Effect of cruising RPM on wear?

Messages
4,624
Location
Ca.
Haha funny!

Just speaking of averages the average rig needs something "like" 20 HP at freeway speed which is one reason they last so long.

The informant has the pareto principle reversed.
 
Messages
4,624
Location
Ca.
BITOG. :)

If you think about it, though, airplane and marine engines run near full power most of the time and do ok. Same with small engines in generators and whatnot.

Haha probably!

ehhh the marine stuff maybe the low HP stuff, but not the high end stuff. A 500-800 HP marine big block wants a top end job at about 250 hours and the bottom end usally between 500-1000.

I think most of the Lycoming and continentals want an overhaul at about 2000 hours which is pretty good if you consider distance traveled and speed over ground.
 
Messages
1,671
Location
California
As long as RPMs are within a "normal" cruising range (e.g. 1500-3500 RPM), I believe load has a much larger impact on wear than does RPM.
Engines are generally happiest when loafing along in top gear... moderate RPM, low load, good oil pressure and good airflow across radiator.
 

Avery4

Thread starter
Messages
554
Location
USA
Thank you to everyone who responded, great information! What is the general rule of thumb for how high in the RPM range someone can cruise for extended periods of time without causing damage or excessive wear? I have heard half of redline as a general rule of thumb, but if that was true I think there would be a lot more dead cars out there since some pretty common cars turn 4K or more going 80 and last quite a while.
 
Messages
1,965
Location
USA
What is the general rule of thumb for how high in the RPM range someone can cruise for extended periods of time without causing damage or excessive wear?

That's impossible for anyone to even speculate on unless they have access to the design stress analysis and even then the RPM has to be measured against a load (torque)- any answer without that information is a meaningless guess by an unqualified person and has no legitimate data or science to support it.
 
Messages
1,107
Location
Minneapolis
My dad ordered a '78 G30 with what he expected to be the tallest rear end Chevy had. Instead, he got the shortest.

It was horrible to ride in. It would hit 3rd gear by 15 mph, and just started screaming after that. By the time I started driving it as a bus for my friends in high school, I never wanted to go more than 60 mph. I guess it worked out. But to the point, the engine was perfectly fine at about 160k when the body was too rotten to continue.

My '91 318is would spin at 4100 @ 80 mph. Also ran perfectly at 160k when a drunk driver killed the car.
 
Messages
1,054
Location
Wash, DC
My 1975 Civic had no tachometer. I wanted a motorcycle, but my mother insisted I get something with two extra wheels.
It had a 4-speed stick and on a couple cross country drives I'm sure it was running 4000+ for hours on end..
I'd fill the crankcase with Castrol 20w-50 for said road trips, 10w-40 otherwise.
Eventually, the body surrounding that little sewing machine of a motor rotted away.
 

Avery4

Thread starter
Messages
554
Location
USA
Thanks for the responses, I appreciate all of your input. At what point should one be concerned about a high cruising RPM causing damage or excessive wear when the gearing has been modified to run at a higher than stock RPM? In the Honda community it isn't too uncommon for someone to install a very high final drive diff like a 5.06 in their already short geared transmission, which results in cruising at 4500 RPM or sometimes more going 80. Lots of Honda's cruise at around 4K at 80 MPH and their engines seem to last forever when properly maintained, but obviously at some point the sustained high RPMs would start to do damage over time.
 
Messages
4,709
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 ...

Your Subaru, do you have an H4 (maybe a 2.2 or a 2.5L) or an H6 (perhaps a 3.0 though there are also 3.3 and 3.6Ls?)
 
Messages
3,183
Location
Western S.C.
Your Subaru, do you have an H4 (maybe a 2.2 or a 2.5L) or an H6 (perhaps a 3.0 though there are also 3.3 and 3.6Ls?)
?? You can't see the list in my signature? That was long ago, a 1972 model with the 1267 cm³ (82mm bore × 60mm stroke) push-rod H4. That sort of top gear ratio was typical in small cars for years.
 
Messages
460
Location
AZ
I've always thought this is why Diesels last so much longer, lower overall RPM. i have 2 with well over 300K and a few with over 700K. no signs of stopping.
Engines wear out. The more times the piston has to go up and down has to mathematically wear it out faster then on that does 1/4 as many times.
there has to be a number at what point the rings are just flat worn out, what that number is i have no idea. but i've sure it's around somewhere.
the best thing for an engine or any mechanical devise is let it run, get it warm and all the tolerances within spec and let it run and they usually run for a long long time. vs the short trips or use cycles.
speech to txt usually hates me. Sorry for the errors if present.
 

