Minimum driveshaft lengths

TreverStevens

TreverStevens

Knows enough to be dangerous
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I know this isn't exactly a straight-forward answer, there's a lot that goes into it. So, I'll start with the basics and you guys can give me feedback and I can fill in the gaps on anything I didn't specify initially.

My Magnum V8 swap is nearing an end. The past 2 days have been spent positioning the engine in the frame so I can get the engine mounts welded in place. With all of that I have gone from an AX-5 transmission to the NV3550 out of a Rubicon so it has the 241R transfer case behind it. I'm putting a Ford 8.8 in the rear and hopefully within the next week (or so) having the Barnes axle truss welded in. As for right now I have about 3-4" of extra room on the backside of the heads before they're against the firewall. Now I know it's fairly common for people that are doing the Magnum swap to beat in the firewall in certain spots to get everything to fit. That's not my issue (currently). (This is where a lot of non-techincal terms are going to be used due to my lack of knowledge of the actual names) The hole where my shift lever goes in the top of the transmission is about 5-6" in front of the hole in the floorboard that the shifter is suppose to pass through. I don't really mind having to cut or fabricate an alternate hole in the floorboard to compensate for the difference and I'm leaning towards that option right now because I'm more worried about if I push the engine back as far as I can (plus extra room after altering the firewall) how short will my driveshaft be? I can't pull a true measurement right now of the distance from the back of the TC to the yoke of the axle because I don't have the previous axle in currently and even so, that wouldn't be the exact same measurement as I'll have in the end.

So back to the original question, how short can my rear DS be? Would you push the engine back as far as possible and attempt to make the hole in the tub and the trans match up? Or should I leave everything slightly forward (we'll say 2-4") and have a little longer DS? I know the second option requires cutting the opening in the tub and possibly bending the shift lever a little too so I'm not punching the dash every time I go into 1st, 3rd, and 5th gears. Oh ya, I have a 4" suspension lift, getting custom driveshafts made, adding a 1" body lift (probably) to help tuck the trans up a little more, and adjustable upper and lower control arms in the rear. Don't know if any of that changes thoughts or what not. Thanks again for the input.
 
Here's a picture for better reference. The "eye" of the safety pin is roughly where the center of the shift tower translates to. (there's duct tape over the hole right now to keep debris out. So that's why you don't see into the bowels of the trans right now.)

20190902_211130.jpg
 
With everything in place, what is the distance from rear axle u joint to t case u joint? Might give a better idea of what is too short. With a 4 inch lift a little more rear length would be a good thing.

What are you doing for a fan/radiator? My 5.9 is about a 1/2 inch from the firewall and I run a clutch fan with a homemade shroud.
 
It isn't so much length it's the angle of operation. Measure the angle of the tcase flange and angle of the driveshaft to get the working angle. I think Spicer recommends 15 degrees or less. Above that life of the DC is shortened.
 
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A post on JF.
Do you have a tummy tuck along with the 4.5" lift? Any idea what the operating angle on the double cardan joint is at ride height? Based on the several conversations I've had with guys at Tom woods, the longevity and smoothness of the centering ball decreases the closer the operating angle is to 22*. I forget which guy it was, but he said any angle higher than 22* and the life of the driveshaft falls off a cliff.

From Shawn at Tom Woods from the same thread on JF.

Finally I will address the cause of the original poster's issues. Angle is a big factor here. If you have a 4.5" lift and a 2" tummy tuck, that means that as far as the drive shaft is concerned you have about a 6.5" lift. This is across a span of about 16"-17", so the angle can get real steep real quick. As angle increases, life expectancy decreases. On average most vehicles' drive shafts will turn about 2,500 times per mile driven. That means that if you drive just 10,000 miles, your drive shaft turns 25,000,000 times. That's a lot. When the shaft is running at an angle that is really pushing the limits of what it is capable of, it will wear out more quickly. I wish I could say that we have found the magic solution to this but we haven't. A big lift on a short Jeep means a short life on the drive shaft. That being said, there may be an element of just plain bad luck at play here. Often we will talk to customers who never grease their cv center ball, drive their Jeep every day, and still they will go years without any issues. You may also want to check and make sure that it is not binding under full droop. If this happens, even momentarily, it can pry the ball loose from its seat. This in turn will expedite wear of the ball and socket
 
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Paul Ruggles said:
With everything in place, what is the distance from rear axle u joint to t case u joint? Might give a better idea of what is too short. With a 4 inch lift a little more rear length would be a good thing.

