Need advice for overland build


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domv813

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Trailer, 4 cylinder, and mountains don't seem like a winning combo. If you keep the 4 banger in it get 5.13 and try not to add too much weight.

I realize it is underpowered but I do like the jeep. Is it a lost cost to put money into this jeep with what I want to do it?
 

Alex01

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I realize it is underpowered but I do like the jeep. Is it a lost cost to put money into this jeep with what I want to do it?
I haven't driven a 4 cylinder TJ that with 5.13 gearing so I don't really know. I can't imagine long grades would be much fun though.
 

Steel City 06

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It depends. Do you want to sustain freeway speeds for significant periods if time? If so, you'll really want a bigger engine. If you're more interested in back roads and off road, the 4 cylinder is acceptable.

Would you be willing to consider an engine swap? It takes $5k at the minimum (plus a lot of work) to do right, but would leave you with something better than a 6 cylinder and certainly better than a 4 cylinder.
 
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domv813

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It depends. Do you want to sustain freeway speeds for significant periods if time? If so, you'll really want a bigger engine. If you're more interested in back roads and off road, the 4 cylinder is acceptable.

Would you be willing to consider an engine swap? It takes $5k at the minimum (plus a lot of work) to do right, but would leave you with something better than a 6 cylinder and certainly better than a 4 cylinder.
As long as I could maintain 65 mph I would be okay with it. I was able to keep around 55-60 in the mountains with the trailer. Maybe eventually an engine swap but that would be a couple years down the road and I do not have the time/space to do it myself unfortunately. With the trailer I never get out of 3rd gear much so maybe a re gear could get me enough to be able to tow around 65 for all but super steep grades?
 
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Steel City 06

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As long as I could maintain 65 mph I would be okay with it. I was able to keep around 55-60 in the mountains with the trailer. Maybe eventually an engine swap but that would be a couple years down the road and I do not have the time/space to do it myself unfortunately. With the trailer I never get out of 3rd gear much so maybe a re gear could get me enough to be able to tow around 65 for all but super steep grades?
The best thing you can do before buying any gears is to play around with a good gear ratio calculator. Here is the best one we use here:

You'll need to put in all of your information. Here is a run down:

You probably have an AX-5 transmission. This was exclusive to the 4 cylinder TJs and has different ratios than the AX-15 people talk about for the 4.0.

You have a NP231 transfer case.
No underdrive.

Gear ratio: Play with this number. 5.13 is what I recommend; you can compare it to your stock 4.10s or other options like 4.88 or even 4.56 (not recommended).

Tire size: enter either your current or target tire size based on what results you want to see. Subtract 1" from the tire diameter (e.g., put in 32" for a 33" tire) to account for tire deflection. (A more accurate measurement is to use the revolutions per mile listed on your tires specifications.)

Then take a look at your results. Some things to note:

The engine puts out maximum torque at 3250 RPM. The closer you can get to this RPM, the more torque you can efficiently put to the ground. If you can target this for freeway cruising in the top (5th gear), that would be ideal.

The engine puts out maximum power at 5250 RPM. Redline is about 5500 RPM. Ideally you would have a gear in the range of the maximum power RPM at the desired speed to climb hills as fast as possible.

Plugging in 32" tires (33" nominal) and 5.13 gears gets you very close to ideal in those scenarios. At 65 MPH, you'd be running about 3000 RPM in 5th, 3500 RPM in 4th, and 5100 RPM in 5th. So you'd have a good cruising gear, a good intermediate gear for towing and hills, and a power hill for passing and climbing hills.

Keeping 32" tires instead of 33s (plugging in 31") brings you even better results. At 65, you would be running 3100, 3600, and 5250 RPM in 5th, 4th, and 3rd respectively. So you have gears close to both peak torque and peak power, plus an intermediate gear.

A note about peak torque and power:

Engines are generally most efficient at generating mechanical power at full throttle at peak torque. Hence why that metric is so often used when talking about frequent hauling. At a given road speed, you actually deliver maximum torque to the ground at peak horsepower, because even though the engine isn't producing as much torque above peak torque RPM, the gear reduction more than makes up for it. Above peak horsepower, the drop in engine torque becomes so significant that the increase in gear reduction cannot keep up. Generally automakers set the redline as the point at which the engine stops producing useful power when compared to lower RPMs. Since we don't have CVTs and instead only have 3-6 gears, this is set somewhat over peak horsepower RPM. With a 5 speed transmission at a given road speed, I would say that the gear that gets you closest to redline is probably going to deliver the most torque to the wheels.

Here is a handy chart:
1605919513650.png

The closer you are to peak torque, the more efficiently you will move when the pedal is floored. The closer you are to peak horsepower, the more propulsive force you can deliver to the ground, and the faster you can accelerate or climb a hill.
 

Mr. Bills

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I realize it is underpowered but I do like the jeep. Is it a lost cause to put money into this jeep with what I want to do it?

It depends. What exactly do you want to change?

Next, what will that cost in terms of money, time and the energy expended? Is that within your budget? If you sold this jeep and purchased another with the engine, transmission and axle assemblies you want (e.g., Dana 44 vs. Dana 30 or Dana 35), would it cost more or less to get to your desired end result? Would the project be finished faster?

As a general rule, my opinion is that one should be circumspect about embarking on any project that requires engine transplants, different transmissions, major fabrication and serious skills unless one possesses those skills or has buckets full of money to pay someone who does. Sometimes paying more money to buy a rig already close to the desired end result is the best bargain. Sometimes not. As I said, it depends. It may be that when all is said and done it makes sense to enjoy your four banger for awhile pretty much as is, with tires better matched to the existing gearing, while you figure out how you want your next jeep to be equipped based on your own experience as a jeeper.