Avery4

Thread starter
Messages
554
Location
USA
I've always thought this is why Diesels last so much longer, lower overall RPM. i have 2 with well over 300K and a few with over 700K. no signs of stopping.
Engines wear out. The more times the piston has to go up and down has to mathematically wear it out faster then on that does 1/4 as many times.
there has to be a number at what point the rings are just flat worn out, what that number is i have no idea. but i've sure it's around somewhere.
the best thing for an engine or any mechanical devise is let it run, get it warm and all the tolerances within spec and let it run and they usually run for a long long time. vs the short trips or use cycles.
speech to txt usually hates me. Sorry for the errors if present.
Thank you for the information! As I said in post #17, the way diesels tend to be used (in trucks that drive a lot of highway miles) does definitely contribute to them lasting so long. But one thing to keep in mind is that the RPM diesel engines run at is actually pretty similar to gasoline engines a lot of the time. Let's say a diesel engine in a large truck redlines at 2500 RPM and typically cruises at 1200 RPM, that's about half of its redline and is relatively similar to a gas engine that redlines at 6K RPM and cruises around 3K RPM.

Assuming the same RPM, large diesel engines are usually much more stressed than gasoline engines since they are usually built with very heavy parts and typically have very long strokes compared to gasoline engines. So while cruising at 2500 RPM is completely reasonable for a typical gas engine, it is over the redline of a lot of large diesel engines. For that reason, I would think that the RPM an engine is running at relative to its redline would be more important than its RPM compared to a totally different engine's RPM.
 
Messages
263
Location
STL, MO
Thank you for the information! As I said in post #17, the way diesels tend to be used (in trucks that drive a lot of highway miles) does definitely contribute to them lasting so long. But one thing to keep in mind is that the RPM diesel engines run at is actually pretty similar to gasoline engines a lot of the time. Let's say a diesel engine in a large truck redlines at 2500 RPM and typically cruises at 1200 RPM, that's about half of its redline and is relatively similar to a gas engine that redlines at 6K RPM and cruises around 3K RPM.

Assuming the same RPM, large diesel engines are usually much more stressed than gasoline engines since they are usually built with very heavy parts and typically have very long strokes compared to gasoline engines. So while cruising at 2500 RPM is completely reasonable for a typical gas engine, it is over the redline of a lot of large diesel engines. For that reason, I would think that the RPM an engine is running at relative to its redline would be more important than its RPM compared to a totally different engine's RPM.
In addition to RPM in relation to max RPM, load is a factor as well. A lightly loaded engine cruising down the highway at 4000 RPM will last far longer than the same engine heavily loaded at 1000 RPM. This sort of plays into what you are saying, but adds something additional. I just think there is a lot more to it than RPM alone defining the lifespan of an engine.
 
Messages
2,328
Location
Seattle-ish, WA
Assuming the same RPM, large diesel engines are usually much more stressed than gasoline engines since they are usually built with very heavy parts and typically have very long strokes compared to gasoline engines.

Yes, but... They are built for it. Apples to apples, it seems given longevity shown between tear-downs, commercial grade diesels are over-built for their applications vs. gas engines.

But on another angle, to give an answer wrt your first post, I read something interesting last year on this topic that stuck with me. In an article on the topic of gas engine wear, an authority* stated that 5,000 cruising highway miles would be about equivalent in wear to 800 city miles. A very clear and memorable analogy.

* - unfortunately, I can't recall the article in which I read this, and didn't book mark it.
 

Avery4

Thread starter
Messages
554
Location
USA
Yes, but... They are built for it. Apples to apples, it seems given longevity shown between tear-downs, commercial grade diesels are over-built for their applications vs. gas engines.

But on another angle, to give an answer wrt your first post, I read something interesting last year on this topic that stuck with me. In an article on the topic of gas engine wear, an authority* stated that 5,000 cruising highway miles would be about equivalent in wear to 800 city miles. A very clear and memorable analogy.

* - unfortunately, I can't recall the article in which I read this, and didn't book mark it.
That makes sense. I wonder how much of the apparent longevity advantage that heavy duty diesels have over passenger car engines is as a result of being overbuilt compared to being used differently (less startups and heat cycles, more highway miles, more miles per year, maintenance being more likely to happen on time, etc).

Realistically I just don't see engine durability as being much of a problem these days since my experience suggests that properly maintaining and not abusing an engine is really all that is needed to get it to outlast basically everything else on the vehicle. 500K reasonably trouble free miles really isn't that much to ask for out of a properly maintained engine these days. However, not many people even keep their cars half that long, making engine longevity basically a non issue for the average person.
 
Messages
35,696
Location
NY
In the perfect world common sense tells me the less an engine turns to do its job the less wear it will have. But I would also "think" the slower an engine turns there's a possibility more deposits can build up and cause rings to coke up and possibly damage the cylinders. Where the faster turning engine might take longer for the deposits to build up, or be less prone to it.

I often think about a 4 cylinder engine and a V8. In theory a 4 cylinder engine has to rev about twice as much as a V8 to travel the same distance. I've seen many well maintained 4 and 6 cylinder engines outlive V8s. So the answer isn't as cut and dry as common sense would lead me to believe.
 
Messages
1,054
Location
Wash, DC
Let's say a diesel engine in a large truck redlines at 2500 RPM and typically cruises at 1200 RPM, that's about half of its redline and is relatively similar to a gas engine that redlines at 6K RPM and cruises around 3K RPM.
Average piston speed is also a factor.
Think of a small 4 engine with an 80mm stroke at 3000 rpm vs a big diesel with a 160mm stroke at 1500 rpm.
 
Top