IMO, there are a few major reasons...

When you lift the vehicle you increase the vertical distance between the planes of the front output of the t-case and the axle pinion. Anything that raises the pinion decreases that distance and is a good thing. The less angle you have the longer joints last - single or double. Also, the less angle the more abuse (torque) the joint will take before failing. Bigger/stickier tires 35"+, gearing - both t-case/axle, turbo/superchargers, higher displacement engine swaps, t-case swaps/doubler box install - consider all of these before making your axle choice because they all will increase the stress on your drive line - especially the u-joints. If you can see ANY of these things in the future it's better to build your axles correctly NOW than have to redo everything later.

Note, some of these combination absolutely require a HP and/or exclude the ability to run at road speeds with the front axle engaged, or have some other (one or more) irritating drawback(s).

Depending on how you're planning on driving with the front axle engaged also makes a difference. The HP lubricates differently than the LP. The HP, IMHO, works better when the pinion is run at extreme angles. If you're planning on using the front on the highway (up around 50 MPH) then the less angle there is the better the diff will oil itself and the more likely it will (if set up correctly) run vibration free.

This is VERY important if you're planning on running deep gears at road speeds: 5.xx+:1. Even with full synthetic gear oil there is a LOT of heat generated in a differential running at road speeds. If there isn't adequate oiling then you will do EXACTLY what I did - weld the pinion to the housing. This isn't as much of an issue on the trail where you're bouncing around and sloshing oil everywhere and the speeds are VERY low - but, of course that depends on trail conditions. If it's 105-110F degrees when you're wheeling all the time, then....

Getting the drive shaft to run without vibrating at road speed - if this is a concern - is important. At those speeds, especially if driven frequently, you will beat the crud out of your pinion bearing, t-case output shaft bearings, u-joints and drive shaft slip joint in short order.

Drive shaft protection is usually very important. Anything that raises the drive shaft at the pinion makes it less likely the DS will get hit or you'll land on it. Lots of big rocks on tight trails and shelves will make a hash of a low riding DS, t-case and pinion bearing quickly.

Not all front Dana 44s are the same, either. If I remember correctly the axle tubes on the HP are 1/2" thick. The LP are 1/4" or 3/8" - something like that, anyway they're the same thickness as the tubes on the Dana 44 rear axle. Again, if I'm not mistaken, that includes that XJ axle.

As some of the guys have pointed out it's possible in some cases to use a LP. I just depends on what you're doing with your vehicle. The best advice I can give is to think ahead when building your vehicle. It'll save lots of cash and headaches down the road if you do that. In most cases you probably can use either a HP or LP but there are some cases where either a HP will last longer, or in some way decrease costs (short term or long term) or you have no choice and you MUST use a HP.

Edward

Why would run a rock crawler / offroad truck in 4 on the highway? <img src="/forums/images/graemlins/confused.gif" alt="" />

Mine's also my work vehicle - on call 24/7. That's why I mentioned what I did above. Some people do use their verhicels for DDs. Anyway, occasionally it gets real scummy here in Denver in the winter. When we get a couple feet of snow out here you'll see ALL the crawlers come out of the woodwork. Its like a giant neon invitation sign to play! <img src="/forums/images/graemlins/smile.gif" alt="" />

Edward

Ohhh thats right.. snow. <img src="/forums/images/graemlins/lol.gif" alt="" /> <img src="/forums/images/graemlins/smile.gif" alt="" />

Edward thanks for the explanation... and I am trying to think the whole set up prior to buy things... also I have another question... my plan is to install 5.13 gears with 35"x12.5x15 tires... with my 4 banger 2.6 L... this set up is right?, or should I install 33x12.5x15?

Thanks
JorgeA

what terrain will you be doing?
5.13s should be just fine with 35s

Unless you plan on upgrading the axle shafts, u joints, etc. I wouldn't go over 35s on the stock axles.

5.13s with help with the power issues of the 2.6, but your MPG is going to go down the crapper. But you pay to play. 4.88s are probably more along the lines if you're going to run 33"s. 5.13s or 5.29s I would go with 34s or 35s.

No problem. You're welcome.

