3:21 vs 3:92 Gears

[Dusty1948]

I agree. I was comparing the 6.2 to my old 5.3, not the Hemi. Sorry for the confusion there...

Ah. No problem.

Best regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 045634 miles.

 

[Willwork4truck]

There's a really long thread relating to this just use the search bar above and type in:

An engineer's Ultimate Guide to 3.21 vs 3.92 axle ratio​

It has more than you'll ever want to know re; this question.

Thank you for showing him that thread. This is one of the most popular questions (despite the search function) and it’s constantly rehashed.

I’m sure its a legit query on his part yet... if he has the time to read 1K posts on it he will find that there’s little agreement save for very heavy or very light towing/very little usage.

Guess I had to be the one to say it... .
Not trying to be a jerk, well, I do get accused of it some...

 

[jimchi]

If you look at the Torque/HP curves for the GM 6.2 and the 5.7 Hemi, it's really remarkable how close they really are. On both engines torque and horsepower cross at just about the same 4100 RPM. The 5.7 actually delivers its peak torque at about 3950 and the GM 6.2 at about 4200. In the low RPM range (1000 RPM) the 5.7 Hemi starts producing horsepower and torque just a little bit sooner. At 1500 RPM the 6.2 is about 25 lb. ft. ahead of the 5.7 Hemi. The 6.2 has a flatter torque curve. Across the RPM range the GM 6.2 is never more than <>35 lb. ft. torque ahead of the 5.7 according to published engine graphs. So the advantage of the GM 6.2 motor is really the result of its raw larger displacement of 0.5L.

Best regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 045634 miles.

FYI, HP and torque always cross at 5252 rpm regardless of engine, because HP = torque * rpm* unit constant. More detailed explination here:

EDIT: to clarify, 5252 when measuring in lb-ft

 

[Willwork4truck]

I live in Palm Bay Florida & try as I might, reaching out 250 miles, in inventory Rams with 3:21 gears are like hens teeth. Gotta wonder why....not everyone tows something that heavy that often.

Weird
Plenty here in NC
Must be all those Fl mountain passes...

 

[Willwork4truck]

If I wasn't hauling a 7500 lb boat and fishing gear, I'd have gotten the 3.21 rear. I do a lot of highway driving along with my towing, and the MPG just sucks with the 3.92s.

It would be great if they offered the 3.55 rear with the Hemi. That would kind of sit in the sweet spot for my personal towing needs and at least net better than the 17.5 MPG I get on the highway.

This... x2

 

[Dusty1948]

FYI, HP and torque always cross at 5252 rpm regardless of engine, because HP = torque * rpm* unit constant. More detailed explination here:

EDIT: to clarify, 5252 when measuring in lb-ft

Except looking at actual dyno graphs it doesn't happen. While I won't challenge his math, the theoretical is devoid of varying mechanical influences produced by an internal combustion engine. We know we can alter torque in an engine of any size by changing the piston stroke, camshaft timing, and even increasing or decreasing weight of the flywheel. Maybe in the 'free space' theoretical it "always" happens at 5252 RPM. But in practical terms it doesn't.

Best regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 045634 miles.

 

[jimchi]

Except looking at actual dyno graphs it doesn't happen. While I won't challenge his math, the theoretical is devoid of varying mechanical influences produced by an internal combustion engine. We know we can alter torque in an engine of any size by changing the piston stroke, camshaft timing, and even increasing or decreasing weight of the flywheel. Maybe in the 'free space' theoretical it "always" happens at 5252 RPM. But in practical terms it doesn't.

Best regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 045634 miles.

Do you have examples that back up your hypothesis?

EDIT: Just to summarize the linked video, power (as in horsepower) is a quantity calculated from the measured quantities of torque and rpm. While torque (and therefore power) may peak at different RPM vs. a published graph in the real world, the two curves always cross at 5252 RPM when measuring torque in lb-ft. Again, that is because HP is calculated, even on a real world dyno. Here are some graphs that back this up:
C8 Corvette (red lines = baseline, blue lines = aftermarket exhuast)
Mazda RX7 (a rotary!)
Subaru WRX

 

[silver billet]

Do you have examples that back up your hypothesis?

