To me, running a poorly balanced engine at 3k rpm for hours at a time is one thing. However, you can also run into cooling issues in the trans, t-case, and diff's from running such a higher rpm. You are also more likely to run into vibration issues because of the drive shaft speed. Yes the power is nice.

But there is no way you will get decent mileage with 4.88's and 33's on the HWY. Around town you may get a bit better than stock if you drive it right. But not on the HWY. The math isnt there to run those kind of RPM and still get good fuel economy unless the speedo isn't calibrated for the actual tire size (not advertised, but actual, which for a KM2 is about 35.5").

He can go with what ever he wants. But personally I think there is a fad to over gear in this forum.

 

RPM isn't going to have a direct correlation to efficiency. At higher rpm and on the same road and to maintain the same speed at higher rpm you are going to run more negative pressure in the manifold. The end result is that you are using less fuel per stroke, while it might be moderately more than at a lower rpm.

Gear oil isn't something I would much worry about, outside of using a quality product from the likes of Lucas, Royal Purple, RedLine or the like. I would be more concerned about transmission fluid in an automatic, and there what's going to really inflate the temperatures is unlocking the converter. I'd also be leery of running an automatic without a transmission cooler, as there have been far too many chrysler transmission that have burst into flames. I wouldn't want that as a potential day wrecker.

Lastly I'd love for there to be an aftermarket damper for the 3.8L. It would squash a lot of fears about twisting the engine a little bit. With under drive it would also reduce the rpm the accessories are spun at. With the drive shafts, if you have a vibration issue at 3,000 rpm it's something that needs fixed and it's still going to be present at lower rpm although you might not feel it.

 

Oh I agree. There is a point to where a higher RPM will require less throttle to produce the same amount of power as a lower RPM that requires more throttle. Volumetric Efficiency comes into play here, and typically you want to cruise close to peak torque. As that is where the engine tends to be the most efficient. Although the 4k RPM peak torque of the 3.8L V6 is hardly an RPM you want to cruise at.

A balance shaft would be very much welcome. Every other V6 that has shared rod journals has them (well, most made in the last two decades anyway). It would really smooth things out. This is one of the reasons I have been a big fan of independent rod journals, such as an i6 has, or Fords newer V6's. It makes for a much smoother engine, and power strokes can occur more often, depending on how the crank is laid out.

But the way I feel is this. The above poster says the OD kicks out if you are under 2600-2700 rpm. But with any stock JK, they wont see that RPM unless you are going nearly 90mph. And thats only if you have 4.10 gears. Those with 3.73's have to be going even faster. And I do not see the Chrysler engineers designing the OD to have such a limited use. And yes, larger tires will draw off more power, even if re-geared. And you do need to go slightly deeper than stock gearing to keep the same efficiency if you do have larger tires. But there is a point where you can go to far. And yes you get more power. But at the same time you are greatly increasing the amount of wear on other components that won't show up for some time down the road.

 

All you need to get an automatic transmission to kick down with larger tires is a sniff of a hill or grade. It doesn't kick down because it's not going fast enough, but because it can't make enough power to conquer the grade or crosswind at the given speed. Chrysler engineers have a lot of people to answer to, and one of the more important ones is the EPA who stick that little estimate number on the sticker.

 

I wonder why NorthRidge 4x4 only offers 4.88 or 5.13 gears? I want 4.56 for my X. What do I have to do to get this?

 

Just call David, he'll make it happen.

 

So it is possible? Would I need new carriers or what? I run 3.21 now.

 

Call Dave and he will sort out all that you need....but yes, you need a new carrier....this may open up a debate once again by a few that have done the re-gear by retaining the stock 3.21 carrier, but I stand by the industry standard after talking to the individuals that make the gears and confirm that yes a new carrier is required.

 

Akright. I will talk to Dave about it when I get some cash saved up.

Recent order - stubby antenna, 17x8 Mojave teflon, 285/70R17 Duratracs. Next up is 2" BDS BB.

 

You should only need a new front carrier. The JK 44 uses a 'thick' gear on their 4.10 and lower (higher numerically) gearing. For example, if wanted to put a new diff in a non-Rubi with 4.10 gears , you would buy a carrier that fit 3.73 down gearing, not 3.92 up gearing.

Normally 44 cases (carriers) move the ring gear flange to allow for proper alignment of the ring and pinion. The lower the gear (higher numerically), the smaller the pinion head. This means that the carrier needs to moved closer to the pinion centerline. The 3.92 and up carrier moves the ring gear flange closer to the pinion centerline. The Rubi 44 gear is thicker (like a shim) so you don't need the carrier with the flange moved closer to the pinion centerline.

For your front carrier, check the new Eaton ELocker for the Dana 30. It is getting great reviews and is worth considering. The Dana 44 ELocker is still weaker than competing products so stay away from it except for fairly stock applications.

Scott