...Could you explain what your concerns are with the air filtration?...I trail ride motorcycles and when I follow someone in the dust, I try to stay out of the throttle....
Also, if engine lugging isn't taking place due to light brush or an oversized tractor, would you expect to save any appreciable life on your equipment by throttling down a little? A car example would be running at say 1000 rpms below redline say around 5000 rpms (well within designed specifications) traveling at a low speed of say 30 mph vs the same 30 mph at a comfortable 1700 rpms (higher gear). I will experiment with my select shift truck and watch the instantaneous mpg and report back the fuel savings.
On the topic of air filtration, I mostly meant simply that there is some amount of dust that gets past the filter, no matter what. Some filters work better than others, and filtration is a balance of factors. For instance, our dirt bikes use oiled foam, usually, as the filter medium, even though they are maintenance intensive, don't filter particularly well, and are expensive to service and replace. However, they are able to be made to fit the small, tight, oddly-shaped airboxes that modern motorcycle design requires, and flow enough air to feed the high horsepower engines. They also don't become unusable if moistened by water, as paper filters do. The design requirements of other parts of the bike, then, are dictating what filter style we use.
On some tractors there are air filters sold that are supposed to fit, but in reality do not. (Specifically, NAPA sells a filter for one of my Yanmar machines that is about 1/8" too short) I have seen many tractors with oil bath air filters running without any oil in the reservoir, and seen machines without covers over the air filter box. Sucking dirt into the system, to me, must be much more damaging than running the engine at its design RPM.
If lower RPM makes less dust, and lowers the filtering load on the intake system, I don't see how that wouldn't help. My rotary cutter doesn't work unless it is spun up fairly fast, and if it is cutting well, it is making dust. That's my machine in my conditions, though, not a universally applicable law.
I don't think the majority of wear in a tractor engine comes from RPM. As an example: the current industrial version of the Yanmar 3T72 in my tractor is rated at 23.6 horsepower, at 3600 RPM rather than 18 horsepower at 2600 RPM in the tractor. The service intervals on the industrial engines are usually longer, and the expected life is, as well. The operating environment for an industrial engine is probably presumed to be much cleaner than that of a tractor. Engine RPM doesn't seem to be the longevity issue in design, within some bounds, but intermittent power ratings are higher than continuous ratings across the board, while engine RPM is frequently the same.
I don't think that fuel consumption correlates directly to engine life. The car engine at 4 to 5 thousand RPM will certainly burn more fuel at cruise speed vs being in overdrive. Also, car engines are not really intended to operate for more than brief periods at maximum RPM or power, whereas the tractor engines are designed to do exactly that. I think the different requirements of the designs lead to different outcomes. In other words, I don't think running my tractor engine at its rated 2600 RPM all day mowing is equivalent to running my pickup all day at 5000 RPM. I think it is more equivalent to running an Cessna 172 at 2500 RPM all day, or the pickup at over the range of 1500 to 4500 RPM. This is their intended operating range, and the life expectancy would be more in line with what the factory expects.
Lycoming, for instance, bases their cruise RPM based on fractional power settings, not cruise RPM per se. As I recall, the 172 I flew had an engine manual that recommended 65% or less power output for maximum engine life, with oil, cylinder head, and exhaust temperature ranges suggested. The RPM for that power setting varied, of course, over a few hundred RPM, about like a tractor's recommended operating range.
In that context, then, I would expect any engine life benefits to be from reduced power settings, not from the lower RPM itself. This is all conjecture on my part, based on some evidence. I'm certainly not an expert, but it makes sense to me.
I think in the overall sum, it is much more important to keep the air, oil, and fuel fresh and clean, and the engine and fluids at recommended operating temperatures than anything else. I think operating at lower engine speed presents more opportunity for harm than operating at high speed, but that the other factors matter more.
