5.13 or 5.38
02-24-2011, 02:44 AM
#11
JK Jedi Master
You're going to find your engine reving high when driving on the Interstate. The chart posted earlier shows that. I have just upgraded to 35s and 4.88s from 33s and 4.10s. And I definitely like the improvement.
For you, you're already suffering with the 35s on 4.10s (I did both upgrades at the same time), so you'll see a pretty dramatic improvement in performance with a bump to 4.88s. Just from mechanical advantage alone (the gear change), you'll experience a 19% bump in torque ((4.88-4.10)/4.10x100). If a CAI or a programmer or a Fitch Catalyst POS could give you a 19% bump in torque, everyone would buy one!
On top of that, you'll be revving at a better part of the 3.8L engine's HP/torque curve. At Interstate speed of 70 MPH, you'll move from 2316 RPM to 2756 RPM (see the chart earlier). If you look at the 3.8L HP/torque charts (kandn.com/dynocharts/57-1553_dyno.pdf), you'll see that is a bump from about 50HP/110 ft-lbs to 90HP/170 ft-lbs ... that's an 80% improvement in HP and a 55% improvement in torque! When you need to pass someone on a two lane highway, your Jeep will just do it.
And, these improvements are even more dramatic than at first blush, because the penalty you must pay to overcome friction in the drivetrain is not a set percentage of the total HP/torque--it's already paid whether you have very little HP/torque or tons. IOW: Most of the gain will make it to the wheels. This is huge!
Finally, for the 5.38s: Note that when going to 5.38s from 5.13s, you're on a much more level part of the HP curve, and a flat part of the torque curve (see the K & N charts linked earlier). So, you'll see a slight bump in mechanical advantage over a 4.88 (5 percent), but very little from being on a better part of the HP/torque curve. The expense you must pay for this is reduced gas mileage, and slightly more fragile gears.
For you, you're already suffering with the 35s on 4.10s (I did both upgrades at the same time), so you'll see a pretty dramatic improvement in performance with a bump to 4.88s. Just from mechanical advantage alone (the gear change), you'll experience a 19% bump in torque ((4.88-4.10)/4.10x100). If a CAI or a programmer or a Fitch Catalyst POS could give you a 19% bump in torque, everyone would buy one!
On top of that, you'll be revving at a better part of the 3.8L engine's HP/torque curve. At Interstate speed of 70 MPH, you'll move from 2316 RPM to 2756 RPM (see the chart earlier). If you look at the 3.8L HP/torque charts (kandn.com/dynocharts/57-1553_dyno.pdf), you'll see that is a bump from about 50HP/110 ft-lbs to 90HP/170 ft-lbs ... that's an 80% improvement in HP and a 55% improvement in torque! When you need to pass someone on a two lane highway, your Jeep will just do it.
And, these improvements are even more dramatic than at first blush, because the penalty you must pay to overcome friction in the drivetrain is not a set percentage of the total HP/torque--it's already paid whether you have very little HP/torque or tons. IOW: Most of the gain will make it to the wheels. This is huge!
Finally, for the 5.38s: Note that when going to 5.38s from 5.13s, you're on a much more level part of the HP curve, and a flat part of the torque curve (see the K & N charts linked earlier). So, you'll see a slight bump in mechanical advantage over a 4.88 (5 percent), but very little from being on a better part of the HP/torque curve. The expense you must pay for this is reduced gas mileage, and slightly more fragile gears.
Last edited by Mark Doiron; 02-24-2011 at 06:40 AM. Reason: Deleted HP from the mechanical advantage paragraph (doh!)
02-24-2011, 03:21 AM
#12
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02-24-2011, 07:15 AM
#14
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I am running 37" KM2 with 5.13 gears and a 6 speed. Hwy is great about 3000 rpms at 75. I have a lot of hills where I live and the gears really helped keep constant speeds. Also mpg about 14.5 -15 half street and half hwy driving. The gears are the best mod so far!!!
