how to increase my tow capacity??

[th3m4nm4rcu5]

I have a 1989 jeep comanche MJ and I'm not sure which model it is. 

4.0 I6 Renix engine. 

5 speed manual - Ax-15 trans i believe (2wd)

upgraded intake to custom cold air K & N

i custom built a rear bumper with a hitch in the bumper, heavy duty.

new spark plugs, wires, ignition coil. new CPS. just did Valve cover gasket and painted the valve cover. 

 

 

 

the problem: I was towing a single axel trailer with a bunch of trash for my brother to the dump, and the truck was like losing speed in 5th gear on the highway, i mean i would have too down shift to 4th, even 3rd, just to keep up with traffic. I would say that that trailer and all didnt weigh even 2,000 lbs. and thats terrible for such a beast little truck.

 

from what i understand, the reason for the crappy tow capability, is due to the gearing in the stock axel? i think i must have a rear dana 30 or 35, not sure.

 

My question: what can i upgrade the rear axel too to get better towing capabilites? and do i have to upgrade the front axel as well? its a (2wd). and my other question is, say i have a 27 spline driveshaft, and i get a dana 44 rear axel or the crystler 8.25" rear axel, and they are like 29-30 spline, how can i make my driveshaft fit that? or where would i start to get one that would? I had planned on converting the MJ to a 4x4 during some point in its life, i was just going kill this 2wd trans first. I also want to lift my MJ about 6.5" in the end, to run atleast 33's. and i have a 4BT cummins engine ready for her, just going to wait until the I6 dies, and i plan on doing a swap. (maybe) 

 

 

but for now all i want to do is tow my boat to the lake this summer, which is a little over 2100 lbs i would say, and i plan on doing it weekly, 2 hour drives. so what do i need to do guys? and where to find a axel for it?

 

thats for the help guys, huge comanche fan! and new to the fourm!

 

[gogmorgo]

You're over-thinking this.

With that drivetrain combination, you've got rear end ratio of 3.07. That's the highest ratio available from the factory. Easiest thing to do would be to find a rear axle with lower gears. 3.55 is common, but if you're planning on going to taller tires you might want to go even deeper, maybe a 4.10 to go with 33's.

The factory tow package was the receiver hitch, automatic trans (came with 3.55's) and a Dana 44 rear axle. It was good for 5000lbs.

 

Your rear axle is a Dana 35, which is notoriously weak. I wouldn't bother regearing it. Best to try to find another rear axle with the ratio you want. Finding the heavy duty MJ axles isn't going to be easy, and you'll pay through the nose for them if the seller thinks he knows what he had. XJ axles will be the correct width and will reuse your driveshaft's ujoint, but you'll need to weld on new perches... Likely not an issue for you.

Common swaps are of course the heavy-duty XJ/MJ axles, the Dana44 and AMC20, also the later Chrysler 8.25. Ford 8.8's are also common, although they're a touch narrower.

You won't need to touch your 2wd front beam axle unless you're swapping in 4x4.

The spline count on the axle shafts has nothing to do with your driveshaft. That only matters for matching up the internal components of the diff.

 

My advise would be to figure out what you want the end product to be, locate all the parts you'll need, and then put them all in at the same time, otherwise you'll find yourself pulling everything apart over and over.

Another thing you can do, especially in the planning stage, is to take a look through the project threads on here to get ideas, see what other people are doing and how it works out for them.

[th3m4nm4rcu5]

Thanks gogmorgo! This has been very helpful. I've been researching axels today. Funny actually my dad just got a donor Ford Explorer with an 8.8 rear axel he said I can have! I hear good things about the 8.8 -

 

Question is. I do plan on going 4x4 one day, just was going to use this 2wd transmission up first. So when I do my axel swap, my pinion angel for my perches and shock mounts will be different Angels when I go 4x4 in the future, correct? Due to the transfer case, I hear your pinion angle is based on the angle of the transfer case. Without one I'm guessing it will be different. Due to the fact the driveshaft will go directly to the transmission beings it's 2wd. Or would a 2 piece driveshaft be alright when I go 4x4 in the future and not have to change my pinion angle later on.

