Another 3.21 vs 3.92 comparison

[riccnick]

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.

I don't see the page that you screenshot-ed for some reason. I can get to the Feature Guide but it looks slightly different. (At least on my end)

 

[riccnick]

Or am I wrong?

You're right, and wrong, and so is everyone in this thread.

When speed is static, and the transmission gear is the same, the 3.92 trucks rev 22% higher than a 3.21 truck.

When rpm is static and the transmission gear is the same, the 3.92 trucks don't have to work as hard to accomplish the same toque at the wheels, however ground speed will be lower.

When speed is static and the transmission gear can change, a 3.92 and 3.21 truck can come very close to achieving the same mechanical advantage at a very similar engine rpm, until you reach the limits of either the ratio spread, or the engine's operating rpm. (That was the point of the chart I made, for context in the other thread)

 

[silver billet]

You're right, and wrong, and so is everyone in this thread.

When speed is static, and the transmission gear is the same, the 3.92 trucks rev 22% higher than a 3.21 truck.

When rpm is static and the transmission gear is the same, the 3.92 trucks don't have to work as hard to accomplish the same toque at the wheels, however ground speed will be lower.

When speed is static and the transmission gear can change, a 3.92 and 3.21 truck can come very close to achieving the same mechanical advantage at a very similar engine rpm. (That was the point of the chart I made, for context in the other thread)

You lost me there, sorry I'm a little slow. That seems an odd way to look at it.

My view is; at any given speed below (say) 55 mph, both trucks can use gears 1 - 7 as needed. RPM's will go up and down while shifting. Only until the 3.92 enters gear 8 does the situation change, because at that speed the 3.92 has run out of gears and can only rev up. The 3.21 will upshift later, and then rev lower for the same speed.

Before that point, the 3.92 is working less hard, with less stress, to get the same speed out of the truck, because it can apply more force to the ground. It's meaningless to compare RPM's between the two without knowing what speed you will be staying at; there are certain speeds where the 3.92 has just upshifted and is now revving lower than the 3.21.

 

[devildodge]

I don't see the page that you screenshot-ed for some reason. I can get to the Feature Guide but it looks slightly different. (At least on my end)

Yes. That is not it. I got it from FCA media site...but think I can find it now....nope

 

[Rob5589]

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

Actually it makes perfect sense, when you think about it. More gears means less rpm drop off every time it shifts. I will provide this example; my old Silverado had the 4L60E 4 speed, 3.73, and 4.8. In tow mode towing my jeep, it would shift at say 25 mph from 1-2. The rpm's would drop from around 3100 to 1700. That significant drop off would cause it to bog in 2nd until it reached around 2200 rpm when it would begin to enter it's torque curve. Then say it shifts from 2-3 @ 50 mph, 3300-2000 rpm, again, it would bog down until reaching the beginning of it's tq curve.
Using the same parameters but with the 8HP75, the drop offs are ~1000 rpm 1-2, ~ 400 rpm 2-3; I stopped there since the 4L60 is a 3 spd with 4th as OD. But here is the big take away, the mph in the 8 spd at similar rpm's are only 16 and 28 mph. So you are shifting sooner but in a much more progressive way. Acceleration is smoother, more controlled, and the engine stays within it's tq curve much closer.

As far as your Wrangler, it has the aerodynamics of a shoe box Mine did fairly decent but it did have the 2.4. 31's, 4.88, 6 spd, it would knock down 19 on the hwy all day long. Even trailing around it was 16-17.

 

[SpeedyV]

Where did you find that!?!?

I have the specs document too. I guess we should host these things in the resources section. What do you think, @devildodge ?

 

[geotex1]

Your mathematical assessment is only looking at one factor. Real decisions are made by multi-dimensional comparisons and then decision made suited to the need. We engineers often assess these via pair-wise comparison and plotting a Pareto to reveal rankings. Seat of the pants could be a metric, but I'd rather turn that into a true assessment of comparative acceleration for instance.

If you're an occasional tower, your final evaluation of the dimensions may show 3.21 is your best choice. If you tow daily anything in the weight range lawfully requiring traile brakes, I would be surprised if an evaluation of all dimensions resulted in anything other than 3.92s. I miss the days of being able to select from 4 to 5, sometimes even 6 stock gear ratios, but you can thank governmental requirements for that level of choice no longer available to us...

Have a look at my signature... I do a "little" towing... So YMMV from mine!

