RPMs affect on engine longevity

[Razl]

I know in general, higher engine RPMs results in a shorter engine life. But why? Is it due to A)higher stress level of parts causing increased wear or B)the accelerated rate of wear directly related to accerated accumulation of engine revolutions? To clarify, in B), higher RPM don't actually cause more engine wear, you are just reaching the end of an engine's life that much quicker. Just curious

 

[Drew99GT]

I dunno about this theory cause of all the high strung econo boxes out there that turn well over 3,000 rpm on the highway and go hundreds of thousands of miles. Honda Civic and Toyota Corolla come to mind.

 

[rpn453]

For most cases, I'd guess A. For engines whose trannies are geared lower but the car isn't driven hard at all, it could be B. I think in most cases much more wear would occur from driving hard than driving at high rpm's though. Some cars rev at 4000 rpm on the highway, while some run at 1400 rpm. They all seem to last. For a given car at a given speed; at half the revs the engine has to produce twice the torque to move the car (for a reasonably flat torque curve such as in a naturally aspirated gas engine).

 

[XS650]

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Originally posted by Drew99GT: I dunno about this theory cause of all the high strung econo boxes out there that turn well over 3,000 rpm on the highway and go hundreds of thousands of miles. Honda Civic and Toyota Corolla come to mind.

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The fast turning engines usually have shorter strokes so their pistons may not be moving much faster than a bigger engine at lower RPM. Also the smaller engine's parts are smaller and lighter, that plus the shorter stroke results in loss internal acceleration forces.

 

[Gary Allan]

Well, I'd think, in general, if you measured the piston travel per mile that you would (probably) find like wear among like rates ..assuming all other things were equal. Maybe someone with more smoke will chime in here.

 

[TallPaul]

Very low rpms can affect engine life too, if you are down in the lugging range a lot. The (presumably CAFE driven) shift points in my owners manual would have my F150 4.9L running pretty slow all the time and lugging if I ever put my foot into it.

 

[rpn453]

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Originally posted by rpn453: For a given car at a given speed; at half the revs the engine has to produce twice the torque to move the car (for a reasonably flat torque curve such as in a naturally aspirated gas engine).

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Oops; I put the part in brackets on thinking I had written something different. I guess my mind was going in too many directions at once! The statement is true for any engine or motor, assuming no major differences in drivetrain losses.

 

[ATS-V]

Also, high rev engines are generally in lighter cars. Less mass to move, therefor less work.

 

[JHZR2]

at the same time, consider OTR trucks. Lots of overdrive, low RPMs, steady-state driving. Those trucks go a million miles or more, very often. Id suggest that longevity is mainly a function of power output. a 3000 lb. car, going 60 mph needs ~60hp or so. That same 3000 lb car accelerating to 60 MPH, particularly if not driven gently, is using much more than 60 hp. That car, whether its geared high or low, going along at 3500 rpm or 1500 RPM, still needs more or less the same amount of power, even if the mechanical advantage from the gearing is wildly different. In a word, both power requirements are low. Ill bet it takes more power to contantly accelerate away from stoplights to 30 mph, than it does to cruise at 60 mph. that power output puts thermal (remember, an IC engine is only about 30% efficient, so for every 100 hp, there iw 200 hp worth of waste heat emitted and transferred through the moving parts) and mechanical stresses on the parts, not to mention the oil. an econobox that does 3500 rpm at 65 mph and has 200k miles likely has seen less overall stress than a more powerful, slower revving car that does stop and go all the time, especially if they have a heavy foot... JMH

 

[Curious Kid]

I pondered something along these lines some time ago. Lugging an engine perhaps translates to transition from hydrodynamic lubrication in the main and connecting rod bearings, as well as low cooling oil and coolant flow rates (developement of hot spots). Maximum pistion velocity and relative forces exerted upon the bearings and transfer parts vary among displacement and stroke and rpm, with smaller engines tending to be able to run at higher rpm vs. larger displacement units...as mentioned above. I then thought about piston circumference, and how wear rates seemed to favor larger bores, which seems to make sense with how truck engines could last so long (I can't recall how I concluded this). But, these trucks are designed to use a heavier weight oil (generally higher HTHS), operate longer per start, and also don't have to deal with cylinder washdown like gasoline engines do, due to the nature of direct injection that modern diesels have. Diesel fuel also has greater lubricant value, but I'm not sure how that might be applicable if it burns completely/desent washdown. For myself - so many variables, so little knowledge. Interesting.

 

[moribundman]

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I know in general, higher engine RPMs results in a shorter engine life.

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What do you consider "higher RPM"? I'd say 5k RPM and up. Some engines have premature engine component failure when they are run with low RPM. I consider too low RPM (as soon as the engine is being lugged) more detrimental to long engine life than high RPM. Speed will also figure in. High RPM at lower speed will certainly not be as hard on an engine as high RPM at top speed.

 

[1sttruck]

I too think that lugging is worse than higher RPM, especially when 'higher RPM' is often still well within regular operating conditions. For instance I prefer to keep the Taurus in drive until hitting a sustained 65mph where I'll shift into OD, preferring to let it run up to 3000 rpm instead of lugging it in overdrive 'for better mileage'. I'll take some additional piston travel over lugging the powertrain, which affects the tranny as well as the lower end. The truck has a diesel and they REALLY don't like being lugged. As someone else mentioned piston speed is what you need to watch, where a 'low rpm' long stroke engine can end up with 'high' piston speed at fairly low rpm compared to an oversquare short stroke engine that can rev all day long with no problems. Other than the turbo on my truck the highest sustained rpms that I've been around were when using an ultracentrifuge. It had a titanium cage in case a rotor came off. Everything was balanced down to the milligram level, it was run at a decent vacuum close to freezing, sometimes for 48 hrs at 60k rpm.