What is the factory tire size and axle gears for your vehicle? Automatic or 5-speed? Transmission 1st gear ratio?

Edward

I don?t really know the original tire set up... is came with 16" wheels.. the engine is a 2.6L 4 cylinders carburated, manual 4 speed transmission, all 1982 Gen I. I Assume that gear ratio is 4.623 ...

JorgeA.

DRaider90, dropping gears will help like you said but for a 2.6L with a manual transmission 4.88s are not deep enough. Assuming a 30" stock tire 5.13s are the minimum. If you pick the right ratio your MPGs do NOT have to go into the toilette. Under 'gearing' and over 'gearing' are probably the biggest cause of drivability issues I see - specifically poor millage.




There are two ways to make your vehicle move. The American way - HP and torque - and the Japanese way - gearing. There is a big difference between tweaking a 2.0L ricer and tweaking a 350+CID muscle car. Now days you also get to toss a computer into the mix just for fun.

The basic idea when dealing with gear ratios and tire sizes on a stock to moderately modified, naturally aspirated (especially computer controlled) engine is to 'make the car think nothing has been changed'. Manufactures build 'slop' into all vehicles - there is always some leeway +/- from the stock configuration and you want to stay within that zone in order to keep everything happy. If you can select your combination of gears and tires to keep the end result within the mentioned 'zone' then the vehicle will perform substantially the same as it did before the changes.

x+y+z=47

In other words, if your stock configuration = 47, you can change x, y, and/or z but the answer at the end of the day still needs to equal 47. If you can manage that your gas millage and driveability will remain close to stock.

The first thing I recommend is figure out what the stock tire size was in diameter - you really need this information. Figure out what your new tire diameter will be. You can get real picky here and actually measure the diameter - just 'cause it SAYS 35" doesn't mean it is. 4.62 sounds about right but I'd make sure - this is not the place to make a mistake. Once you have all the information click on the link below and punch your data into "Find new Gear Ratio with new tire size".

Calculate Gear Ratio, Tire Size, etc.

Just for grins: 31" original tire, 35" new tire, 4.62:1 original axle ratio = 5.21:1

The calculator will feed you back the mathematical gear ratio you need to exactly match the factory installed tire size and ratio. A lot of the time the ratio will be between available gear sets so some compromise is necessary (see example). Doing what we do (bigger, heavier, wider tires) it's usually best to go to the next deepest (numerically higher) set of available gears.

Again, for grins: next lowest is 5.29:1

So, now you should have a ratio to work with. Now is the time to make additional tweaks to that ratio to compensate for altitude (beyond what the manufacture expected the vehicle to be driven at), weight, aerodynamic efficiency (roof racks, bull bars), driving pattern (lots of stop and go or lots of highway. for lots of stop and go you can go deeper and for lots of highway you can go taller - numerically smaller number), engine size/age and engine (including exhaust/intake) modifications (those that affect torque output). This will end up being your ACTUAL, 'real life' ratio.

So, our example ratio of 5.29:1 turns into: more weight @ 6,000FT average altitude = 5.38:1 A 5.5ish:1 would probably be a better choice if there's lots of stop-n-go (and the engine has 450K miles on it:) as a DD.

A good - general - rule of thumb is one step deeper (numerically higher) on a reasonably stock vehicle. 'Reasonably stock' = a small roof rack, bull bar w/winch, with some millage, running at around 4-5000FT and w/ moderately wider tires.

There is a limit on how deep you can go with gears. Obviously, if they're not available then that's the end of that. With deeper (higher numerically) gear sets there is less contact area from the pinion to the ring. Running 6:1 gears on 44"s with a 500HP engine is probably not such a good idea. So, even if the manufacture selection were infinite the number of practical selections is not. Running 5:1+ gears also transfers all the torque to the axle shafts and everything connected to them outboard of the differentials. Gear reduction is a wonderful thing but it's very easy to unintentionally exceed the operational specs of parts. <img src="/forums/images/graemlins/frown.gif" alt="" /> High quality axle shafts and u-joints are always a very good idea.

There are variables that go into the calculation for serious gear ratio/tire selection that I didn't get into that still should be considered but the above is a good place to start.

Edward