With more and more automatic transmissions, especially 8 and 10 speeds, on the market nowadays, I have noticed many of them tend to be agressive with upshifting and keeping revs low. One good example is my 23 F150 Tremor 3.5 ecoboost. It has the 10R80 transmission with 3.73 rear end. With 33" tires, under coasting conditions, it'll be in 10th gear as low as 45mph. Or 6th gear at 20 mph. Almost at 1000 rpm! The engine does not struggle, and if i lightly accelerate itll happily hold the high gear and get up to speed. Sometime's it will add boost, even under low revs. And if I stab the throttle a bit more, it will eventually downshift. It's not knocking and I only feel very slight vibrations, however my traditional "sense" tells me this can't be good for the engine, trans, or driveline! It feels and sounds a lot happier gently accelerating and cruising at 2k rpm than 1200 rpm!
From my understanding, many automatics these days have this sort of shift strategy for emissions and fuel economy reasons. Along with more agressive torque converter lockup. However, I want to discuss some of the negative effects this can have long-term. To be clear, I am not discussing traditional "lugging" in the sense of high load at low rpm, but generally low rpms throughout low load and moderate load conditions. And I've implied this but i want to focus on gasoline engines specifically.
The question I want to discuss is.. given the fact modern cars are running at lower revs, and traditional wisdom tells us more rpm = more wear, but are the lower rpms really helping reduce LONG TERM wear? Or are they better off, under light driving conditions, running at slightly higher RPMs, say in the example of my F150, 2.5k rpm vs 1.2k rpm? Want to discuss some points that I feel why lower rpms may mean MORE wear and tear, and I'm curious to hear what others have to say about this topic.
Boundary lubrication condition
We know from the Stribeck curve, lower speeds mean boundary lubrication conditions are more prevalent. Meaning you are relying on the properties of the oil coating itself to protect metal surfaces, rather than an oil film being formed between two surfaces. There is MORE friction and wear under boundary conditions, especially under load (note the higher torque output at lower RPM required to maintain the same HP output), and you are relying on additives such as ZDDP to protect your engine. Even if we assume additives can signifciantly reduce wear under boundary conditions, from my understanding additives such as ZDDP are heat activated, so this may mean even more wear after a cold start! Which is worrisome when you consider the majority of engine wear happens before operating temperatures are achieved.
Higher torque outputs
Given the same power demand/HP, running at lower revs obviously means more torque per rev.. meaning you are having higher cylinder pressures and putting more strain on bottom end components. But at higher revs, you get marginally higher wear on valvetrain components. However, at moderate RPM ranges such as 2-3k rpm I'm sure this effect is more linear rather than exponential. To me, it seems like this is a good tradeoff, to prevent unnecessary strain on cylinder walls, piston skirts and rings, crankshaft, bearings, rods, etc. Plus, I'd rather have to do a timing job than have to rebuild or swap an engine. And I know im not “lugging” the engine in the traditional sense of High Load/Low RPM, but you're still asking for a high torque per rev output with potentially less oil film support. I'm sure that's not nothing over 150k, 200k, or 300k+ miles etc.
LSPI
While modern engines are generally good at careful timing maps, precise fuel injection, or downshifting to better operating conditions, to avoid LSPI generally, but at the end of the day, these rely on knock sensors. As such, there has to be minimal knock even on a well tuned engine, at these low rpm conditions. The driver may not notice, but its there, and I'm sure over long term this cannot be good.
Carbon buildup
There is incomplete combustion at lower RPMs, and with more and more engines going to direct injection, with no fuel washing the top of intake valves, this is a valid concern. I'm glad my truck has port fuel injection along with direct injection- DI is used in conjunction with PI, mostly under higher loads, and PI is used alone under low loads.
So, what does everyone think about this? I tend to drive my truck in sport mode because of these concerns. It's a little more jumpy on the throttle, but it holds the revs just a little higher in what I believe to be the "sweet spot"- under low load normal driving conditions - 1800 - 2500 rpm. Unlike the 1200-1500 rpm normal mode prefers under low load. The engine feels more responsive, and overall sounds and feels a lot happier. However, I do wonder if this is putting unncessary wear, contrary to my beliefs. After all, everything turns twice as much at 2500 rpm vs 1250 rpm. I'm happy to hear what everyone thinks about this.
