Another 3.21 vs 3.92 comparison

[Richard320]

Bigger tires = 3.92
Racing = 3.92
Towing anything more than 5k wet probably = 3.92
Just out “having fun” = 3.92

Old grandpa driver never having a v8 that had more than 210 hp = 3.21
What was on the lot that the wife wanted color wise = 3.21
Don’t tow anymore (compared to 20 years ago) = 3.21
The 8 speed tranny vs the old days 3 and 4 speeds = 3.21

Yeah, that would be me. I haven't driven a V8 regularly since 1980.... Mom's 1970 Plymouth Fury Wagon. Although it was a Sport with the bulges on the hood, the blackout grill, and a Roadrunner 383. But I digress. I don't tow, but about once a year something comes up when I wish I had a hitch. I decided I wanted factory trailer brake wiring on my new truck just in case. 8 forward gears is overkill in my opinion. The second overdrive is not. I've been driving a Wrangler with 4.10 gears and a 5 speed for 18 years and I am always reaching for the next gear on the freeway, and it isn't there.

Putting around town, the 3.92s might actually deliver better mileage. Getting 3 tons moving after every stop takes a lot of torgue. However, I've also seen the mileage drops on my Wrangler. 16 highway driven conservatively,. Try to drive 75 and it drops to 14 or less. I'm quite pleased so far with my highway mileage, almost 22 mpg. I'm not sure I'd get that high with the lower gears as the engine would be running 22% faster

 

[Cr250Ram]

My calculations put it at an 18% different in RPM. I have always wondered if wear and tear for non-towing is reduced with the higher (lower #) gears? Diesels have a long life which is said to be *partly* from their lower RPMs. A truck with 3.21's engine is rotating 18% slower than a 3.92 truck over the same given distance. I get under heavy load the 3.92s will help with heat, but in situations where heat isn't raised (ie commuting w/o a load) wouldn't the 3.21s offer better long term reliability due to the overall less engine/drive train rotations. The truck is rated to tow 8,000, surely there is enough torque to pull the truck along without over stressing the drivetrain.

 

[SpeedyV]

My calculations put it at an 18% different in RPM. I have always wondered if wear and tear for non-towing is reduced with the higher (lower #) gears? Diesels have a long life which is said to be *partly* from their lower RPMs. A truck with 3.21's engine is rotating 18% slower than a 3.92 truck over the same given distance. I get under heavy load the 3.92s will help with heat, but in situations where heat isn't raised (ie commuting w/o a load) wouldn't the 3.21s offer better long term reliability due to the overall less engine/drive train rotations. The truck is rated to tow 8,000, surely there is enough torque to pull the truck along without over stressing the drivetrain.

Way too many variables to make a simple comparison. If two otherwise-identical trucks, one 3.21 and one 3.92, were to be driven until failure at 75 mph in 8th gear, the 3.92 would theoretically wear out first. But when you add in the complexities of an 8-speed transmission, loading variations due to options, driver inputs, climate, road conditions, trailering (or lack thereof), average speeds and durations driven, etc...the math isn’t that simple.

 

[riccnick]

My calculations put it at an 18% different in RPM. I have always wondered if wear and tear for non-towing is reduced with the higher (lower #) gears? Diesels have a long life which is said to be *partly* from their lower RPMs. A truck with 3.21's engine is rotating 18% slower than a 3.92 truck over the same given distance. I get under heavy load the 3.92s will help with heat, but in situations where heat isn't raised (ie commuting w/o a load) wouldn't the 3.21s offer better long term reliability due to the overall less engine/drive train rotations. The truck is rated to tow 8,000, surely there is enough torque to pull the truck along without over stressing the drivetrain.

3.92 is 22% more than 3.21. If you're worried about MORE rpm's, you gotta do the math in the correct direction.

Assuming the same transmission gear and same ground speed, at 2,000 RPM in a 3.21 truck, a 3.92 truck would run 2,442 RPM.

However, your example regardless of actual percentage, doesn't work in real life, because it assumes that a 3.92 truck and a 3.21 truck will be in the same gear all the time. There's gear overlap too that can be considered. A 3.21 truck running 1 gear lower in the transmission has either the same or slightly more mechanical advantage as the 3.92 truck running in X gear.

I made a chart to convey this in a simpler manner. The better explanation is in the other diff ratio thread.



And to make BOTH of our points moot, the powertrains in these trucks, and any modern vehicle, are abused and tested for millions of miles in conditions that span beyond what's achievable on this planet. A few RPM's here and there over the relatively short lifespan most vehicles actually see (when compared to how they're durability tested) is the last thing any owner should have to worry about. Calculating the fuel saving affect of running Nitrogen in your tires is actually a better use of your time, lol.