 

[Gmoosevt]

Agree with all. If you are towing heavy stuff, get the 3.92 or a 2500/3500. But I also think the vast majority of readers/bloggers here - with light duty 1500’s - are mostly just doing occasional towing of utility trailers, fishing boats, and maybe a camper. Stuff under 5000#’s - and towed occasionally, not frequently. And probably 50% of them are flat landers as well. They are somehow made to believe that spending extra money on a 3.92 and suffering the mpg hit is what they need to do if they do any towing at all... which just isn’t the case. But again, yes, for the 20% that might be towing heavier stuff, more often, and/or where there might be +6% grades... then for certain, the 3.92 will make the towing a better experience - for them and their engine/trany. I’ve kept an eye on the TFL truck guys - who have been doing a good bit of ‘real world’ moderately controlled testing (which I put more credence in than a bunch of ad hoc blog contributors with huge variability in their data collection conditions). They have found that hemi 3.92 trucks - and especially the Rebels - are falling well below the Maroney posted mpg by 2 or more mpg. Not so with V8 3.21’s... where actual real-world highway and combined mpg is at or above the sticker rating. It’s true that Rebels are higher, and with bigger rolling mass tires, and lack the +35 mph aero effects - and they all have the 3.92 - so of course they are going to suffer a lot on mpg and never get close to their Maroney. But regular crew cabs with the hemi and 3.92 are suffering a bit as well. It is very possible that owners with 3.92’s are using their trucks differently than those with 3.21’s... but since the 3.21 gas mileage is turning out to be better - my engineering intuition is that the 3.21 is keeping the V8 hemi at a better efficiency point for more of the engine hours and work demand than those fitted with a 3.92. This makes sense, since the 3.21 fitted engine will run at lower rpm more often (less friction and pumping losses) and it’s broad and flat torque curve is plenty adequate for a high percentage of drive time... if not nearly 100% of it.


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[Gmoosevt]

Hey, no problem, no ‘fense’ taken. I think we both know the dynamic load details of an internal combustion engine, and can fill-up lots of white boards with the calcs. After all, the physics involved are not debatable, and buy-and-large the internal combustion piston engine hasn’t changed for a 100 years... other than a steady stream of continuous improvements in materials, combustion enhancements, and controls over the engine’s dynamic range. But how many of the blog participants are MEs with a long career in combustion engines and drive trains like us. Some maybe. Most not. They are quickly bored listening to you and I, and they don’t really care about all the statics, dynamics, thermo, heat transfer, fluids and physics principles involved. I prefer to just provide digestible information. Does it require over-simplification? Yes, absolutely. Can someone state operating condition cases where the simplification might not hold true? Yes, absolutely (e.g. towing 8,000 lbs with a light duty truck up a steep grade in hot weather at 7000 ft altitude, with 87 octane, 35” tires, and a transmission that has 3 overdrive gears... or whatever). My principle simplification point is that in general, for two identical engines, the one that is run at lower rpm for its lifetime will have less wear than the engine run at higher rpm. Most of that wear comes from friction. Higher rpm = higher friction. A 3.92 will run an engine at a higher rpm average over its life than a 3.21. If things are taken to the extreme, the simplified principles make more sense. For example: NASCAR runs the same engine blocks as the family truckster. Albeit every other part on the car and engine is different - but which engine lasts longer? The Nascar one run at 10k rpm? ...or Jim and Joan Middleclass’s engine? One engine lasts about 5 hours before overhaul is needed. Jim and Joan’s engine will last 5000 hours. It’s why in my world - of huge internal combustion engines in utility plants - where 30 year lifetimes are necessary - with 24x7 continuous duty cycles - we could get more output power by increasing the rpm - but the engine wouldn’t last, and the efficiency loss of trying to cool it from the frictional and combustion heat kills its financials. So, the engines are designed to run at low rpm in a sweet spot of torque and really tall gear reducers are used to drive the generators and compressors and such. Why? It’s all about long lifetime expectancy of the engine by keeping engine rpms and internal dynamic loads as low as possible, and efficiency as high as possible. We could use 1000 hp NASCAR engines too, and get the same output to the compressors and generators - with much different gearing - but we’d have to replace the engine every few hours - due to bearing, ring, and valve wear. It’s the principle reason why a 5.7 hemi with a 3.21 gets better gas mileage vs a 3.92, and why the 3.21 engine probably has a slightly lower probability of wear and repair events over its service life. I have no physical evidence to that latter claim, but certainly the white-board calcs would lead to that conclusion every time. So if someone asks: 3.21 or 3.92? My answer is 3.21, unless you are towing heavy loads (>5000 lbs) frequently, in somewhat hilly regions. Why? because you’ll get better mpg and likely less repairs over the life of the engine - assuming you are shooting for that 5000 hour-plus life cycle. If you really, really want good mpg and long-life cycle, get a diesel (next discussion.? argh.). Why does a diesel usually last longer? Simplistic reason: lower rpm.