EDIT: Just to summarize the linked video, power (as in horsepower) is a quantity calculated from the measured quantities of torque and rpm. While torque (and therefore power) may peak at different RPM vs. a published graph in the real world, the two curves always cross at 5252 RPM when measuring torque in lb-ft. Again, that is because HP is calculated, even on a real world dyno. Here are some graphs that back this up:
C8 Corvette (red lines = baseline, blue lines = aftermarket exhuast)
Mazda RX7 (a rotary!)
Subaru WRX

Yes exactly, HP is a function of torque and RPMs, the number 5252 is actually part of the equation so it's not a mistake that the lines cross at that point. The exact equation is: Horsepower = Torque x RPM / 5252

I'd be interested in seeing a torque curve of the update 5.0 from Ford, apparently it's numbers are meeting/beating the 5.7 now. That's NOT good.

 

[Dusty1948]

Do you have examples that back up your hypothesis?

EDIT: Just to summarize the linked video, power (as in horsepower) is a quantity calculated from the measured quantities of torque and rpm. While torque (and therefore power) may peak at different RPM vs. a published graph in the real world, the two curves always cross at 5252 RPM when measuring torque in lb-ft. Again, that is because HP is calculated, even on a real world dyno. Here are some graphs that back this up:
C8 Corvette (red lines = baseline, blue lines = aftermarket exhuast)
Mazda RX7 (a rotary!)
Subaru WRX

It's not an hypothesis. In the previous post I included two engine HP/Torque graphs that clearly indicate torque and horsepower crossing at less than 5252 RPM.

Regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 045634 miles.

 

[HemiDude]

I have the 3.92 hemi, non-e. Don't tow much weight but I feel like this gear would be best if I did. This summer I got over 22 miles to the gallon. According to some wisdom on the site if I had 321s I would've pushed far better than that. Be your own judge

 

[jimchi]

It's not an hypothesis. In the previous post I included two engine HP/Torque graphs that clearly indicate torque and horsepower crossing at less than 5252 RPM.

Regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 045634 miles.

Ah I missed that. I presume the lower crossing value has to do with the SAE J1349 correction factor, which I just learned about. That being said, the HP value is still calculated, and as a result of that calculation the curves cross at the same point in both your graphs (~4600rpm).

 

[BowDown]

Except looking at actual dyno graphs it doesn't happen. While I won't challenge his math, the theoretical is devoid of varying mechanical influences produced by an internal combustion engine. We know we can alter torque in an engine of any size by changing the piston stroke, camshaft timing, and even increasing or decreasing weight of the flywheel. Maybe in the 'free space' theoretical it "always" happens at 5252 RPM. But in practical terms it doesn't.

Best regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 045634 miles.

HP and TQ always cross at 5250 no exceptions.
My guess is that each one of those curves were done independently as in the best HP curve that day and the best TQ curve that day, not necessarily in the same dyno pull. Someone simply didn't take the time to overlay the graphas properly.
An indicator of what I'm saying is the beginning rpm of each pull, 1250 RPM is way low to start a pull, most start at 2500 rpm due to dyno loading inaccuracies below 2500. If you look at that graph and assume that its off by a 1000 rpm, the curves cross exactly where they should.
Of the hundreds of pulls I have on my car, none start below 2000 RPM for that reason and you can see anomalies below 2400 rpm chassis or engine dyno, doesn't matter, they dont start at 1200 rpm

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superchips.com

 

[Dusty1948]

HP and TQ always cross at 5250 no exceptions.
My guess is that each one of those curves were done independently as in the best HP curve that day and the best TQ curve that day, not necessarily in the same dyno pull. Someone simply didn't take the time to overlay the graphas properly.
An indicator of what I'm saying is the beginning rpm of each pull, 1250 RPM is way low to start a pull, most start at 2500 rpm due to dyno loading inaccuracies below 2500. If you look at that graph and assume that its off by a 1000 rpm, the curves cross exactly where they should.
Of the hundreds of pulls I have on my car, none start below 2000 RPM for that reason and you can see anomalies below 2400 rpm chassis or engine dyno, doesn't matter, they dont start at 1200 rpm

Well, the HP/Torque graph was SAE certified. Do you think they are that sloppy?

Regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 045634 miles.

 

[BowDown]

Ah I missed that. I presume the lower crossing value has to do with the SAE J1349 correction factor, which I just learned about. That being said, the HP value is still calculated, and as a result of that calculation the curves cross at the same point in both your graphs (~4600rpm).

No, they plotted the graph wrong. You can adjust the plots/rpm/wheel mph on most dynos

Well, the HP/Torque graph was SAE certified. Do you think they are that sloppy?

Regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 045634 miles.

I think they unintentionally plotted or scaled the graphs wrong, thats adjustable (the scaling) on every dyno. Its either scaled wrong (rpm increments) or they pasted the curves (multiple curves) over a graph sloppily.
I don't know what they were doing, they could have scaled it wrong or (my guess as to what they actually did) they took the best HP curve that day and the best TQ curve that day and overlaid them without paying attention to the RPM scaling.