 

[Gmoosevt]

The math may not be simple, but the engineering principles are. For two identical engines, doing the same ‘within-spec’ work and traveling the same mileage, the one running at higher rpm will create more friction and thus will require more maintenance to sustain its life as long as the lower rev’ing engine. Generally, it is not only fair to say, but in real life true: a big V-8 will last longer with a 3.21 vs a 3.92, and it will get better gas mileage over its life. That’s just a fact. This of course assumes the truck is doing the normal light duty work of towing once in a while, running with a load once in a while, but the majority of its miles are just going down the road. i.e. the use that 90% of light duties get. I owned and drove many a ‘boat’ back in the 60’s. It was always amazing to me how good the mpg’s were in big Caddies, Pontiacs, and Fords (all with big V-8’s of course). They had tall gears, and would just float down the road like a coasting battleship... not dissimilar to what it now feels like in a 5.7 Ram crew cab with a 3.21. I like that kinda low rev cruise ride, and I like the +20 avg mpg I’m getting for all combined driving. And it tows my boats and campers just fine, for the minimal 2000 miles per year involved with that infrequent work. I’ll take the 3.21 any day. My trucks last 20 years. If my light duty was a work truck, and I was towing a loaded landscape trailer every day, or a horse trailer or something like that, frequently, then sure, a 3.92 fits the bill... but then again, a 2500 or 3500 would fit the bill much better, for regular towing and loading work.


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[riccnick]

The math may not be simple, but the engineering principles are. For two identical engines, doing the same ‘within-spec’ work and traveling the same mileage, the one running at higher rpm will create more friction and thus will require more maintenance to sustain its life as long as the lower rev’ing engine. Generally, it is not only fair to say, but in real life true: a big V-8 will last longer with a 3.21 vs a 3.92, and it will get better gas mileage over its life. That’s just a fact. This of course assumes the truck is doing the normal light duty work of towing once in a while, running with a load once in a while, but the majority of its miles are just going down the road. i.e. the use that 90% of light duties get. I owned and drove many a ‘boat’ back in the 60’s. It was always amazing to me how good the mpg’s were in big Caddies, Pontiacs, and Fords (all with big V-8’s of course). They had tall gears, and would just float down the road like a coasting battleship... not dissimilar to what it now feels like in a 5.7 Ram crew cab with a 3.21. I like that kinda low rev cruise ride, and I like the +20 avg mpg I’m getting for all combined driving. And it tows my boats and campers just fine, for the minimal 2000 miles per year involved with that infrequent work. I’ll take the 3.21 any day. My trucks last 20 years. If my light duty was a work truck, and I was towing a loaded landscape trailer every day, or a horse trailer or something like that, frequently, then sure, a 3.92 fits the bill... but then again, a 2500 or 3500 would fit the bill much better, for regular towing and loading work.


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shorter gears also equal lower mechanical advantage though, so the assumed wear cannot be directly compared strictly based on rpm's / distance traveled.

The amount of energy required to move the truck is the same regardless of gear ratio. How "hard" the engine is working to achieve that energy level is strictly a matter of efficiency of the combustion cycle at whatever rpm is being considered. Lower rpm doesn't mean fewer forces are at work internally. And if anything, because HP is the calculation of how much Work that Torque is doing at any given moment, getting the most work out of each spin of the crankshaft is by definition done at peak HP, which is a very high RPM. Anything less than that could be considered non-essential wear due to less than optimal Work output.

 

[Cr250Ram]

Does anyone know the transmission ratios for the 19 1500’s 8hp75? I keep finding conflicting info. 4.71 vs 5.0 (1st).

 

[riccnick]

It's in the chart, 5.000:1 is correct.

 

[Cr250Ram]

It's in the chart, 5.000:1 is correct.

Do you know where this chart is? I’ve searched and can’t find it.

 

[riccnick]

Do you know where this chart is? I’ve searched and can’t find it.

I'll try and find it again. It was originally posted in the original 3.21 vs 3.92 thread, and I know I went and verified it myself as well before making this chart. The numbers are good, I'll see if I can get the proof for ya.

 

[Cr250Ram]

I'll try and find it again. It was originally posted in the original 3.21 vs 3.92 thread, and I know I went and verified it myself as well before making this chart. The numbers are good, I'll see if I can get the proof for ya.

http://www.wk2jeeps.com/pdf/8HP75_Data_sheeet.pdf
I found this but it doesn’t specify which transmission ratio spread out trucks have (7.071 vs 7.813).
https://media.fcanorthamerica.com/pdf.do?id=18758
This says ours have the same spread as the 850RE transmission, which has the 7.071 from my research, ie 1st gear is 4.714 vs 5.0

 

[riccnick]

http://www.wk2jeeps.com/pdf/8HP75_Data_sheeet.pdf
I found this but it doesn’t specify which transmission ratio spread out trucks have (7.071 vs 7.813).
https://media.fcanorthamerica.com/pdf.do?id=18758
This says ours have the same spread as the 850RE transmission, which has the 7.071 from my research, ie 1st gear is 4.714 vs 5.0

Pretty sure that data sheet from ZF is wrong, maybe it was translated incorrectly. I think it was supposed to be a comparison chart between the two trans, however, they copied the data set to both sheets. Everything I've read elsewhere states the 8HP75 has the wider spread and deeper first gear, and the 8HP70 is the old trans with the narrower spread.