[91Pioneer]

There's more to towing capacity than power and axles. Think cooling, braking, suspension etc.

[johnj92131]

You really do need to think about how much towing you are going to do.  If hauling trash to the dump once a year, then just use third and fourth gears with your present set up and keep your speeds down below 55 mph.  If you are going to want to tow 5000 lbs a 1000 miles every month, get something else. 

 

My Comanche come from the factory with all the options to "properly" tow 5000 lbs.  Automatic, Long Bed Dana 44, Metric ton/Big Ton package, all the H.D. factory stuff, frame mounted hitch,  I added an equalizing hitch to complete the package.  The Gross Combined Vehicle Weight for my truck is about 9500 lbs.  But it is not something I want to use for regular towing of 5000 lbs.  On the other hand, it works just fine to haul crap to the dump and even to move a full trailer of household stuff 3-400 miles, once in a while.

 

But if you want to improve your Comanche for towing, get a 4.10 rear end and put some disk brakes on the rear.  Buy a frame mounted hitch, Your bumper mounts are the weak spot, that is why you want the frame mounted hitch.  Remember, when you are towing, brakes are REALLY important, so is leaving way more room from other cars on the road.  It is no fun when you are trying to stop quickly and you have 4500 lbs behind you pushing you down the hill. Never mind a long down grade on the interstates.

[JeepsOLot]

I would do the WJ brake booster/master cylinder mod, to get as much braking power as you can.

[1stDeuce]

If I followed correctly, you're unhappy with the towing performance of a truck that isn't set up for towing at all.   As pointed out above, the easiest thing to do is just use 3rd and 4th for the occasional trailer towing.  5th is certainly not for towing much at all with a Renix 4.0L.   

 

If you really plan on a diesel swap, then your current axle ratio is actually fairly ideal.  Get going on it !! :)

(Note: a 4BT is a HUGE engine for a Comanche, unless you're going to lift it ~6")

 

The 8.8 out of an Explorer is a great swap if it has better gearing.  It's a bit narrower than your current axle, but wheel spacers fix that.  Pinion angle will not change when you swap it to 4wd, as you don't change the angle of the powertrain at all by adding a t-case.   Even with some lift, the powertrain angle still stays the same, and so should the pinion angle.

 

And I'll agree that improvement to the brakes would be a good idea if you plan to tow a lot without trailer brakes.  (Or even if you don't, really!)  

I'll got out on a fairly thin limb though and say that the 10" drum brakes that came on the back of the early trucks are fairly capable when properly adjusted.  Going to discs without changing the master and prop valve actually nets you LESS brake performance.  This is because drum brakes "self energize" and therefore make more brake torque with less hydraulic pressure than discs require to get the same brake torque.  

 

Therefore, if you convert to discs, do a WJ master/booster/prop swap.   If you keep drums, then the '96 master/booster/prop swap works out well.

 

Good luck!!

[mikekaz1]

id say its pretty common to not be able to use "overdrive" while towing in most any vehicle. no matter how bad @$$, or good of a suspension and parts jeep may have given the MJ... when it comes down to it its still just a midsize pickup.

[th3m4nm4rcu5]

Thanks for all the info guys, I mainly will be towing my boat to and from the lake all summer long. About 1900 lbs. so I don't need to max my towing out just get it about 2500-3000 lbs towing capacity would be nice. Working on my brake booster swap now. My suspension should be decent enough to tow a 16 ft boat. But I plan on lifting soon, so it's going to get re done. And the chummins project is long In the future.i don't believe in pulling a good engine out lol.drive her til she dies haha

[75sv1]

^{If you look at the specs, the manual trans is limited to 2500 lbs, the auto to 5,000 lbs. I am not sure as to why you are slipping in 5 th gear. I would agree with the others, larger brakes should be on the list. I think others do not have a problem with higher loads with a manual, but you have to be smooth with shifting. Wondering if it is a preassure plate problem, and the extra load shows it. With an auto, you should have  an auxiliary cooler.}

[Eagle]

There's a difference between towing capacity and engine performance, and it seems to me that the primary complaint in the original post here was about engine performance.