What are you doing for a fan/radiator? My 5.9 is about a 1/2 inch from the firewall and I run a clutch fan with a homemade shroud.
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I don't have a rear axle currently because I'm swapping the rear end with all of my other changes because I didn't want to get new driveshafts knowing my Dana 35 was going to blow at any given moment. I could re-enstall my Dana 35 to get a rough estimate of the distances and angles. But with the 8.8 going in and now having adjustable upper and lowers I'm going to push the axle rearward a little (about an inch). I don't have a flat skid, but planning on doing a 1" BL to give me a little extra room under the tub for everything going in.

As for the radiator, as of right now I'm planning on using a Superior radiator with their electric fan. Right now I have about 2" from the firewall to the corner of the head on the passenger's side. A little more room on the driver's side, but I also know I'm not completely square right now. Finishing up centering the tail of the TC so the engine is sitting in straight.
 
I'm avoiding welding the truss on the rear axle until I have the engine, trans, and TC in place so I can adjust my angles back there a little to accommodate everything better.
 
@Paul Ruggles I'm guessing you used the AA mount kit? Did you key the mounts off the dowels into the holes in the frame like they intended? Or did you cut them off and place your mounts further forward or backwards?
 
I used Aa mounts moved back to allow more room for a engine driven fan. Doing this also made using Ram manifolds impossible. I have a set of early 1980s truck center dump manifolds that I am going to try next. Cheap block hugger headers suck.

With a 4 inch lift I would probably leave the engine as aa intended. Mock up a dummy shifter to see if the current location works. Did you mock up exhaust manifolds yet?

If I were to do a magnum swap again, a grand cherokee based setup like @RangerRick probably is the way to go. Starter on passenger side makes exhaust routing much better.
 
I have a complete 1998 ZJ V8 donor available if anyone wants to do a V8 swap. It's a 5.2L 4x4 Laredo that runs and drove until the power steering leak got so bad I had to park it.

Also going to have a 2nd Rock Jock G2 Dana 60 rear axle available soon. Was a second order for another V8 conversion that isn't going to happen at this point, one V8 TJ is enough. It was never fully assembled and driven on, just used for mock-ups of a friends TJ who wanted a V8 like I have. Running a high pinion RockJock 60 takes at least 2 to 2.5" out of the lift allowing for a more reasonable drive shaft angle and will drastically prolong your rear shafts lifespan. My last Dana 44 low pinion TJ would only get about 10-12K miles before I had to send the drive shaft back to Tom Woods and have him rebuild it again. The centering ball would squeak and the U joint caps would spin in the yolk because of the short high angles. I would clean and lubricate it after every outing, didn't matter the angle was too much. Going to the Dana 60 high pinion of the RockJock has made a huge difference in driveline angles and longevity.

The transmission, power steering and all the parts needed for a good swap that will look factory is included in the ZJ donor when you buy the complete donor vehicle FWIW.

RR
 
I think I'm going to install a Savvy 1.25" body lift. I know that makes my driveline angles even more steep, but I think it will allow me to put the transmission where it needs to go. Since I posted last week I kind of started the engine placement process over again because I realized my UCF skid plate wouldn't line up and support the trans and TC like it needs to. The trans and shifter tower is now pretty much lined up with the hole in the floorboard, still a little forward, but with little to no modification to the hole in the tub it will work, as well as the skid plate. The engine is about 2" further back than the spot AA intended them to go. I have about 3/4" between the back of the head and the firewall. But my trans and TC are about 3/4" too low for the skid, hence the need for a body lift.

Unfortunately, I had already bought and painted a set of block huggers before I saw all the negative remarks about them. So I'll run them until they rot out or whatever then I'll look into other options.
 
TreverStevens said:
Wouldn't it though if I'm having to push my underbody driveline components upward?
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How does raising the body change what the drive line is doing? Other things may change the drive line but is isn't specificly the body lift.
 
jjvw said:
How does raising the body change what the drive line is doing? Other things may change the drive line but is isn't specificly the body lift.
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I'm doing the BL so I can tuck my trans and TC further up because I'm doing a shallower skid plate. Hence why the "guts" in my belly are getting raised.
 
A body lift is not a tummy tuck. The lift will not change shaft length. A tuck will. But not necessarily enough to need new driveshafts.
 
X2--Body lift has nothing to do with driveline angles. A shallower skid, yes.
 
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