 

[Gmoosevt]

Hey, I’m a mechanical engineer too, with lots of experience designing and monitoring engines - especially big power plant diesels. Slower rpm engines last longer than higher rpm engines due to less friction and load stresses. Keep it simple, honest, and real for the readers.


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[raven_DT]

Hey, I’m a mechanical engineer too, with lots of experience designing and monitoring engines - especially big power plant diesels. Slower rpm engines last longer than higher rpm engines due to less friction and load stresses. Keep it simple, honest, and real for the readers.


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ME here as well. The problem is that its not an apples to apples comparison for towing. With the 3.92s you are running closer (by a couple of hundred rpms) to the meat of the torque curve while towing. When your towing, that may mean the difference between a downshift or two when you need to accelerate and/or climb a grade. At that point you would probably be turning higher rpms in 4-5th gear with a 3.21 than a 3.92 in 5th or 6th (I haven't seen the ZF gear ratios). Previously, I would lockout 6th gear (@65mph or less) in my 2017 2005 CTD w 3.42s to keep the 6.7 running at its peak torque output. Otherwise I would get a 6-5 downshift on any grades while towing. I always wanted 3.73s, now they are standard in those trucks...wonder why. It think that 3.21s are probably fine (based on my towing experience) for most (majority) people, especially if your towing 5K or less. If you are towing 5K to 8K, I would say 3.92s offer a better towing experience. Anything 8K and above, really calls for a 3/4 or 1 ton truck. Can you you tow 10K with your 1500 (while meeting your max payload #s)? Sure, but the towing experience is much more pleasant and safer with the larger truck. Just a data point, my 2019 BigHorn (4x4, 3.92, CC) gets around 17mpg, while my 2017 2500 CTD (4x4) consistently averaged about 19mpg usually with 6tg gear locked out (not towing). When Ram comes out with a 10sp transmission for its 2500/3500 lineup...

 

[devildodge]

The answer is 4.71

 

[raven_DT]

Does anyone have a torque/hp curve for the 2019 5.7 (w/o etorque)? Wonder why Ram doesn't make that readily available.

 

[riccnick]

The answer is 4.71

View attachment 33616

Where did you find that!?!?

 

[riccnick]

Slower rpm engines last longer than higher rpm engines due to less friction and load stresses.

What's the equations for lifespan in regards to rpm given that friction and load are equal and static?

Keep it simple, honest, and real for the readers.

I'm sure as you know, you lose honestly as you increase simplicity. Noting in life is simple, especially not in Engineering. This isn't a black and white scenario. And it's just as dishonest and not real to make blanket statements like yours as it is to spew info like mine to someone who may not understand and use the info the wrong way.

And by definition, in the truck with the shorter gears, each spin of the crank has to do more in order to achieve the same result at the wheels. Is that not higher load? Force of Friction increases with load too, right?

With your logic (and others using it as well), looking at two trucks identically equipped pulling the same load for a perceived lifetime, you're actually saying the 3.21 truck's engine would last longer purely due to the fact that it's rpms would be 18% lower (if it could run the exact same transmission gear in every situation).

I don't know for sure, but I just get this feeling that it's not the case.

(I know you're not the first person to take the stance you took, I'm just conversing here against the sides of the convo, not against any person in particular, It's not a you vs me thing, I just quoted you for context of the convo.)

 

[devildodge]

Where did you find that!?!?

The feature guide that is located here. If you don't find it by the time I get a chance I will repost the link.

 

[silver billet]

What's the equations for lifespan in regards to rpm given that friction and load are equal and static?



I'm sure as you know, you lose honestly as you increase simplicity. Noting in life is simple, especially not in Engineering. This isn't a black and white scenario. And it's just as dishonest and not real to make blanket statements like yours as it is to spew info like mine to someone who may not understand and use the info the wrong way.

And by definition, in the truck with the shorter gears, each spin of the crank has to do more in order to achieve the same result at the wheels. Is that not higher load? Force of Friction increases with load too, right?

With your logic (and others using it as well), looking at two trucks identically equipped pulling the same load for a perceived lifetime, you're actually saying the 3.21 truck's engine would last longer purely due to the fact that it's rpms would be 18% lower (if it could run the exact same transmission gear in every situation).

I don't know for sure, but I just get this feeling that it's not the case.

(I know you're not the first person to take the stance you took, I'm just conversing here against the sides of the convo, not against any person in particular, It's not a you vs me thing, I just quoted you for context of the convo.)

Instead of focusing on the higher rpm of the 3.92, we should be asking "which engine/gear ratio" has to work harder to pull the trailer over the lifetime of the truck. Higher RPM's only affects the last gear (8) since at that point the 3.92 can't upshift any further. Before that point, the 3.92 can just upshift into the next gear and then drop the RPMs again. My guess is that the 3.92 has less stress on the engine pulling the same load, because at any given rpm, the 3.92 is putting more force on the road than the 3.21 is. Or am I wrong?