Best of luck with your project. More pictures of your trailer would be fantastic.
 
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domv813

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It depends. What exactly do you want to change?

Next, what will that cost in terms of money, time and the energy expended? Is that within your budget? If you sold this jeep and purchased another with the engine, transmission and axle assemblies you want (e.g., Dana 44 vs. Dana 30 or Dana 35), would it cost more or less to get to your desired end result? Would the project be finished faster?

As a general rule, my opinion is that one should be circumspect about embarking on any project that requires engine transplants, different transmissions, major fabrication and serious skills unless one possesses those skills or has buckets full of money to pay someone who does. Sometimes paying more money to buy a rig already close to the desired end result is the best bargain. Sometimes not. As I said, it depends. It may be that when all is said and done it makes sense to enjoy your four banger for awhile pretty much as is, with tires better matched to the existing gearing, while you figure out how you want your next jeep to be equipped based on your own experience as a jeeper.


Best of luck with your project. More pictures of your trailer would be fantastic.
All I am going for is to be able to pull my little trailer up to 2-2.5 max hours on the road for camping and use the jeep to get it to some off road campsites when we get there. As long as I can keep 65 pulling on flat/slight grad interstate I am happy. What is your opinion on the 2.5 being able to make those goals? I realize the 4L would obviously do it better but does the 2.5L have a chance?
 
OP
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domv813

New Member
Joined
Nov 18, 2020
Messages
11
Location
Knoxville, TN
The best thing you can do before buying any gears is to play around with a good gear ratio calculator. Here is the best one we use here:

You'll need to put in all of your information. Here is a run down:

You probably have an AX-5 transmission. This was exclusive to the 4 cylinder TJs and has different ratios than the AX-15 people talk about for the 4.0.

You have a NP231 transfer case.
No underdrive.

Gear ratio: Play with this number. 5.13 is what I recommend; you can compare it to your stock 4.10s or other options like 4.88 or even 4.56 (not recommended).

Tire size: enter either your current or target tire size based on what results you want to see. Subtract 1" from the tire diameter (e.g., put in 32" for a 33" tire) to account for tire deflection. (A more accurate measurement is to use the revolutions per mile listed on your tires specifications.)

Then take a look at your results. Some things to note:

The engine puts out maximum torque at 3250 RPM. The closer you can get to this RPM, the more torque you can efficiently put to the ground. If you can target this for freeway cruising in the top (5th gear), that would be ideal.

The engine puts out maximum power at 5250 RPM. Redline is about 5500 RPM. Ideally you would have a gear in the range of the maximum power RPM at the desired speed to climb hills as fast as possible.

Plugging in 32" tires (33" nominal) and 5.13 gears gets you very close to ideal in those scenarios. At 65 MPH, you'd be running about 3000 RPM in 5th, 3500 RPM in 4th, and 5100 RPM in 5th. So you'd have a good cruising gear, a good intermediate gear for towing and hills, and a power hill for passing and climbing hills.

Keeping 32" tires instead of 33s (plugging in 31") brings you even better results. At 65, you would be running 3100, 3600, and 5250 RPM in 5th, 4th, and 3rd respectively. So you have gears close to both peak torque and peak power, plus an intermediate gear.

A note about peak torque and power:

Engines are generally most efficient at generating mechanical power at full throttle at peak torque. Hence why that metric is so often used when talking about frequent hauling. At a given road speed, you actually deliver maximum torque to the ground at peak horsepower, because even though the engine isn't producing as much torque above peak torque RPM, the gear reduction more than makes up for it. Above peak horsepower, the drop in engine torque becomes so significant that the increase in gear reduction cannot keep up. Generally automakers set the redline as the point at which the engine stops producing useful power when compared to lower RPMs. Since we don't have CVTs and instead only have 3-6 gears, this is set somewhat over peak horsepower RPM. With a 5 speed transmission at a given road speed, I would say that the gear that gets you closest to redline is probably going to deliver the most torque to the wheels.

Here is a handy chart:
View attachment 204738
The closer you are to peak torque, the more efficiently you will move when the pedal is floored. The closer you are to peak horsepower, the more propulsive force you can deliver to the ground, and the faster you can accelerate or climb a hill.
Thank you for the detailed explanation this helps a ton!
 
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Mr. Bills

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All I am going for is to be able to pull my little trailer up to 2-2.5 max hours on the road for camping and use the jeep to get it to some off road campsites when we get there. As long as I can keep 65 pulling on flat/slight grad interstate I am happy. What is your opinion on the 2.5 being able to make those goals? I realize the 4L would obviously do it better but does the 2.5L have a chance?

I think the key is going to be using tires more appropriate in diameter for the gear ratio you already have and keeping the weight down as much as possible.

If you think a four banger can't haul a 6' trailer, remember that WWII four banger jeeps regularly towed them with both jeep and trailer completely overloaded. Top speed was only about 45 mph, but they could do it all day long. I used to tow a Bantam trailer loaded with gear behind a CJ-5 four banger and thought nothing of driving 5 hours on the interstate and Hwy 101 to the Pismo dunes. Speed on the highway was probably about 60, less going over the Grapevine.

I am of the belief that far too many jeepers create laundry lists of modifications and start throwing money at their rigs before they have even learned what it will or won't do in its current state. That can turn out to be a very expensive mistake when the do-overs begin.

I'm starting to sound like my father, God rest his soul. Maybe I'm entitled; I turn 70 shortly.