To celebrate my joining as a Premium Member, I thought I'd start a fight. :) Actually, that's the last thing I'm trying to do, but I would like to start a thread discussing the "best" RPM for the various diesel engines we have powering our vehicles. Assuming you're interested in such things, perhaps you could humor me by sitting back with your favorite beverage (recommended for your own mental health) whilst I ramble a bit::party018:
I think we can assume that, for any given engine and load (weight & air resistance combined), there's an RPM (or at least a relatively narrow range) that the engine is most efficient at, and also isn't harmful to the long-term durability of the engine. (In other words, there's an engine RPM that would result in the best combination of fuel efficiency, longevity, and ability to supply the torque necessary to do the job at hand.) I would also like to assume, for simplicity's sake, that to begin with we can ignore ambient temperature, gradient (zero RPM in neutral is most efficient going downhill, right?), wind (that's covered under load), etc.
To further lay out some suggested "ground rules", I'd like to try to categorize the stuff we throw out here as Fact, Theory, Experience (or "anecdotal evidence", if you prefer), Opinion, or Comment/Question. (We all know people who will swear that there's no difference between their opinion and fact, but I hope we can avoid too much of that in this discussion... :hehe:) To that end, I would propose the following definitions:
Fact - Something that can easily be verified to be true by "common knowledge", published works, mathematical formulas (not to be confused with "statistics"), or we can just get a general consensus that at least 90% or so of the participants here agree on, even. I'm not trying to get too technical here - I think we'll mostly be able to agree on whether something is a fact or not, or at least resolve the confusion pretty quickly...
Theory - Something that we believe to be true and have "evidence" to support, whether it be seemingly trustworthy statements by reliable sources (like Cummins (and NOT Ram) where Cummins engines are concerned, for example), or by our own Experience (definition coming in a moment), or by statistics or formulas that use variables that aren't necessarily Facts, but seem credible.
Experience - Something that we've observed personally or know has been observed by a credible source and can be verified to be repeatable. (I only throw in "repeatable" because of what we're trying to accomplish here. For example, "I got struck by lightning while I was sitting on the toilet in my basement" may be true, and may be evidence that it's possible, but it's certainly misleading to say the experience suggests that the next time I or anybody else sits on the toilet in a basement, they're going to get struck by lightning...)
Opinion - Something we believe because we just think it's true, or wish it were. Given this definition, there's probably not many of us that have any opinions, because we almost always think we believe what we believe because we've got good reason to, but if it's an Opinion based on Experience, let's at least say that, and elaborate on the Experience in question because, as they say (and pardon the pun here): Your mileage may vary. (In other words, I might regularly get 25 MPG with a particular rig on a particular trip because I'm always taking the loaded rig in one direction, which happens to be downhill the whole way, and empty on the return trip. If I'm not astute enough to know that, and honest enough to tell you, my "Experience" is probably worse that worthless to you - it's not just useless, more to the point, it's misleading. And the fact that it's repeatable is a negative, not a positive, in this case, unless "the rest of the story" is told.)
Unknown - Let's also allow for a place to throw in things that we think are relevant, but we're intentionally not characterizing them as Fact, Theory, Experience or Opinion - we just want it on the table for consideration by "the brain trust" here.
Comment/Question - None of the above, just something that needs to be said to further the discussion, or a question to be thrown out for consideration.
So with that long-winded preamble, I'll say here and now that this post is meant to be a more serious continuation of a discussion that's been going on in the 2013 forum here regarding axle ratios, Cummins recommendations, etc. Some of what I'm including here is spread across several threads, but it was mostly off-topic there anyway. To get us started, here's a Fact (and the purpose of this discussion is to get feedback - who knows, perhaps somebody who really KNOWS something will one day get directed here and chime in and tell us something we DON'T know):
Fact 1 - Cummins says the following things about gearing recommendations (the things they say are themselves probably best categorized as Theory or Unknown, because I haven't seen anything "proving" them - in fact, if I knew they could be categorized as Fact, I probably wouldn't have started this thread, though I'd still want more details):
Given an ISB 6.7L engine in an RV of 10,000 to 30,000 pounds GVW,
1) Your RPM at your target cruise speed should be between 1900 and 2400.