 

Certainly, the stock 3.07 gear ratio that came from the factory with the 5-speed 4.0L trucks was a bad choice. You say you only want to tow 2,500 to 3,000 pounds, but that's more than the load capacity of even the metric ton option models, and if you get to 3,000 pounds it's nearly double the weight of the truck. You can't double the weight and expect the performance not to take a hit.

 

The torque peak for the '89 4.0L engine was 2,400 RPM. Because of the axle gearing and the overdrive 5th gear, at any normal highway speed you're actually running below the torque peak in 5th gear. Assuming standard-ish tires, 60 MPH in 5th gear is only about 1700 RPM. You don't hit the torque peak in 5th until you're up to 85 MPH. To get the engine turning closer to the torque peak at sane towing speeds, you need to change your gearing. 3.73 would be a good choice, but not available in used axles that will fit the MJ. 4.10 is also a good choice, and was used in XJs and MJs with the 4-cylinder engine.

[mikekaz1]

I never quite understood the tow capacity difference from manual to automatic.....

[Megadan]

I never quite understood the tow capacity difference from manual to automatic.....

Gearing and trans (for BA10) and/or clutch holding capacity would be my 2 guesses,

 

I can't imagine the clutch really being an issue, but asking it to hold the force of more than double the vehicle weight while accelerating would put quite a strain on it.

[Eagle]

I never quite understood the tow capacity difference from manual to automatic.....

Automatics don't have manually (or "pedally") operated clutches. It's easy to slip a clutch and burn it out when trying to get started with a heavy load in tow.

[75sv1]

I've kind of noticed a jerkiness with a heavy load. So, there is sort of an impact on parts with a manual. Not an expert with towing though. The Ford 8.8 does come in 3:73. Probably limited slip available. Also, 4:10.

Also, an auto doesn't have a direct linkage, the torque converter is a hydraulic coupling, so it cushions the start up. It didn't have much problems with one of my XJs twoing some 2400 lbs cars on a dolly. Well, looking in the rearview mirror and seeing a car on your bumper.

[Megadan]

I've had no problem towing with my manual, but... I am a truck driver.  Yes a big diesel tractor and a smaller gas engine pickup are different animals, but the same game applies to the clutch.  Honestly, if I were going to be regularly towing heavy loads with one of these trucks, I would consider a small upgrade (Stage 1) in the clutch department.

[Eagle]

An 18-wheeler has multiple granny gears to get the loaded rig rolling, and a fat torque curve that picks up at under 1,000 RPM. Big trucks are geared properly from the factory, and have enough gears in the transmission, to allow the driver to keep the RPMs in the fat part of the power band.

 

The OP here is running the wrong gears, never approaching the torque peak on the highway, and then wondering why the engine feels like it doesn't have any power. The answer is simple -- because it doesn't have any power.

 

To my amazement, I couldn't find an actual horsepower/torque graph for the early 4.0L engines anywhere on the Internet. They used to be everywhere, such that I never thought to download and save a copy. Now I'm really sorry. I was able to find one for a '99 version, here: http://www.automobile-catalog.com/wykres_power.php

 

Note that the torque curve on this one is not as flat as for the Renix engines, and the torque peak is up at 3,500 RPM rather than 2,400 RPM. What is common for both, however, is that there's nothing happening below 2,000 RPM. In the graph, if we look at 1,800 RPM (65 MPH on stock tires with stock gearing), the torque is about 240 Newton-Meters (which is about 177 foot-pounds), and the horsepower is a mere 65 (or so) horsepower.