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[Willwork4truck]

Ha ha, when I want to hear all engineer talk that about any topic I go visit my SIL, an ME who does stress work on planes. I can’t have a reasonable conversation with him as it devolves into technical terms very quickly. “Look, I didn’t take materials and stress classes in college so slow down...”
Anyway, thanks for both your your contributions. It is and will continue to be good fodder for discussion, as will gas v diesel in the 2500/3500 HD forums. (Although a few will try to justify their diesel 1500 purchases here).
Since I’m in the low hills NC area and don’t tow, my head said buy a 3.21 even though my heart said get the 3.92 (in case...) They both would have agreed on 3.55 however that wasn’t offered. Ford is smarter there, they selectively offered 3.55 and 3.73 for the F150, in addition to the 3.2 something that was standard on lower trims.

 

[devildodge]

Once again the only difference between 3.21 and 3.92 is GCWR. 13900 and 17000.

If you want gas mileage get the HFE...pretty much useless for anything else. Otherwise, get the truck you want and enjoy having a 1500. I can not justify a 1500 so I have a 2500, and I got 3.73 gears

If you tow over 5000 lbs think about 3.92. If you live in hilly terrain think about 3.92. If you want a better seat of the pantsrockete ship think about the 3.92.

If you just like the size and comfort of a truck think about the 3.21. If you plan to maximize MPG with your driving habits think about the 3.21.

I will be getting 4.10 on my next truck. Probably go 3500 if I buy new...unless I decide to purchase a Powerwagon

 

[SpeedyV]

Once again the only difference between 3.21 and 3.92 is GCWR. 13900 and 17000.

If you want gas mileage get the HFE...pretty much useless for anything else. Otherwise, get the truck you want and enjoy having a 1500. I can not justify a 1500 so I have a 2500, and I got 3.73 gears

If you tow over 5000 lbs think about 3.92. If you live in hilly terrain think about 3.92. If you want a better seat of the pantsrockete ship think about the 3.92.

If you just like the size and comfort of a truck think about the 3.21. If you plan to maximize MPG with your driving habits think about the 3.21.

I will be getting 4.10 on my next truck. Probably go 3500 if I buy new...unless I decide to purchase a Powerwagon

Power Wagon FTW...

 

[silver billet]

It’s the principle reason why a 5.7 hemi with a 3.21 gets better gas mileage vs a 3.92, and why the 3.21 engine probably has a slightly lower probability of wear and repair events over its service life. I have no physical evidence to that latter claim, but certainly the white-board calcs would lead to that conclusion every time. So if someone asks: 3.21 or 3.92? My answer is 3.21, unless you are towing heavy loads (>5000 lbs) frequently, in somewhat hilly regions. Why? because you’ll get better mpg and likely less repairs over the life of the engine - assuming you are shooting for that 5000 hour-plus life cycle.

I don't think you can make that claim. You're simply looking at RPM's, saying "the 3.92 will rev higher", reving higher is worse for lifetime expectancy than revving lower (which is a true statement in isolation by itself), therefore 3.21 is the longer lasting option.

That's extremely simplistic. There are other factors at play, such as, the 3.92 will work less hard and be under less stress than the 3.21 under any sort of load, or during regular acceleration without a load. If the 3.92 can hold a gear wheras the same speed/load/hill would force the 3.21 to constantly downshift, that is going to affect lifetime expectancy as well, my gut feeling is that would be worse than reving a couple hundred rpms higher. Even at the same rpm, the 3.92 has to be putting less stress on the motor, no?

As for the diesel lasting longer; again you simplistically say "it revs lower". My non-engineer-butt feeling is telling me that's probably more a case that they are built much much stronger to withstand the higher compression.

I think if we look at the evidence and past history, there is no data to support that one engine with a one gear ratio lasts longer than the other. If there is any sort of data, it must be so close as to be a non issue. Driving style and regular maintenance probably has much more affect than what gear ratio you choose.

My viewpoint is: either gear ratio will work for the vast majority of buyers. If you want to maximize mpg, get the 3.21. If you want to maximize towing power, get the 3.92. It's really that simple.

 

[Moe1962]

I'm hoping someone can clear up something for me about this 3.92 VS 3.21 deal. So, I understand the gas mileage difference, towing in a hilly environment and efficiency, however, I don't understand how two trucks, outfitted the same, except gear ratio, and one has a higher towing capacity than the other. Help me understand how a gear ratio will allow you to tow more.

