Hi I am looking at he torque curves for the 3.8 I have a 2008, rubi 4 dr with 4:10s and an auto < I understand the the tire size graphs are related to rpm which is derived from the horse power curves. For all of you who are much more knowledge able than I how does the torque curve play into this ? I see that the torques only varies maybe 20 ft lbs form about 2000 rpm to 3400 rpm. Yet the hp varies quite a bit more. so looking at tires and torques what does tell us?

 

hp is derived from torq and rpm. So the longer your torq carries out the greater hp you have. You can have a ton of torq but if it drops off al low rpm you have low hp. You can also have high hp and low torq if your torq carries out over high rpms.
the formula is HP= torqXrpm/5252

 

The 3.8L torque curve doesn't start to flatten until about 2500 rpm.

 

This is why you want to regear low enough to run at least 2500 rpm at 70 mph.

 

This is something I put together regarding a very specific request a few years back. But, I think it will help answer OP's question about horsepower, torque and wheel size. I'm posting it unedited (I saved a copy on my hard drive for future use), so please ignore references to the earlier thread (about CAIs and Fitch Catalyst stuff) ...




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 (.pdf at http://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 [horsepower/toque chart linked above]). 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.

 

Lost of great info in this thread. I am thinking of going to 35’s and trying to decide between 4.56 and 4.88 My question is will this info be the same for a manual transmission 3.8 jku.

 

My JKU is a manual transmission with the 3.8L motor and 4.88 gears, 35" tires. It's a slug, but does get places--turned 277,000 miles just yesterday, and two weeks ago ...

 

Wow it’s a slug even with the 4:88! That’s good to know. What is your rpm range in 6th at 65-70mph?

 

About 3,000 RPM. Bear in mind that my Jeep is heavy--well over 6,000 pounds. And I'm comparing it to my Gladiator, which is light and has the 3.6L with 4.10 gears (on the same 35 inch tires). Both have manual transmissions, but the gearing in the Gladiator is weird--short 1st and 2nd and tall in 5th and 6th. ...