How can I get more power out of my 4.0?

Bonetech34

Bonetech34

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San Antonio, Texas
Hey guys, I've got a 2006 Jeep Wrangler Unlimited LJ with a 3 in. Lift. I just bought 33 x 10.5R 15 LT BF Goodrich Mud Terrain KM3's. I lose power(rpm's) when I'm on the highway. Seems like whe going up a hill is when it happens more. I've got an i6 4.0 under the hood with 69000 miles. Dana 44 Rear Dana 30 Front. When I had my 79 gmc Jimmy, I took care of the problem by putting a 3/4 Cam when I rebuilt the 350 it had. Any suggestions on how to get my power back?
 
Your axles are geared to the factory ratio which is only correct for the factory size tires. The 33" tires you installed reduced the engine rpms at any given speed so the engine is not developing as much power as it did with the factory size tires. Your rpms are so low from the factory axle gearing with the 33" tires your engine is now lugging around town and on the highway.

Your factory axles were geared to a 3.73 ratio and you need to regear them to 5.13 to get the engine rpms back up enough for the 33's. 5.13 will give appropriate highway cruising rpms and restore its drivability where it will again be fun to drive. Don't let some shop yahoo talk you out of 5.13, he doesn't know your 42RLE automatic transmission ratios and why 5.13 is the correct ratio for a TJ with your transmission and 33"tires. He's only thinking of his V8 powered pickup truck where 4.11 works fine for 33" tires.
 
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I have TJ 2004, same engine as yours and I have 4 speed auto (don't know whether you have manual or auto). Also have Dana 30 and Dana 44 rear.
I am running 31x10.5xR15 BF Goodrich KO2 tires. I have 4.56 gears now, and it does not feel underpowered on highway at any speed. I get 2500RPM at 70MPH. Reaches 80MPH if it is not climb, but I try to keep it under 70MPH.
It will never drive like my 280HP Turbo Kia AWD 2021, but still does not feel a powerless at all.
Previously I had 3.73 gears and was a complete dog with 32 inch tires. Will get 2000RPM at 70MPH, which just did not work for highway speeds. With 33s, it would be barely drivable on highway.
As Jerry said, if you have 2006 Auto, you need 4.56 for 31'', 4.88 for 32'' and 5.13'' for 33''. After you have the right gearing as listed, try it, and see whether you still feel like you need to get stronger engine. I doubt that you will really feel a need for different engine with correct gearing. I definitely don't feel like doing stroker right now. It is not a sports car, and it was never intended to be one. 4.0 with correct gearing for your tires will get you anywhere you need, just don't expect it to accelerate too fast.
Don't listen to anybody you tells you otherwise for gearing ratios, they don't have a clue about the 42RLE transmission at it's 0.69 ratio overdrive. Before deciding on correct gearing, I should have spent bunch of time reading here, this way I would go to 5.13 with 33'' inch tires, and not to 4.56 which limits me to 31''. Even the Rubicon Auto was supposed to come out of the factory with 4.56 gears and not 4.10, BTW.
 
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Bonetech34 said:
Hey guys, I've got a 2006 Jeep Wrangler Unlimited LJ with a 3 in. Lift. I just bought 33 x 10.5R 15 LT BF Goodrich Mud Terrain KM3's. I lose power(rpm's) when I'm on the highway. Seems like whe going up a hill is when it happens more. I've got an i6 4.0 under the hood with 69000 miles. Dana 44 Rear Dana 30 Front. When I had my 79 gmc Jimmy, I took care of the problem by putting a 3/4 Cam when I rebuilt the 350 it had. Any suggestions on how to get my power back?
Click to expand...

Regear for your transmission and tire size and do Jezza’s turbo kit or engine swap.
 
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Jerry Bransford said:
I doubt he'd feel like he needs a turbo or bigger engine after regearing to 5.13.
Click to expand...

I thought about a crazy idea of having 5.13 gears with my 31s, this would put me into 2750+ area for 70MPH. This would definitely increase the available torque and horsepower at any given speed, but then it will drink gas like my neighbor Ivan from my USSR childhood was drinking vodka.
 
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2750 rpms is not an excessive highway cruising rpm. Not even close. Don't compare those rpms to the rpms needed by lightweight cars with small tires or with V8 engines. I wish my current highway rpms with 35's and 5.38 gears with the 42RLE automatic transmission were closer to 2750 like I used to run in my previous TJ at 70 mph with a 5-speed and 4.88 gears with 35's. The 4.0 loves to run all day that rpm. My previous TJ's highway cruising rpms rpms were significantly higher than that after I converted that 5-speed to the 32RH 3-speed automatic.
 
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NOTNSUV said:
Considerably higher rpm than your estimate.
http://www.grimmjeeper.com/gears.html
Click to expand...
With his 4-speed automatic 42RLE, 5.13, and 31" tires it shows 2700 rpms at 70 mph.
Capture.JPG
 
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TexasTJ2004 said:
I thought about a crazy idea of having 5.13 gears with my 31s, this would put me into 2750+ area for 70MPH. This would definitely increase the available torque and horsepower at any given speed, but then it will drink gas like my neighbor Ivan from my USSR childhood was drinking vodka.
Click to expand...

5.13 gears with the 6-speed and 31’s your rpm’s would be way higher than 2750 at 70mph. If your final tire size is 35’s you should be fine as far as gas mileage. What I’d be concerned about is harmonic vibes. The driveshaft would really be spinning fast at 70mph (3,894rpm with 31’s; 3,449rpm’s with 35’s).
 
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This thread reminded me of my old 98' 32rh auto with 4.56 gears and 32s. Gas mileage sucked! That could be a result of my driving habits with the extra "perceived" power I had. It felt like a Prius turbo with a full rack of luggage on top. :rolleyes:
 
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JMT said:
5.13 gears with the 6-speed and 31’s your rpm’s would be way higher than 2750 at 70mph. If your final tire size is 35’s you should be fine as far as gas mileage. What I’d be concerned about is harmonic vibes. The driveshaft would really be spinning fast at 70mph (3,894rpm with 31’s; 3,449rpm’s with 35’s).
Click to expand...

I have 4 speed auto, not 6 speed manual.
6 speed has overdrive of like 0.81 or something, which is really different from my 4 speed’s 0.69.
As for harmonics - when I drove my TJ with 4 speed, 31 inch and 4.56 gears 80MPH on highway (here in Houstonn area we have places with speed limit of 75MPH and really good roads), I was getting ~2850RPM. My driveshaft rotation speed was 2850/0.69=4130. Didn’t feel too much vibration. The engine was screaming, the transmission started feel hot, but very controllable and didn’t feel wrong.
 
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