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Old 05-17-2017, 07:35 PM
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3.21 gear comparison

After joining up on the forum here, I've noticed quite a bit of disdain for the factory 3.21 gear selection by FCA. It got me thinking about how a couple rigs might compare from a statistical comparison.

Here's a quick comparison between two rigs I've recently owned:

First- an '05 Jeep TJ Rubicon 4speed auto with 4.11 gears and stock 32" tires

Second- a '16 JK Sport 5 speed auto with 3.21 gears and stock 32" tires

The TJ has a curb weight of about 3750 lbs and the JK has a curb weight of about 3900 lbs

The TJ's 4.0 puts out 190 hp and 235 ft lbs of TQ

The JK's 3.6 puts out 285 hp and 260 ft lbs of TQ

The final drive ratio of the TJ auto gears with 4.11's is:
1st- 11.67
2nd- 6.45
3rd- 4.11
4th 2.84

The final drive ratio of the JK auto gears with 3.21's is:
1st- 11.52
2nd- 7.03
3rd- 4.53
4th- 3.21
5th- 2.66

The TJ Rubicon has always been highly regarded from a driveability standpoint and I don't recall much criticism about being under-geared in stock form. It was always the benchmark of acceptable gearing for 32" tires.

Yet by the stats above, the final drive ratio's of the JK auto are very comparable to that of the TJ Rubi, the curb weight is only a shade more, and the hp and torque of the JK are increased.

Obviously the increased capability of the low range TC and lockers in the Rubicon would be a difference maker in off-road crawling, but for the sake of daily street driving performance, these two rigs are very comparable on paper.

I'll go on record as stating I feel that 32-33" tires are probably the max I'd feel comfortable running on the JK with 3.21's from a performance standpoint

Feel free to set me straight if I'm missing something here.

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Old 05-17-2017, 07:44 PM   #2
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Peak torque isn't the metric you should be looking at. Low end torque at part throttle is far more important than peak torque at wide open throttle. The 4.0L shines there compared to a smaller displacement v6.

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Old 05-17-2017, 07:49 PM   #3
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You can cut it any which way you want, but the JK is still undergeared with 3.21 unless you have the stock Sport 29 inch tires.
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Old 05-17-2017, 07:52 PM
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Peak torque isn't the metric you should be looking at. Low end torque at part throttle is far more important than peak torque at wide open throttle. The 4.0L shines there compared to a smaller displacement v6.
Not so much. The 3.6l gets to 250 ft lbs at a mere 2000 rpm and stays above 250 all across it's curve. Very stable torque curve.
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Old 05-17-2017, 07:53 PM   #5
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Not so much. The 3.6l gets to 250 ft lbs at a mere 2000 rpm and stays above 250 all across it's curve. Very stable torque curve.
Again, that is at wide open throttle. I don't know about you, but that's not typically how I drive around. At part throttle the online motor will create more torque.
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Old 05-17-2017, 07:55 PM
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You can cut it any which way you want, but the JK is still undergeared with 3.21 unless you have the stock Sport 29 inch tires.
By comparison, would you suggest the TJ Rubicon was also under-geared in stock form? The final drive numbers are almost identical, yet it was underpowered in comparison.
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Old 05-17-2017, 08:00 PM
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Again, that is at wide open throttle. I don't know about you, but that's not typically how I drive around. At part throttle the online motor will create more torque.
2000 rpm is wide open throttle? The 3.6l creates 250 ft lbs of torque from a very low 2000 rpm all the way up to nearly 6000 rpm.
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Old 05-17-2017, 08:03 PM   #8
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2000 rpm is wide open throttle? The 3.6l creates 250 ft lbs of torque from a very low 2000 rpm all the way up to nearly 6000 rpm.
Dyno pulls are performed at wide open throttle the entire rev range. That is how dyno pulls work. A dyno pull at part throttle will tell you essentially nothing as each person running the engine on the dyno could have a different throttle position. At part throttle cruising around at 2000rpm you are not making 250lb-ft of torque.
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Old 05-17-2017, 08:12 PM
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Dyno pulls are performed at wide open throttle the entire rev range. That is how dyno pulls work. A dyno pull at part throttle will tell you essentially nothing as each person running the engine on the dyno could have a different throttle position. At part throttle cruising around at 2000rpm you are not making 250lb-ft of torque.
That's an interesting perspective. So you are stating that a singular engine can produce different torque figures at the same 2000 RPM?
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Old 05-17-2017, 08:18 PM   #10
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That's an interesting perspective. So you are stating that a singular engine can produce different torque figures at the same 2000 RPM?
Yes, that's exactly what I'm saying. Volumetric efficiency changes with throttle position, and the amount of air present in the combustion chamber changes as a result. That changes fuel requirements for the engine at a specific rpm because more air requires more fuel to keep from going lean, and as a result creates more power. The heads and intake design on the pentastar lends itself to higher power levels at higher rpm and at wider throttle openings.
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Old 05-17-2017, 08:29 PM
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Yes, that's exactly what I'm saying. Volumetric efficiency changes with throttle position, and the amount of air present in the combustion chamber changes as a result. That changes fuel requirements for the engine at a specific rpm because more air requires more fuel to keep from going lean, and as a result creates more power. The heads and intake design on the pentastar lends itself to higher power levels at higher rpm and at wider throttle openings.
The 2000 rpm reading was taken on its way to wide open throttle, not at wide open throttle.
I've never witnessed a dyno pull where the operator mashed the pedal to the floor from the get-go. It's done on a smooth and steady application of the throttle throughout the rpm range.

