Ideal Cruising RPM for 4.0 from Gearing - JeepForum.com

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post #1 of 25 Old 07-10-2010, 10:33 PM Thread Starter
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Ideal Cruising RPM for 4.0 from Gearing

I'm looking into regearing my LJ. I am wondering what is the best cruising RPM that I should shoot for. Mainly mine is a DD and will see hunting trails. I do a fair amount of freeway driving as well. I'm running the 42 RLE auto (.69 final OD gear) on 305x55R20 (33x12's). OD is about useless as it can barely keep itself in on a flat. Current diff ratio is 3.73, and I"m aiming at a cruise MPH of 70. I'm thinking around 2200 to 2300 rpms for OD. Any input is appreciated!

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post #2 of 25 Old 07-11-2010, 07:13 AM
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Target 2700-2800. That's where I was sit on 35 (4.56) and it runs great. I've been on the full gamut of tire/gear combinations and this has proven the most ideal for me as a highway DD.

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post #3 of 25 Old 07-11-2010, 07:24 AM
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post #4 of 25 Old 07-11-2010, 07:35 AM
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Quote:
Originally Posted by VnutZ View Post
Target 2700-2800. That's where I was sit on 35 (4.56) and it runs great. I've been on the full gamut of tire/gear combinations and this has proven the most ideal for me as a highway DD.
You have a manual, the OP has the 42RLE, different animal. It's that 0.69 OD that requires lower gearing (higher numerically).

OP, for 33's with the 6-cylinder and 4-speed auto the recommendation is 4.88.

http://www.jeepforum.com/forum/f9/diy-alignment-kit-steps-w-pics-2264465/
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post #5 of 25 Old 07-11-2010, 08:02 AM
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go straight to 4.88s. do not pass go, do not collect $200. it will be better, but many say 5.13s would be better yet. 35s needs 5.38s or 5.89s. the deepest your D30 can go is 4.88.

the 42RLE's gearing is horrible, it comes undergeared from the factory. a 5-speed manual, with 3.73s and 35s turns the same highway RPMs as a stock 42RLE/3.73s and 30s.
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post #6 of 25 Old 07-11-2010, 08:11 AM
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Like everyone said, just go to 4.88 or get a waggy 44 for the front and drop in something deeper (Since you don't have the rubi lockers you can as high as 5.89, but 5.13 or 5.38 would be better for 33"s). Being undergeared from the factory sucks on anything but a flat or slight downhill highway.

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post #7 of 25 Old 05-26-2011, 06:05 AM Thread Starter
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*bump* I found a guy to do the regearing and dropped off the cash to order the parts. Tentatively it'll be done around 11 June 11. Here's the final set up: 4.88's, (still have the 33's) with Detroit TrueTracs for front and rear. I did the calculations on 4lo.com and my cruising speed will only hit approx 2400 RPM at 75 (interstate driving...maybe) with that set up.

I was wondering why is 2600-2700 is the best cruising RPM for the 4.0? Most sites, including 4lo.com and Quadratec Knowledge center recommend that 2600-2700 RPM?


Secondly, and finally, why is 2600-2700 RPM sought after for the 4.0 when the factory RPM limit is 3,000 RPM? Any input is appreciated, and I hope this thread helps someone else out too!
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post #8 of 25 Old 05-26-2011, 06:20 AM
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The RPM limit is well above 3K RPMS. Here's the peak hp / torque ratings for the six:

4.0L I-6 242 cu in - 2000-2006
* 190 hp @ 4,600 RPM
* 235 ft/lbs @ 3,200 RPM

The reason people shoot for 2600-2700 (or higher) is that's the spot in the power band that has the best efficiency, particularly with larger tires.
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post #9 of 25 Old 05-26-2011, 06:23 AM
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Rotational mass + brick shape + 4.0 = you need some RPM's to cruise efficiently and to provide the motor with adequate leverage. At 70 mph, the resistance against the shape of the jeep is working hard against you. IMO, the 4.0 needs to be at an absolute minimum of 2400 and preferably in the 2600-2800 range at 70 mph. Lower is not better in this case as you've seen with the 42rle lugger. The 4.0 reaches the point of peak volumetric efficiency at about 3100 RPM, so running closer to that is more efficient than lugging it. Regardless, you can't expect to get acceptable fuel mileage at 70 mph due to wind resistance, even if optimally geared. After about 60 mph, the dropoff is sharp.

