Low RPM dangerous for engine longevity?

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New cars with high efficiency transmissions keep the revs so low to save fuel & reduce emissions. I wonder if the engine crank & connecting rod bearings are achieving full hydrodynamic separation of these surfaces? Or are these low revving engines frequently operating with elasohydrodynamic or worse, boundary oil flow at these critical bearings?

I know the finishing of the crank, cam surfaces is far better today, but what else have the OEM engineers done to address this characteristic of low rev operation? Greater pressures from the oil pump? Asking more of the bearing makers? Or is it all up to the tribologists?

I have a 2013 Nissan Maxima SV with the VQ35DE engine & CVT transmission, in my city driving, rarely does the engine exceed 1500 rpms, in-fact it spends most of the time at 1100 rpm. Good for fuel economy, and this engine has lots of torque response from down low, but I wonder about the flip side of this low revs for longevity. CVT with a high torque V6 allows for low & steady engine speed much less up & down of a traditional automatic.

FWIW, I use Pennzoil Platinum 5W30 with either a Wix or Nissan oil filter with changes at 6 months max. I will do a UOA at my next change. And no, I have no plans to try the Ester Oil from Nissan!

Thanks again for your thoughts.
Ken
 
I was taught that a mid size car only need to use 30 hp to cruise on highway speed (i.e. 75mph) despite having 150-200hp for acceleration need.

If the engine is designed for everyday 100hp use for acceleration at 4k rpm, running it at 1500 rpm and 30hp during cruising should be no problem at all. The computer knows to downshift, cut throttle, etc, if it is not good for engine life or fuel economy.
 
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Thats a good question. You would have to think, low rpm, low oil pressure, high stresses...that doesn't sound like a combination for long life. I am no engineer though.
 
Wouldn't this line of thought be in contrast to the ubiquitous "ECO" button?
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Originally Posted By: gregk24
Thats a good question. You would have to think, low rpm, low oil pressure, high stresses ...that doesn't sound like a combination for long life. I am no engineer though.


Not high stress.

Running at low rpm and oil pressure (which isn't low if the oil pump is engine driven and designed into the rpm's worst case need) for the best fuel economy of the need (tall gear) means the car is not in need of power (acceleration, climbing).

This is especially true with automatic transmission (will downshift and has torque converter) and drive by wire engine (will decide whether to open up throttle or increase fuel mixture or retard ignition timing to keep the engine happy).
 
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Well, I can go 55 mph at only 1100 rpms in my 6 speed Corvette. My owner's manual says not to lug the engine and defines that as when the engine bucks or under 900 rpms.

My 2014 Impala runs at about 1100 rpms on the highway also.

I would say this is not a problem.
 
Not a problem at all. Many an 18 wheeler out there with well over 1 million miles that tend to run only at the 1800 rpm mark driving 70 mph and typically is run between 1000 and 2000 rpm.

Some of the biggest diesel engines in the world operate on cargo ships and at least one that I found only operates in the 22-102 rpm range and produces over 100,000 horsepower and over 5.6 million lb-ft of torque.
 
Those Nissans are awful with that. They lug the engine around and there just isn't enough balancing on the engine ... horrible vibrations into the cabin.
 
Originally Posted By: bullwinkle
Another reason to love manual transmissions & hate automatics-I'd rather do my own gear selection!


Ahhhh yes, my exact words....two years ago.
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Originally Posted By: Miller88
Those Nissans are awful with that. They lug the engine around and there just isn't enough balancing on the engine ... horrible vibrations into the cabin.


Not so, all is sweet & smooth. I was really impressed at how much nicer a CVT is with this V6 compared to how the Altima CVT with the 4cyl behaves.

Maxima is very, very smoooooth drivetrain, at WOT it makes nice noises too!
 
I drove a rental with an 8 speed AT a while back and it felt like it was lugging a lot of the time. That can't be good for the engine, or anything else for that matter.
 
Inertial forces on the bearings are going to be a major portion of the bearing load and are obviously going to be much lower at low rpms.

Lugging vibrations are the sum of the drivetrain interaction with the body and not going to be indicative of the situation at any individual bearing.
 
Back in the mid 1990's I operated a Northwest Dragline ( Crane ) in a sandpit powered by a Murphy engine. As I recall it was built around 1943. That Dragline ran every day, all day long wide open at about 800rpm with no issues. It's all about how the engineers designed the system.

As a matter of fact, I'm getting a little misty thinking about that old beast right now...
 
Originally Posted By: gregk24
Thats a good question. You would have to think, low rpm, low oil pressure, high stresses...that doesn't sound like a combination for long life. I am no engineer though.
You left out the most important one,,, load.
 
Originally Posted By: CT8
Originally Posted By: gregk24
Thats a good question. You would have to think, low rpm, low oil pressure, high stresses...that doesn't sound like a combination for long life. I am no engineer though.
You left out the most important one,,, load.
Sure, I can putt around in the wife's xB on the side roads & flat ground in 5th @ 25 MPH-but that's with virtually no throttle, too-different story going uphill or merging!
 
If low RPM were dangerous for engine longevity then I don't think our old Chrysler inline and slants sixes would have, respectively, made it to 278 and 189K. That 4.0 lived most of it's life at 1100 rpm on 5w-30, still occasionally reminisce about its satisfying valve train clacking at low speed.

Wish I would have known about rustproofing back then...
 
It makes sense but I doubt it. My Falcon would be an excellent example of that, it changes gear below 2000rpm, and cruises at 1500-1700rpm at highway speeds and stays at those revs going up fairly steep inclines, yet 1,000,000km (620,000 miles) is about the norm before these things need an overhaul!
Only thing low revving engines are prone to is carbon build up, but a good old italian tune up every now and then fixes that!
 
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