2) The recommended RPM for the best combination of Power and Economy is 2150 RPM.
3) Engine speeds of less than 1900 RPM at the target cruise speed or 1600 RPM at 55 mph should be avoided.
Note: I know I'm repeating myself, but I didn't label those three statements as Fact - I only said it was a Fact that Cummins says them - they come from their PowerSpec program, for what it's worth...
Fact 2 - There's no way to stay within Cummins' Guidelines above with stock tires, a typical 6-speed transmission (I'm using the AS69RC for reference) and 3.42 diffs, unless your intended cruise speed is 84 mph (well, 83.435 mph, but what's 0.565 mph amongst friends?), or you don't intend to use 6th gear. (Frankly, even if you are running these speeds, I'm pretty certain that it would be worth asking whether Cummins has factored in the wind loads at those speeds, because I serously doubt they really expect your normal cruise speed to be 84 mph in that type of vehicle.) If you go with 3.73 diffs, you'll need to plan on running 77 mph as your target cruise speed. With 4.10 diffs, your cruise speed would have to be at least 70 mph to use 6th gear.
Experience - I've owned (not counting 18-wheelers, straight trucks, gas pickups, gas and diesel cars, diesel construction equipment, etc.) '93 and '03 4x4 SRW Rams with 5.9L Cummins engines, NV-5600 6-speed manual transmissions (yeah, I know the '93 didn't come with that, but mine had one in it after about 80k miles) and 3.07 and 3.73 diff ratios, respectively. I used both of them to pull loads that way exceeded what they were supposed to pull (over 25k numerous times - and I used a 3-axle trailer with 9k axles, 18k-rated brakes on all three, so I had braking capacity that far exceeded the total load, so you can call me a little out of bounds, if you insist, but please don't accuse me of having a death wish). I should also throw in that my usage for both trucks was about the same, approximately 20% towing very heavy loads, bobtailing the rest of the time. Ok, so far, this is all Fact, but here's the Experience part: Both trucks did great. In mountains, otherwise, hot weather, cold weather, etc. I don't remember what fuel mileage I got, unfortunately - I'm looking for the logbooks, which I've got laying around somewhere. Neither of them had any engine trouble of any sort in the first 150k miles - I didn't own them after that, so I don't know how they fared later. Based on my Experience, I could offer the Opinion that 3.73 is the best differential ratio and 3.07 is the second best. I won't, but I could. I'm perfectly willing to allow for the possibility that if either of them had 4.10 diffs, I'd say that was the best. Or 3.42's, or 4.88's, you get where I'm going with this. I have no way of knowing what's best, even though I have a lot of Experience. If forced to, I could only offer the Opinion that I think going from 3.07 to 3.73 was a change in the right direction, because it towed heavy loads better, and did get better fuel mileage overall (but they were also quite different 5.9L engines). (Some would say my Experience means that last statement has to be a Fact, but I'm way too much of a stickler for detail to be that cavalier about my Facts. :stirpot:) What I'd really like, if it could be had, is some Facts, or at least Theories, that provide a basis for determining the "best" RPM for a given engine. I know it's not the same for the typical ISB as it is for the typical ISX. The Cummins recommendations cited above seem to indicate 2150 RPM as the "best", but is it the same for a 10k pickup as it is for a 10k-30k RV? I don't know. Is it the same for a pickup bob-tailing at 80 MPH as it is for the pickup towing 15K at 70 MPH? I don't know (but I doubt it).
