Effect of cruising RPM on wear?

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A former Chrysler engineer posted on this forum a couple of years ago discussing the different impacts oil viscosity has on the bottom end vs the upper end. In short, he mentioned that valve trains last longer with thinner oils, while the bottom held up better with thick oils. where his pos was relevant here was that in the v6 pentastar, the timing chain did not become hydrodynamic until over 1500-1650 rpm, meaning for that engine, the timing components weren’t completely gliding on oil film until above that speed. Makes me wonder when my eco boost is putzing down the road at 1200.... which I do like, but now wonder about. I don’t wonder about it enough to force a lower gear though.
 

Avery4

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A former Chrysler engineer posted on this forum a couple of years ago discussing the different impacts oil viscosity has on the bottom end vs the upper end. In short, he mentioned that valve trains last longer with thinner oils, while the bottom held up better with thick oils. where his pos was relevant here was that in the v6 pentastar, the timing chain did not become hydrodynamic until over 1500-1650 rpm, meaning for that engine, the timing components weren’t completely gliding on oil film until above that speed. Makes me wonder when my eco boost is putzing down the road at 1200.... which I do like, but now wonder about. I don’t wonder about it enough to force a lower gear though.
That is very interesting. I would think the RPM that the components becomes hydrodynamic at would depend somewhat on oil viscosity as well. My buddy has a 2017 Dodge Ram with an 8 speed transmission and it is very aggressive about trying to use the tallest gear possible, even under decent loads. The truck seriously feels like the engine is being lugged while climbing hills at 1200 RPM. Whether that adversely affects longevity I do not know, but it is annoying. I can't imagine that subjecting the engine to that heavy of a load at such a low RPM is good for it, but I am no engineer.
 
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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.
I have always been told that’s why diesels last so much longer than gas engines, because they cruise up and down the highways at a lower rpm. I’ve read that, and I’ve been told that. So, I picture a diesel truck cruising at 70 mph with the tach reading 1,200, while I’m cruising in my gas engine going 70 with my tach reading 2,000-2,500 rpm’s.

During the course of 100’s of thousands of miles, I can see why this would make a difference like you pointed out.
 
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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.
I would have agreed with you (on the engine longevity part) if we were talking 10-15 years ago, before direct injection and manufacturers using low tension rings to meet CAFE regulations. But now? I think it’s a gamble/toss up, to know just how long these newer engines can go “trouble free”. I’m only speaking from experience...my last two direct injection engines didn’t fair too well with proper maintenance and easy highway driving. One made it to 179,000 miles before the oil consumption became unbearable to me (rings were shot), 5,000 mile oil changes using synthetic. The other made it to 52,000 miles before I decided to trade it in (because it was using oil and misfiring).

And I get it...not all modern engines are going to suffer the same fate as my two, but I still don’t think these modern DI engines are making it to 500,000 miles, trouble free. Maybe the port injected ones, before CAFE regulations, but even then? I still think it’s a harder than you think.
 

Avery4

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I would have agreed with you (on the engine longevity part) if we were talking 10-15 years ago, before direct injection and manufacturers using low tension rings to meet CAFE regulations. But now? I think it’s a gamble/toss up, to know just how long these newer engines can go “trouble free”. I’m only speaking from experience...my last two direct injection engines didn’t fair too well with proper maintenance and easy highway driving. One made it to 179,000 miles before the oil consumption became unbearable to me (rings were shot), 5,000 mile oil changes using synthetic. The other made it to 52,000 miles before I decided to trade it in (because it was using oil and misfiring).

And I get it...not all modern engines are going to suffer the same fate as my two, but I still don’t think these modern DI engines are making it to 500,000 miles, trouble free. Maybe the port injected ones, before CAFE regulations, but even then? I still think it’s a harder than you think.
That's a good point, I wasn't thinking about those new pieces of crap since the newest car in my family is a 2005, but we have no problems getting 200K+ trouble free miles out of an engine without significant oil consumption, loss of compression, etc. It does seem like cars are being built to last less and less though, so I wouldn't be surprised if these new engines don't have that same longevity.
 

Avery4

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Here's another hypothetical question. Let's say someone's transmission fails and either loses overdrive or gets stuck in a low gear for whatever reason and the person can not repair it immediately. Up to about what RPM could they cruise at continuously at before worrying about causing damage or significant wear? I am thinking about half of redline would be a pretty safe rule of thumb?
 
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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.
My Toyota Soarer does 95mph at 4000rpm in overdrive.
 
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its very hard to tell just like others have said; modern cars are tuned for fuel efficiency thus their constant low rpms;

here is my story to add; my previous car was a mazda6i, it was a port injected engine; the car would shift to 5gear (highest gear) pretty much immediately after motion I drove it to well above 200K and hand it over to my dad; now the car has 248K miles; so during my ownership engine and transmission performed excellent. I tried to look on the web for some 14mazda6 model since that is their first year of DI application, but in vain, out of luck

now I own 2 K24W engines both DI; for some reason one burns some oil in between OCI while the other does not; however, they both go again in the lowest gear right upfront; yet, from the sound of the engines they're making, they both are highly candidates for 250K without mayor overhaul; heck you have already examples out there of '13Accords (first year of DI for Honda) that managed 200K miles and asking price for those models go to $9K; discard the price, but it just gives you an idea that everything on the car is still in tip-top condition
 
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