 

I did finally find a copy of an earlier power graph, but it's so small you can't read it. For what it's worth, here it is:

 

 

The torque curve is the upper, flatter line. The curve from lower left to upper right is the horsepower line. If we take the torque peak at 2,400 RPM, you can see that the curve is increasing up to that point, and becomes (relatively) flat only after 2,400 RPM. And, of course, as with the other curve you can see that below 2,000 RPM there just isn't a lot of horsepower being produced. That's why vehicles have tranmissions with multiple gears -- so you can choose a gear that matches the engine speed and power range to the amount of power needed to get the job done.

[Megadan]

I know why they have gears....  You left out the fact that most large diesels have an operational rev range of about 300-400rpm (most trucks operate between 1200-1700 through each gear), which also necessitates multiple gears. 

The renix makes plenty of low end torque to manage towing 5k lbs as long as you drive it intelligently.  I've already pulled over 4 more than once without a single problem with my 5spd.

[mikekaz1]

Eh maybe its a lingering thought of tow trucks and dump trucks. Manuals always feel more competent then their auto equivalents. But as stated. They generally are designed for hauling. Rather then an option as with pickups/suvs

[Eagle]

I know why they have gears....  You left out the fact that most large diesels have an operational rev range of about 300-400rpm (most trucks operate between 1200-1700 through each gear), which also necessitates multiple gears.

By "operational rev range" you are referring to the RPM range where the engine is producing maximum torque. Large diesels actually have a much broader torque range than that (it's a characteristic of diesels in general), but the trucks are equipped with multiple gears to keep the engine in the range right at the torque peak.

 

That's what I'm saying also applies to the Jeep 4.0L. Ideally, for towing you would want to keep the engine close to the torque peak, which is 2400 RPM. If you're going to go off the peak, go for more engine speed because at least then your gaining horsepower in exchange for losing torque. When you drop down to 1800 RPM or less, you don't have either torque or horsepower working for you.

[Megadan]

I don't disagree with you at all. That's why most tow packages have shorter gearing.  with my 235/75-15 tires ideally a set of 3.73's would be in my axles, but my 3.55's work ok.  On flat ground I can go into 5th, but I have to downshift for most hills for obvious reasons

[th3m4nm4rcu5]

^{If you look at the specs, the manual trans is limited to 2500 lbs, the auto to 5,000 lbs. I am not sure as to why you are slipping in 5 th gear. I would agree with the others, larger brakes should be on the list. I think others do not have a problem with higher loads with a manual, but you have to be smooth with shifting. Wondering if it is a preassure plate problem, and the extra load shows it. With an auto, you should have  an auxiliary cooler.}

I'm not slipping, its just loosing speed, everything is fine but 5th gear i with the pedal to the floor, i just was loosing speed

[th3m4nm4rcu5]

i am a very experienced manual driver, its all ive ever owned. i got an 89 nissan pickup 2wd 4 cylinder, and it tows my boat like a champ. and its rated for 5800 lbs towing, its got a clacky timing chain at the moment. I'm going to make some adjustments to the comanche, i mean its got good horse power, i have 4 pin injectors, and cold air intake, duel exhaust. i guarentee I'm pushing 185 HP or more, which should be enough. shes quick with no load 

[Eagle]

... 5th gear i with the pedal to the floor, i just was loosing speed

Because the engine is operating FAR below the torque peak. You need to regear -- and since you mentioned future plans to lift and run larger tires, consider your choice carefully before acting.

[COMJNUT]

I have changed my d35 to a d44, regeared to 3.73, and have 32" tires. With the 32" tires I should have gone to 4.10 gears for towing in the mountains.

 

Pick a d44, 8.8, 8.25, just something beefier than the d35.

Get some very good brakes

It makes a difference when you add brakes to your trailer.

Add a aux cooling fan if you don't already have one, I always have mine on when I'm towing.