Again, just looking at the starting RPM, I can see those graphs are wrong

 

[Dusty1948]

No, they plotted the graph wrong. You can adjust the plots/rpm/wheel mph on most dynos


I think they unintentionally plotted or scaled the graphs wrong, thats adjustable (the scaling) on every dyno. Its either scaled wrong (rpm increments) or they pasted the curves (multiple curves) over a graph sloppily.
I don't know what they were doing, they could have scaled it wrong or (my guess as to what they actually did) they took the best HP curve that day and the best TQ curve that day and overlaid them without paying attention to the RPM scaling.

Again, just looking at the starting RPM, I can see those graphs are wrong

I have to respectfully disagree. The SAE J1349 certification is depended upon by the manufacturers for technical specification and market competitiveness. These tests are not cheap. If a manufacturer thought the results of the certification were inaccurate, you can bet there would be a re-evaluation before the SAE applied and published the certification. Keep in mind that the test, which is on an engine dyno (not a chassis dyno), is performed by a third party and witnessed by a manufacturer AND an SAE representative to ensure test procedure compliance. While the theory may be valid, there are sometimes counter influences in the individual engine dynamics.

Regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 045634 miles.

 

[BowDown]

I have to respectfully disagree. The SAE J1349 certification is depended upon by the manufacturers for technical specification and market competitiveness. These tests are not cheap. If a manufacturer thought the results of the certification were inaccurate, you can bet there would be a re-evaluation before the SAE applied and published the certification. Keep in mind that the test, which is on an engine dyno (not a chassis dyno), is performed by a third party and witnessed by a manufacturer AND an SAE representative to ensure test procedure compliance. While the theory may be valid, there are sometimes counter influences in the individual engine dynamics.

Regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 03 June 2018. Now at: 045634 miles.

I agree but that's not a real dyno graph or even a certified one nor does it mean that those 2 lines are from one dyno. The original operation may well be SAE J1349 certified and I have no reason to believe its not but that representation was not an accurate representation of one dyno pull IMO, the scaling is wrong plain and simple. It looks like a graphic thats the "result" of a real SAE dyno pull but looks nothing like a dyno graph

Here's a real SAE corrected dyno


Notice the starting rpm, notice the anomalies below 3k and this was on smoothing factor 5. That other dyno was waayyy to smooth so I know its a for the public view graph and not the actual graph, no dyno is that smooth.
I have another pull that starts at 2000 rpm and even noiser down low. notice the hp/tq crossover.
The only way to get hp and tq to crossover sooner is by manipulating the graph

 

[BowDown]

Also, here's another SAE certified graph from the same source

Notice where HP and TQ crossover on this certified graph, its just sloppy scaling and the combination of multiple "best" result dyno pulls placed on one graph.
These are so egregious regarding basic engine dyno principles, i'm surprised no one caught them. Peak power and tq crossing at 2600 rpm, umm no.

 

[silver billet]

Dusty, HP is not measured. It's calculated using RPMs and measured Torque. If you're getting graphs where they don't meet at that magic number, somebody got something wrong.

 

[screamin chicken]

This is what people dont get about the 3.92. The 3.92 is only an advantage in 1st gear then its over, its too much gear for the 5.7 after that and you dont stay in a gear long. If the 5.7 could rev to 7K, the 3.92 may have more of an advantage after 1st but the 5.7 just doesn't have enough rpm to take advantage of the gear and stay in power like it does with the 3.21.
I'd put money on the race with a 3.92 truck vs a 3.21 that the race would be far closer than anyone thinks and from a roll, the 3.21 truck drive away from a 3.92 truck simply because it stays in the peak power band longer.
The only advantage to the 3.92 is from a dig and that advantage, I'd bet is less than 1/2 a truck length.
That said, expecting either of these gears to be fast in this truck is funny, I have zero need for this truck to be a 13 second 1/4 mile truck, it would be fun but highly useless. I have a 620 whp Z06 for that

Love the Z06 , I have a Modded, and a big Thunder Racing custom cam in my 2000 WS6

 

[MOONCHASER]

Took delivery of my 2021 Ram Limited E-torque V-8 with 20" rims & 3:21 gears yesterday. I am thrilled with my decision to go with the 3:21 rear gears over the 3:92. Maybe it's because I had a HD2500 Diesel before but this truck has all the off the line power I can imagine needing.