Comment - Here's the practical reason I'm interested in all this: I have a spreadsheet (there are others around, but you can have mine if you PM me with your e-mail address) that I've been tweaking on and off for 20 years to make it easy to experiment with the results of different combinations of axle ratios, transmissions (up to 7 forward gears), under- and over-drives (which doubles the number of forward gears, obviously), and tire sizes at selected speeds and engine RPM ranges. (I've pasted a few screenshots of it below, illustrating some of the data cited in Fact 2.) Basically, you just put the stuff you're interested in in the tables provided in the sheet and pick the ones you want to see the resulting speeds and RPMs for. Suppose I wanted to pick the best differential ratio (that's what brought all this up) for a Ram with 6.7L engine, AS69RC trans, stock tires, etc. Further suppose that I want to tow 15,000 pounds (pick a number, any number) at about 65 MPH when I tow, which is about 75% of my use for the truck, and I want to drive 80 MPH like a mad-man when I'm not towing. If I have some idea of what RPMs the engine "likes best" when under 50% load and 75% load, or some reasonable facsimile thereof, I can figure out what ratio would put me close to that in 6th gear for non-towing and 5th gear for towing, etc. (or something like that). I realize it's also possible that I may also have to temper the "most efficient" RPM a bit with the understanding that "lugging" an engine is harmful to it, so if I want it to last 300k miles, I should pick a slightly higher RPM, etc., thus my use of the term "best" here, not "most efficient". On the flip side, the fewer RPMs you can turn to do a given amount of work without harming the engine, the longer the engine will last, due to lower wear (assuming, in general, that a given amount of wear occurs for each revolution).
Opinion - Referring back to Fact 2 above, something's messed up here somewhere. Even if we assume that a bob-tail pickup at 10,000 GVW doesn't quite fit in the 10k-30k RV range (lower wind load, for one thing, very bottom of the range for another, etc.), the only thing that's going to do (potentially) is lower the bottom of Cummins' "avoid RPMs outside of" range. So, for the sake of argument, let's say it lowers it from 1900 RPM to 1500 RPM (which is a huge difference, but still just below the peak torque spec RPM, which they presumably wouldn't use if they didn't want you to run the engine at that speed). Now, you can use 6th gear with 3.42 diffs at 66 mph - but there's no evidence whatsoever that Cummins "approves" of that, it's just pure speculation.
Comment - I already know that there's a bunch of people here who can say "Yeah, well, ok, Cummins says that (Fact 1), but I've been <insert personal Experience to the contrary here> for <insert how many years here>". To that, I can only say "Me too." :banghead: That only reinforces my reason for introducing this thread. So, if you're going to respond with that (which would be a good thing, because the more "evidence" to the contrary we get here, the more Theory we can produce - maybe), please at least understand that you're only saying you agree with me (or at least that part of this monolog), because I already said that. Perhaps more to my point, based on the comments in the threads over in the 2013 forum, some of us are content to just pretend Cummins didn't say what they said, or cite personal Experience as all the evidence necessary to just ignore it. I really prefer to have better reasons for making decisions on things this important, myself - sorry, I'm just a bit of a nut that way...
Question - Assuming that there's a reason (other than just stupidity/uncaught typographical error/etc., which I think we can discount, and hopefully that reason is a Fact or at least a Theory) for the range Cummins says you should avoid RPMs outside of, what is it? If anyone has the right to "play God" where Cummins engines are concerned, isn't it Cummins? (I know I, for one, would love to do what they tell me to, but at the moment, I'm confused.) Are they hiding emissions-related reasons behind the "power and economy" banner? :S: Is it really true that peak torque is at 1600 RPM but RPMs below 1900 aren't "good"? Are RPMs below 1900 "ok" or even "good" if the engine is only running at 50% load, even though it's rated to produce its peak torque output at that lower RPM? Is the engine really rated to produce peak torque at an RPM that it's not efficient or, worse, even harmful, to run it at? If 1600 RPM is the one somebody chose to rate the engine at, but it's not ok to run the engine under load at that RPM, should I be having severe leg pains, because of how hard it's being pulled? Is it unreasonable to think all this stuff should make sense and have some solid Factual basis?
If you're still reading, you're either spoiling for a fight, or really interested in the topic (maybe both - either way, you've got too much free time). What do you think?
