4-cyl Japanese motorbikes pull around 5,000 RPM going down the freeway. And these engines last well beyond 100k miles when properly maintained. Of course they redline much higher, so those revs are nowhere near redline.

In your example, the lower revs save fuel but they have no impact on engine longevity.
 
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263
Location
STL, MO
I like to use the analogy of a human being in this regard. Will an avid runner's body break down prematurely over someone who walks very little? It's to hard to say, as there are so many other variables in question. The avid runner will consume more energy though. In both cases, considering both are well maintained, the runners body may possibly succumb to joint wear earlier, but the difference would be negligible. There is a greater chance of either person being effected by an outlying or genetic issue before their activity level causes problems.

Now, this is a very generalized analogy (so take it with a grain of salt), but hopefully it gets my point across.

I've never understood some peoples aversion to RPM. ICE parts are made to spin, and have over a hundred years of engineering baked into them to allow them to do so. Raised RPMs are MUCH healthier than lugging, so let them spin.
 

Avery4

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554
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USA
I've never understood some peoples aversion to RPM. ICE parts are made to spin, and have over a hundred years of engineering baked into them to allow them to do so. Raised RPMs are MUCH healthier than lugging, so let them spin.
Thank you. I agree about high RPM being better than lugging. I know people who put their car in 5th or 6th gear and mash it at like 1200 RPM going up hills or accelerating instead of downshifting to an appropriate gear because they think it saves fuel and wear since the RPMs are lower. I tried explaining that lugging the engine isn't fuel efficient and it causes more wear, but it doesn't help. :rolleyes:
 
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1,468
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Perris, CA
I don't think it matters to a significant extent as long as you're not operating the engine constantly at the edge of its limits, like bouncing it off the rev limiter all day. Wife used to have a 3-speed Cavalier that spun over 3k doing about 70 down the freeway, lasted to 237k miles and the engine was still going strong when the transmission failed. Her current Versa is geared even lower, 3k at 60 and 4k at 80. About a month ago we drove about 1,000 miles in it doing 75-80 the whole way, averaged 37 mpg and it's still going strong at 120k.
 

Avery4

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554
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Since the pistons will have moved less in the2k engine than the 3k engine at the same assumed mileage, then it would stand to reason that the 2k engine would have less wear.
It seems that way, but I'm not sure that's necessarily the case. Even though the pistons moved less times, they were under more load the times they did move since the engine's power output would be constant to maintain that speed. So if RPM is lower, the engine would need to deliver more torque. The parts of the engine that aren't affected by load such as the valvetrain and accessories would likely experience more wear with all else being equal.
 
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Alberta
In general terms, my understanding is that higher RPMs wear the valvetrain more, lower RPMs wear the bottom end (cranks and piston bearings) more. In very general terms.
 
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1,965
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USA
Remember, "wear" is both a culmination and combination of numerous factors such as material of construction, heat treatment, tolerance, mass, surface finish, lubrication, load, temperature, torque, deflection, physical alignment, geometry, cumulative fatigue and a host of others in addition to just RPM ( or cycles/stroke for linear motion).

Its hard ( impossible actually) to give general claims and statements with a reasonable degree of accuracy regardless of how logical a single point may be.

I point that out because its a norm in many manufacturing schools of thought to play games with physics in order to cut costs. Many times a lesser grade of material is used with a higher finish/ different hardness/ mass etc. to compensate.

That can and does work to a point but when it gets in the failure curve, it almost gets hyperbolic in speed to failure.
 
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Central Arkansastan
I base mine on the fact that most otr trucks pile up 5-700000 miles on them during their life, and rarely run above 15-1700 rpm. Yes, they are "beefy", but the materials used in their construction is similar in element. The honda/toyota long stroke engines of the mid and late 80's usually accumulated high miles before overhaul, and cruised 70mph at 1500 rpms.
 
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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.

From watching 1/2 ton truck clap out/ leak like sieves over the last 20 years of being in the boating industry - Load is a bigger factor in wear than mere RPM.
 

Avery4

Thread starter
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554
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I base mine on the fact that most otr trucks pile up 5-700000 miles on them during their life, and rarely run above 15-1700 rpm. Yes, they are "beefy", but the materials used in their construction is similar in element.
That is true, but there are other significant differences as well, not the least of which is that their engines redline much lower than typical passenger car engines, so 1500 RPM is usually over half of redline on those engines. Another big difference is that they are driven much more frequently than most people drive their cars, meaning that they accumulate mileage much more quickly and are therefore less likely to be scrapped with low mileage due to rust and other age related problems. The type of driving they do (mostly cruising down the highway) is typically much less stressful on the engine than the type of driving most people do (short trips, traffic, etc).

If cars were driven all day like OTR trucks and maintained correctly I suspect most of their engines would reach 500K miles or more too, there are quite a few instances of cars that made it to 500K miles or more on the original engine without any issues or measurable wear. These days, it is incredibly rare for engines to significantly wear out or catastrophically fail due to internal issues regardless of mileage unless it was neglected or abused. Nearly all of the catastrophic engine failures I have seen and heard of have been from lack of maintenance, broken timing belts, low/no oil, overheating, revving above redline, etc.
 
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3,183
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Western S.C.
The hypothetical lower-speed engine will operate with lower oil temperatures--- among other components.

A couple of data points:

The crank of my Subaru (listed below) rotated about 3700 times per mile in top gear (thus 3700 RPM at 60 mph, proportionate at other speeds), and that engine showed signs of ring wear by 150k miles.
The Mazda crank spun more like 2600 revs/mile, and significant wear never seemed to happen, even after much higher mileage. However, there are a lot of uncontrolled variable in that comparison, including leaded vs. unleaded fuel, very different bore/stroke ratios ...
 

Avery4

Thread starter
Messages
554
Location
USA
The hypothetical lower-speed engine will operate with lower oil temperatures--- among other components.

A couple of data points:

The crank of my Subaru (listed below) rotated about 3700 times per mile in top gear (thus 3700 RPM at 60 mph, proportionate at other speeds), and that engine showed signs of ring wear by 150k miles.
The Mazda crank spun more like 2600 revs/mile, and significant wear never seemed to happen, even after much higher mileage. However, there are a lot of uncontrolled variable in that comparison, including leaded vs. unleaded fuel, very different bore/stroke ratios ...
3700 RPM is high, that's like 4600 RPM going 80 from my calculations! :eek: What did that engine redline at?
 
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3,183
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Western S.C.
3700 RPM is high, that's like 4600 RPM going 80 from my calculations! :eek: What did that engine redline at?
The rated power was at 6400 RPM---despite push-rods. That gearing was typical of small engines in small imported cars before 5-speed "overdrive" transmissions became common circa 1980 or so. My brother's VW Dasher was geared about the same. Some spun even faster, early 70s Corolla, Datsun 1200, or Fiat 128, for example.
 
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