As for factory rpm limit, not sure what you mean by that. If you're saying you never go past 3k, you may want to do that occasionally. Hell, I probably redline mine at least once a day....that's 5k.

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post #10 of 25 Old 05-26-2011, 06:53 AM
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Quote:
Originally Posted by 1C41N0 View Post
*bump* I found a guy to do the regearing and dropped off the cash to order the parts. Tentatively it'll be done around 11 June 11. Here's the final set up: 4.88's, (still have the 33's) with Detroit TrueTracs for front and rear. I did the calculations on 4lo.com and my cruising speed will only hit approx 2400 RPM at 75 (interstate driving...maybe) with that set up.

I was wondering why is 2600-2700 is the best cruising RPM for the 4.0? Most sites, including 4lo.com and Quadratec Knowledge center recommend that 2600-2700 RPM?


Secondly, and finally, why is 2600-2700 RPM sought after for the 4.0 when the factory RPM limit is 3,000 RPM? Any input is appreciated, and I hope this thread helps someone else out too!
Volumetric efficiency. An engine is nothing more than an air pump. Due to things like valve timing, valve configuration, port size and length, etc, etc. the engine will pump the charge in and out at the most efficient rate, at a given RPM. You usually see the VE

Couple that with the amount of power to keep the vehicle moving (gearing) and you produce a likable set up.

Not to be confused with peak HP and torque. If your building a dragster, you want the engine spinning at peak HP at the end of the run. You gear for that. On a torque vehicle, you gear to be driving at peak torque. Cruising you want the vehicle at the bottm end of the peak VE curve.
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post #11 of 25 Old 05-26-2011, 07:16 AM
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Quote:
Originally Posted by 1C41N0 View Post
*bump* I found a guy to do the regearing and dropped off the cash to order the parts. Tentatively it'll be done around 11 June 11. Here's the final set up: 4.88's, (still have the 33's) with Detroit TrueTracs for front and rear. I did the calculations on 4lo.com and my cruising speed will only hit approx 2400 RPM at 75 (interstate driving...maybe) with that set up.
thats the setup I have now and I like it. still sucks in the mountains, but not near as bad as 3.73s/31s.

Quote:
Originally Posted by 1C41N0 View Post
I was wondering why is 2600-2700 is the best cruising RPM for the 4.0? Most sites, including 4lo.com and Quadratec Knowledge center recommend that 2600-2700 RPM?

Secondly, and finally, why is 2600-2700 RPM sought after for the 4.0 when the factory RPM limit is 3,000 RPM? Any input is appreciated, and I hope this thread helps someone else out too!
This might help:
http://www.jeepforum.com/forum/f9/33...6/#post9838564
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post #12 of 25 Old 05-26-2011, 07:40 AM
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Cut the OD off and rpms are fine.
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post #13 of 25 Old 05-26-2011, 10:20 PM
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Quote:
Originally Posted by Unlimited04 View Post

thats the setup I have now and I like it.

]
X2

I am not in the mountains, this setup runs like a champ and so far seemed to improve my MPG around town and with camper in tow
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post #14 of 25 Old 05-30-2011, 05:57 PM Thread Starter
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Thanks for the background info. I"ll update this post after I get it done in approx 2 weeks.
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post #15 of 25 Old 06-11-2011, 08:38 AM Thread Starter
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Dropped the Jeep off at the regear shop, should be done Sunday late afternoon early evening. I'll post results!
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