Unknown - As a bonus, and further food for thought (or fuel for the fire), I'm throwing this link in. It's relevant, interesting, and maybe even a bit informative, but some of it should be read in the context that it's written by a lubricants supplier: HDDEO
Mac
I think we can assume that, for any given engine and load (weight & air resistance combined), there's an RPM (or at least a relatively narrow range) that the engine is most efficient at, and also isn't harmful to the long-term durability of the engine. (In other words, there's an engine RPM that would result in the best combination of fuel efficiency, longevity, and ability to supply the torque necessary to do the job at hand.) I would also like to assume, for simplicity's sake, that to begin with we can ignore ambient temperature, gradient (zero RPM in neutral is most efficient going downhill, right?), wind (that's covered under load), etc.
To further lay out some suggested "ground rules", I'd like to try to categorize the stuff we throw out here as Fact, Theory, Experience (or "anecdotal evidence", if you prefer), Opinion, or Comment/Question. (We all know people who will swear that there's no difference between their opinion and fact, but I hope we can avoid too much of that in this discussion... :hehe:) To that end, I would propose the following definitions:
Fact - Something that can easily be verified to be true by "common knowledge", published works, mathematical formulas (not to be confused with "statistics"), or we can just get a general consensus that at least 90% or so of the participants here agree on, even. I'm not trying to get too technical here - I think we'll mostly be able to agree on whether something is a fact or not, or at least resolve the confusion pretty quickly...
Theory - Something that we believe to be true and have "evidence" to support, whether it be seemingly trustworthy statements by reliable sources (like Cummins (and NOT Ram) where Cummins engines are concerned, for example), or by our own Experience (definition coming in a moment), or by statistics or formulas that use variables that aren't necessarily Facts, but seem credible.
Experience - Something that we've observed personally or know has been observed by a credible source and can be verified to be repeatable. (I only throw in "repeatable" because of what we're trying to accomplish here. For example, "I got struck by lightning while I was sitting on the toilet in my basement" may be true, and may be evidence that it's possible, but it's certainly misleading to say the experience suggests that the next time I or anybody else sits on the toilet in a basement, they're going to get struck by lightning...)
Opinion - Something we believe because we just think it's true, or wish it were. Given this definition, there's probably not many of us that have any opinions, because we almost always think we believe what we believe because we've got good reason to, but if it's an Opinion based on Experience, let's at least say that, and elaborate on the Experience in question because, as they say (and pardon the pun here): Your mileage may vary. (In other words, I might regularly get 25 MPG with a particular rig on a particular trip because I'm always taking the loaded rig in one direction, which happens to be downhill the whole way, and empty on the return trip. If I'm not astute enough to know that, and honest enough to tell you, my "Experience" is probably worse that worthless to you - it's not just useless, more to the point, it's misleading. And the fact that it's repeatable is a negative, not a positive, in this case, unless "the rest of the story" is told.)
Unknown - Let's also allow for a place to throw in things that we think are relevant, but we're intentionally not characterizing them as Fact, Theory, Experience or Opinion - we just want it on the table for consideration by "the brain trust" here.
Comment/Question - None of the above, just something that needs to be said to further the discussion, or a question to be thrown out for consideration.
So with that long-winded preamble, I'll say here and now that this post is meant to be a more serious continuation of a discussion that's been going on in the 2013 forum here regarding axle ratios, Cummins recommendations, etc. Some of what I'm including here is spread across several threads, but it was mostly off-topic there anyway. To get us started, here's a Fact (and the purpose of this discussion is to get feedback - who knows, perhaps somebody who really KNOWS something will one day get directed here and chime in and tell us something we DON'T know):
Fact 1 - Cummins says the following things about gearing recommendations (the things they say are themselves probably best categorized as Theory or Unknown, because I haven't seen anything "proving" them - in fact, if I knew they could be categorized as Fact, I probably wouldn't have started this thread, though I'd still want more details):
Given an ISB 6.7L engine in an RV of 10,000 to 30,000 pounds GVW,
1) Your RPM at your target cruise speed should be between 1900 and 2400.