In any case, the 3.6l motor has one of the most consistent torque values across it's RPM range of any Jeep motor ever offered in a Wrangler. Near peak torque at 2000 rpm is remarkable.
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Old 05-17-2017, 08:35 PM   #12
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The 2000 rpm reading was taken on its way to wide open throttle, not at wide open throttle.
I've never witnessed a dyno pull where the operator mashed the pedal to the floor from the get-go. It's done on a smooth and steady application of the throttle throughout the rpm range.

In any case, the 3.6l motor has one of the most consistent torque values across it's RPM range of any Jeep motor ever offered in a Wrangler. Near peak torque at 2000 rpm is remarkable.
You're almost certainly talking about a chassis dyno, which these numbers do not come from, and what you're seeing is the guy running the car up to whatever is the 1:1 gear in the transmission before he goes full throttle. These numbers are from an engine dyno, and the 2000RPM number you're seeing is absolutely 100% a measurement taken at wide open throttle.
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Old 05-17-2017, 08:50 PM
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You're almost certainly talking about a chassis dyno, which these numbers do not come from, and what you're seeing is the guy running the car up to whatever is the 1:1 gear in the transmission before he goes full throttle. These numbers are from an engine dyno, and the 2000RPM number you're seeing is absolutely 100% a measurement taken at wide open throttle.
Obviously the figures given on a manufacturers dyno chart are values at the crank, not the rear wheels, so my reference dyno operator may have been irrelevant.
In any case, the torque figures of the 4.0l would have been taken in the same fashion as that of the 3.6 as you are suggesting, correct?
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Old 05-17-2017, 08:55 PM   #14
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Obviously the figures given on a manufacturers dyno chart are values at the crank, not the rear wheels.
And by that same token, the torque figures of the 4.0l would have been taken in the same fashion, correct?
Yes. Both figures are taken at the crank. And they are taken at wide open throttle. Part throttle torque is not the same as wide open throttle torque. On the 4.0L at part throttle you have advantages in stroke length and higher rotational mass. This helps you achieve more part throttle torque at lower RPM, hence why the 4.0L Jeeps are seen as more driveable than the 3.6 with the 3.21 gears.
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Old 05-17-2017, 09:08 PM
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Yes. Both figures are taken at the crank. And they are taken at wide open throttle. Part throttle torque is not the same as wide open throttle torque. On the 4.0L at part throttle you have advantages in stroke length and higher rotational mass. This helps you achieve more part throttle torque at lower RPM, hence why the 4.0L Jeeps are seen as more driveable than the 3.6 with the 3.21 gears.
Well I may have been too quick to throw out the torque value comparison and not have taken the real time use range into consideration. I was primarily looking at final drive ratio's being so similar.

It should probably be pointed out that the final drive numbers in my original post only apply to the 2012+ 5 speed JK auto's. The 07-11 JK's would have the same transmission ratio's as the earlier TJ Rubicon and thus would have drastically different final drive ratio's.
Perhaps this lends itself to some of the criticism for the 3.21 gears as the performance of them with the early transmission would undoubtedly be less than stellar.

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