 

[devildodge]

I'm hoping someone can clear up something for me about this 3.92 VS 3.21 deal. So, I understand the gas mileage difference, towing in a hilly environment and efficiency, however, I don't understand how two trucks, outfitted the same, except gear ratio, and one has a higher towing capacity than the other. Help me understand how a gear ratio will allow you to tow more.

See above....the 3.92 is rated for a 17000 GCWR and 3.21 is rated 13900. This is because of how it performs under the conditions of SAE J2807.

Everyone has a different opinion. But it is based on how it tows on hills, under acceleration, and most importantly deceleration and descent.

 

[devildodge]

@Moe1962

Here are the main test methods trucks would be measured on as per J2807:

• Cooling capability on a long highway upgrade modeled on the Davis Dam grade on Arizona SR 68;
• Launch and acceleration performance on a level road and a 12 percent upgrade;
• Combined handling performance – understeer and trailer sway;
• Combined braking performance – stopping distance and parking brake-hold on grade; and
• Structural performance for the vehicle and hitch or hitch receiver.

New calculations for trailer weight ratings: In addition to the performance standards, SAE J2807 also uses a specific set of assumptions to calculate maximum trailer weight ratings:

• For light-duty full-size pickups (GVWR < 8,500 lbs.), SAE J2807 assumes that the tow vehicle includes any options with higher than 33 percent penetration;
• It assumes there is both a driver and passenger in the vehicle, each weighing 150 pounds;
• It assumes that tow vehicles also include up to 70 pounds of aftermarket hitch equipment (where applicable); and
• For conventional trailer towing, SAE J2807 assumes that 10 percent of the trailer weight is on the tongue.

 

[raven_DT]

I'm hoping someone can clear up something for me about this 3.92 VS 3.21 deal. So, I understand the gas mileage difference, towing in a hilly environment and efficiency, however, I don't understand how two trucks, outfitted the same, except gear ratio, and one has a higher towing capacity than the other. Help me understand how a gear ratio will allow you to tow more.

Old but good article that covers the spec:

What Is SAE J2807? What Does It Really Mean For Your Pickup Truck?

The maximum-claimed towing capacity of most pickup trucks is ********. Those numbers are created by manufacturers and vetted by no one. The Society of Automotive Engineers (SAE) wants to change that, with a standard test for measuring truck towing capacity called "J2807."

www.google.com

 

[Moe1962]

Old but good article that covers the spec:

What Is SAE J2807? What Does It Really Mean For Your Pickup Truck?

The maximum-claimed towing capacity of most pickup trucks is ********. Those numbers are created by manufacturers and vetted by no one. The Society of Automotive Engineers (SAE) wants to change that, with a standard test for measuring truck towing capacity called "J2807."

www.google.com

Thanks for the responses and the link. I'm still not quite sure it answers my question, but I will read more on the link provided.

I didn't take into account the cooling capacity, so maybe that's the key to it all, other than towing efficiency, acceleration and deceleration.

 

[Moe1962]

Thanks for the responses and the link. I'm still not quite sure it answers my question, but I will read more on the link provided.

I didn't take into account the cooling capacity, so maybe that's the key to it all, other than towing efficiency, acceleration and deceleration.

I have towed other things in the past with other vehicles, but this is the first time I have researched towing, since I plan to buy a camper and don't want to get myself in a pickle someplace that could be remote. I never really gave it a lot of thought before and probably could have easily injured myself or someone else. Glad this forum is here for us newbies.

 

[Cr250Ram]

I have towed other things in the past with other vehicles, but this is the first time I have researched towing, since I plan to buy a camper and don't want to get myself in a pickle someplace that could be remote. I never really gave it a lot of thought before and probably could have easily injured myself or someone else. Glad this forum is here for us newbies.

How much does the trailer weight? After a ton of research I’ve come to the conclusion you will need a 2500 or higher if you are towing a camping trailer and having your family ride in the truck. That gets even more reinforced if you end up needing to put anything in the trucks bed. I gave up on buying a camping trailer, if I still planned on buying one I’d buy a 2500 gas or a 3500 diesel. The problem mainly boils down to payload. A tradesman CC 4wd 2500 diesel has 2,250 payload. The gas version has about 1,000 more payload. I would want to be able to tow a 10,000 trailer so that’s 1,500 (@15 tongue weight), that leaves 750. Subtract 600 for my family and that’s only 150 left over for bed cover, bed liner, snacks, and any gear you put in the bed. That’s too close for comfort for me.