2) The recommended RPM for the best combination of Power and Economy is 2150 RPM.
3) Engine speeds of less than 1900 RPM at the target cruise speed or 1600 RPM at 55 mph should be avoided.
Note: I know I'm repeating myself, but I didn't label those three statements as Fact - I only said it was a Fact that Cummins says them - they come from their PowerSpec program, for what it's worth...
Fact 2 - There's no way to stay within Cummins' Guidelines above with stock tires, a typical 6-speed transmission (I'm using the AS69RC for reference) and 3.42 diffs, unless your intended cruise speed is 84 mph (well, 83.435 mph, but what's 0.565 mph amongst friends?), or you don't intend to use 6th gear. (Frankly, even if you are running these speeds, I'm pretty certain that it would be worth asking whether Cummins has factored in the wind loads at those speeds, because I serously doubt they really expect your normal cruise speed to be 84 mph in that type of vehicle.) If you go with 3.73 diffs, you'll need to plan on running 77 mph as your target cruise speed. With 4.10 diffs, your cruise speed would have to be at least 70 mph to use 6th gear.
Experience - I've owned (not counting 18-wheelers, straight trucks, gas pickups, gas and diesel cars, diesel construction equipment, etc.) '93 and '03 4x4 SRW Rams with 5.9L Cummins engines, NV-5600 6-speed manual transmissions (yeah, I know the '93 didn't come with that, but mine had one in it after about 80k miles) and 3.07 and 3.73 diff ratios, respectively. I used both of them to pull loads that way exceeded what they were supposed to pull (over 25k numerous times - and I used a 3-axle trailer with 9k axles, 18k-rated brakes on all three, so I had braking capacity that far exceeded the total load, so you can call me a little out of bounds, if you insist, but please don't accuse me of having a death wish). I should also throw in that my usage for both trucks was about the same, approximately 20% towing very heavy loads, bobtailing the rest of the time. Ok, so far, this is all Fact, but here's the Experience part: Both trucks did great. In mountains, otherwise, hot weather, cold weather, etc. I don't remember what fuel mileage I got, unfortunately - I'm looking for the logbooks, which I've got laying around somewhere. Neither of them had any engine trouble of any sort in the first 150k miles - I didn't own them after that, so I don't know how they fared later. Based on my Experience, I could offer the Opinion that 3.73 is the best differential ratio and 3.07 is the second best. I won't, but I could. I'm perfectly willing to allow for the possibility that if either of them had 4.10 diffs, I'd say that was the best. Or 3.42's, or 4.88's, you get where I'm going with this. I have no way of knowing what's best, even though I have a lot of Experience. If forced to, I could only offer the Opinion that I think going from 3.07 to 3.73 was a change in the right direction, because it towed heavy loads better, and did get better fuel mileage overall (but they were also quite different 5.9L engines). (Some would say my Experience means that last statement has to be a Fact, but I'm way too much of a stickler for detail to be that cavalier about my Facts. :stirpot:) What I'd really like, if it could be had, is some Facts, or at least Theories, that provide a basis for determining the "best" RPM for a given engine. I know it's not the same for the typical ISB as it is for the typical ISX. The Cummins recommendations cited above seem to indicate 2150 RPM as the "best", but is it the same for a 10k pickup as it is for a 10k-30k RV? I don't know. Is it the same for a pickup bob-tailing at 80 MPH as it is for the pickup towing 15K at 70 MPH? I don't know (but I doubt it).