 

[y_p_w]

I think if you're talking about conditions in the same engine, higher revs means more stress. But not all engines are the same (duh). Energy goes up exponentially with speed, and I suppose friction and the effect on lubrication also go up exponentially with revs. I recall an article on the sale of retired Ferrari F1 cars. Each car came with a freshly rebuilt engine. They were slightly detuned with a redline 1000 RPM less than the race-ready version. It was claimed that this would double the mileage before a rebuild would be needed. They also had a button to override the more restrictive rev limiter. You could have one of these for about $1.8M + rebuilt engines as needed. I remember back when I owned a 1995 Acura Integra GS-R, the sales literature noted that the engine had the highest piston speed (redline at 8000 RPM) of any production car in the world at the time. I took it there a few times and it was running OK at 135K miles (although it was using a bit of oil).

 

[Big O Dave]

Rod length/stroke ratio comes into play here, too. The lower the ratio (the shorter the connecting rods are in relation to the stroke length), the more lateral force that is exerted on the piston, which wears the block's cylinder bores. This "side motion" is less severe at lower rpm. A classic example is the SBC 400, which had shorter rods than a 350, but a longer stroke. This, when coupled to siamesed cylinder bores (no cooling jackets between the cylinders) and no added tin/nickel in the cast iron, was a recipe for a 70,000 mi engine. These engines, though, would last reasonably long IF they were not revved above 3,400 rpm or so, and proper attention was paid to keeping the cooling system healthy. Just my $.02.

 

[crossbow]

Don't forget that some engines "need" high revs to survive. On certain engines (rx8 comes to mind) carbonization occurs if the engine its "stretched" rather frequently. There is a starting rash of engine failures all from ATX Rx8 owners because the standard shift points that the atx shifts are FAR FAR below the upper rev range. The engine never stretches...gets all carbon'd up...bye bye engine. Additional the ATX rx8 drivers tend to be individuals who just purchased the vehicle for its looks, and in some cases, have never passed 5,000 rpm. So in some cases its not wear thats your enemy, but other things related to engine rpm.

 

[Drew99GT]

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Originally posted by XS650:

quote:

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Originally posted by Drew99GT: I dunno about this theory cause of all the high strung econo boxes out there that turn well over 3,000 rpm on the highway and go hundreds of thousands of miles. Honda Civic and Toyota Corolla come to mind.

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The fast turning engines usually have shorter strokes so their pistons may not be moving much faster than a bigger engine at lower RPM. Also the smaller engine's parts are smaller and lighter, that plus the shorter stroke results in loss internal acceleration forces.

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Not necessarily. In fact, many import engines that run oer 8,000 rpm have way worse rod/stroke ratios than a SBC, meaning the have incredible piston speeds (where do you think the power comes from from such a tiny engine!). Honda's B18C comes to mind (Acura Integra engine).

 

[Tosh]

Warning: Sweeping generalization ahead! Wear is related to friction, which increases with the square of the speed. This obviously is only a small snapshot (and carefully selected, too) of what is involved in designing for friction, wear, lubrication, etc., but it seems to answer the thread-starter's question about engine wear and RPM. I can appreciate the idea of wear not being a linear function dependent only on speed and time, ie total distance traveled. So I don't turn my fan on full speed and feel wasteful for driving fast and wearing out my tires disproportionately faster. But then again, I think I might have a problem obsessing over mechanical things... (Glad I found this forum)

 

[Drew99GT]

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Originally posted by Tosh: But then again, I think I might have a problem obsessing over mechanical things... (Glad I found this forum)

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I KNOW I do and it only gets worse the more I spend at this website.

 

[Doug Hillary]

Hi, some real world revs/wear date exists in the history of heavy diesels A well known diesel engine maker produced a four stroke V8 engine family for some considerable time - perhaps for two decades or more. In this time the engine was developed from a high speed "revver" (3600rpm) with very little torque (high torque is most important in heavy trucks) to a slower speed "lugger" (2400rpm). Originally they were a fuel inefficient engines too! Obviously the engine became turbocharged and (later) intercooled and rose in HP from around 250 to about 400+ (obviously the torque increase was of a greater value/relationship They were originally known as "Christmas" engines as they needed an in-frame overhaul (liner/piston kits, bearings and etc) annually, and usually after about 180-220k kms! As they were slowed down and run at about 2200rpm against 3200rpm the engine life increased dramatically. Now of course some of this was due to newer technologies (and lubricants) but we believe that most was due to the lower revs - they became an engine to rebuild "as needed" and usually at double/triple the previous intervals - around 450kkms or more The real problems in the change were things like driveline vibration, cooling, oil pressure, fuel temperatures and a few others. These were solved by dual pass radiators, increased under bonnet air flow and fan speed, innovative oil coolers and water pumps, air to air intercoolers (instead of air to coolant), better thermostat control and etc. I think these engines are still around today as "throw away" engines used in Airport Fire trucks, Landing craft etc where they develop very high power (500-600hp) and go from zero revs to maximum on demand - often from cold. Their lifespan in hours is very short I believe! In the last development as a truck engine I have seen them reach 800+kkms without a rebuild! They were quite bullet proof in the end Regards Doug

 

[1sttruck]

Sometimes more revs seem to help with engine life. From what I observed the V-twins in motorcycles seem to 'beat themselves to death', especially when hp has been increased, and they seem to need to be rebuilt more often than say a 4 cyl of similar displacement.