Comment - Here's the practical reason I'm interested in all this: I have a spreadsheet (there are others around, but you can have mine if you PM me with your e-mail address) that I've been tweaking on and off for 20 years to make it easy to experiment with the results of different combinations of axle ratios, transmissions (up to 7 forward gears), under- and over-drives (which doubles the number of forward gears, obviously), and tire sizes at selected speeds and engine RPM ranges. (I've pasted a few screenshots of it below, illustrating some of the data cited in Fact 2.) Basically, you just put the stuff you're interested in in the tables provided in the sheet and pick the ones you want to see the resulting speeds and RPMs for. Suppose I wanted to pick the best differential ratio (that's what brought all this up) for a Ram with 6.7L engine, AS69RC trans, stock tires, etc. Further suppose that I want to tow 15,000 pounds (pick a number, any number) at about 65 MPH when I tow, which is about 75% of my use for the truck, and I want to drive 80 MPH like a mad-man when I'm not towing. If I have some idea of what RPMs the engine "likes best" when under 50% load and 75% load, or some reasonable facsimile thereof, I can figure out what ratio would put me close to that in 6th gear for non-towing and 5th gear for towing, etc. (or something like that). I realize it's also possible that I may also have to temper the "most efficient" RPM a bit with the understanding that "lugging" an engine is harmful to it, so if I want it to last 300k miles, I should pick a slightly higher RPM, etc., thus my use of the term "best" here, not "most efficient". On the flip side, the fewer RPMs you can turn to do a given amount of work without harming the engine, the longer the engine will last, due to lower wear (assuming, in general, that a given amount of wear occurs for each revolution).
Opinion - Referring back to Fact 2 above, something's messed up here somewhere. Even if we assume that a bob-tail pickup at 10,000 GVW doesn't quite fit in the 10k-30k RV range (lower wind load, for one thing, very bottom of the range for another, etc.), the only thing that's going to do (potentially) is lower the bottom of Cummins' "avoid RPMs outside of" range. So, for the sake of argument, let's say it lowers it from 1900 RPM to 1500 RPM (which is a huge difference, but still just below the peak torque spec RPM, which they presumably wouldn't use if they didn't want you to run the engine at that speed). Now, you can use 6th gear with 3.42 diffs at 66 mph - but there's no evidence whatsoever that Cummins "approves" of that, it's just pure speculation.
Comment - I already know that there's a bunch of people here who can say "Yeah, well, ok, Cummins says that (Fact 1), but I've been <insert personal Experience to the contrary here> for <insert how many years here>". To that, I can only say "Me too." :banghead: That only reinforces my reason for introducing this thread. So, if you're going to respond with that (which would be a good thing, because the more "evidence" to the contrary we get here, the more Theory we can produce - maybe), please at least understand that you're only saying you agree with me (or at least that part of this monolog), because I already said that. Perhaps more to my point, based on the comments in the threads over in the 2013 forum, some of us are content to just pretend Cummins didn't say what they said, or cite personal Experience as all the evidence necessary to just ignore it. I really prefer to have better reasons for making decisions on things this important, myself - sorry, I'm just a bit of a nut that way...
Question - Assuming that there's a reason (other than just stupidity/uncaught typographical error/etc., which I think we can discount, and hopefully that reason is a Fact or at least a Theory) for the range Cummins says you should avoid RPMs outside of, what is it? If anyone has the right to "play God" where Cummins engines are concerned, isn't it Cummins? (I know I, for one, would love to do what they tell me to, but at the moment, I'm confused.) Are they hiding emissions-related reasons behind the "power and economy" banner? :S: Is it really true that peak torque is at 1600 RPM but RPMs below 1900 aren't "good"? Are RPMs below 1900 "ok" or even "good" if the engine is only running at 50% load, even though it's rated to produce its peak torque output at that lower RPM? Is the engine really rated to produce peak torque at an RPM that it's not efficient or, worse, even harmful, to run it at? If 1600 RPM is the one somebody chose to rate the engine at, but it's not ok to run the engine under load at that RPM, should I be having severe leg pains, because of how hard it's being pulled? Is it unreasonable to think all this stuff should make sense and have some solid Factual basis?
If you're still reading, you're either spoiling for a fight, or really interested in the topic (maybe both - either way, you've got too much free time). What do you think?
Unknown - As a bonus, and further food for thought (or fuel for the fire), I'm throwing this link in. It's relevant, interesting, and maybe even a bit informative, but some of it should be read in the context that it's written by a lubricants supplier: HDDEO
Mac