I can't decide, need opinions

I want to start building my 66 cj-6 and want to do it right one time. I also don't want to spend money on anything I really don't need. This is what I have, clean straight frame, stock leaf spring packs, lots of leafs in those puppies. Rear dana 44 with power lock, and front 27, open. The diff's are 3.73 gear ratio. Freshly rebuilt 225, fresh rebuilt sm420 trans adapted to dana 18 tcase with 3.15 low gears and overdrive, twin sticked. My problem is deciding what axles and gear ratio to run? Stick with the stock axles and fix the inherent problems? Or go large fullsize axles and large tires. I just don't want to spend a ton of money building a custom front differential. I have an extra set of one ton running gear with 5.87 gears and high steer. This rig will be used to travel up in the hills and take wheeling. I want to run a spring over and I already have built my spring packs. What would you guys do? I need to figure this out and make my decision so I can get started. Thanks for your opinions.

 

IMHO, stock axles are fine, or put in a D30 front if you want to run larger tires. Put on discs or 11" drums, do the knuckle stud conversion if you keep the stock front. Since you've already got the OD, run 4.88 or so gears.

Rather than spend money on axles, I'd put money toward a Saginaw conversion if it doesn't already have it.

 

I agree maybe look around for a 71 flanged 44 with 30 spline axles and swap in a locker.

 

Well this matter really boils down to...
How hard core serious do you wanna get into off-road/ rock-crawling usage ?

The SM 420 transmission with its low gear ratio of 7.05 X Terra Low transfer case ratio of 3.15 X 3.73 final drive yields a very respectable crawl ratio at around 83 to 1.
Problem with that combination of components is that your relatively high final drive, axle gearing will create a weak link.
Your propeller shafts and pinion yokes will work at relatively low velocity, especially with any tire over 31".
If you drop in a 5.87 geared axle from an M-715 you will eliminate the propeller shaft strain issue. But...
This final drive will achieve an absolutely CRAZY low ratio of about 130 to 1. That ratio is way too low unless you are running 36" tires or taller.

With your sufficient V-6 Horsepower the overdrive will only become a major asset when used to achieve hi-way speed.
Obviously your jeep project concept must be realized as a complete matched system to be a 1] dependable, 2] effective and 3] efficient machine.

In my very most humble opinion a matched system would be more like D-225/6.39/.75/2.46/4.88 with 31" tires.

The D-225 is plenty of horsepower for on hi-way usage where as a smaller engine is usually sufficient for off-road usage.
Any transmission ratio much below the typical 3 to 1 first gear or lower is of great advantage used off-road.
The overdrive is a major advantage particularly for any small engine jeep due to gear splitting capability.
The overdrive will be of much less importance with higher horsepower engines or when equipped with four speed transmissions.
The common 2.46 is an absolutely great transfer reduction ratio for most off roaders.
This ratio generally allows one to stay in low transfer range when driving on gravel roads at around 30 mph.
A 4.88 final drive with an overdrive engaged generally will allow for an adequate top speed of around 65 MPH for hi-way use age while keeping engine crankshaft rpm's at a good cruising velocity.
31" tall tires fit the wheel wells perfectly. Tire width is seperate consideration.
Narrow tires basically = roll efficiency, while wide tires generally = more traction, higher flotation.
Besides, wide tires increase jeep side stability and increase obstacle impacting.(ouch!)

Man don't you just love all the various possibilities for a JEEP !

 

I did forget to mention I do have a power steering pump and box to go in. I have all the brackets for the 225. Would the dana 27 with the stud conversion hold up ok with 33 9.50 or 34 10.50 tires and the spring over lift? I do have the larger Jeep brakes I think 10 or 11 inch. I would prefer to go with disc. at least on the front. Will the 3.73 gears I have now work ok with at least 33 inch tires. I am thinking of using what I've got. My M truck will run about 3000 rpm at 60 mph with 38 inch tires. The cj might do ok with 36 or taller tires with the overdrive. I doubt I would ever break anything in the military axles. How many jeeps have you seen running full width axles. I was thinking they would help with stability? Or will it be stable enough with a spring over and stock axle width and wider offset wheels. Thanks everyone, decisions, decision.

 

Warloch,

I believe you are referring to this statement that I made:

I really don't see any contradiction concerning acceptable speed for a jeep. Note; I stated "around 65 mph".
I only see a difference in our "rated calculation". The method by which we rate the numbers.
I vaguely used the designation of 65 mph as a commonly acceptable speed for the jeep. Not as a specific standard.
I agree one can certainly opt for more speed well beyond 65 mph. But I do not see any of these speeds (including 65 mph) as any sort of standard.
My rated calculation is based upon what I perceive as an acceptable standard. Here then is how I see it:

Jeep speed is related to the harnessed force of the engine; specifically the crankshaft velocity.
I have not seen a dynomometer chart of 225 engine performance to use as a proper reference., but I do know the Dauntless engine will be most efficient at around 2400 rpm. The figure of 2400 rpm coincides with service manual specifications. It is at this specific crankshaft velocity that this engine produces its maximum torque output.
So it is at this crankshaft velocity that the 225 will be most efficient in its operation.

Engine crankshaft velocity is certainly achievable that will readily allow for vehicle speeds well over 70 mph.
But this increased crankshaft velocity yields a slightly shortened engine service life and a slightly lower expectation for maximum efficiency.
That is why I used 65 mph as a reference. Actually I should have noted the speed as 59 mph to be specific to my standard.

Here then I'll do the math right before your very eyes.
2400 (crankshaft velocity) ~ .75 (overdrive) X 8 (pinion input) ~ 39 (ring gear) X 31" tire X 3.14 (pi) X .97 (tire radius reduction due to load)
X 63360 (inches per mile) ~ 60 (minutes per hour) = 59 mph (maximum efficient cruising speed)
Here then you can see that there is no sense in operating the jeep at a sustained cruise speed below the designated speed of 59 mph.
In my opinion this gearing combination promotes an optimum in cruising efficiency for my expected useage.

Sorry Fisherman,
Did not intend to hijack your post but jeep concerns just happen. Carry on !

 

No proboem I appreciate the information greatly. I understand as well. I am only trying to figure out as much as possible before starting whole heartedly. Actually all this figuring is time well spent and hopefully I can get it right in my head and on paper before actually building anything. I like to have a full plan, or one that is as figured out as I can get. I am considering spending the money for a sping under lift now. Maybe 3 " and using the stock axles for now. I can upgrade running gear or rebuild as need be down the road. I think if I go this route I'll be keeping the 3.73 gearing until I can drive it a little bit. That will determine what exactly I want for gears lockers etc. I just need now to make up my mind about suspension. Then I can continue on with the frame boxing, etc. Thanks for your opinions, keep em coming.

 

Ive got 33's, 3.73's, a 3-speed (3:1 first), and a teralow 3.15 t-case. (running a 304 v8) The cralwing is good for all but the most technical wheeling. The gears are also a little tall for street use. I think if I had 4.10's or 4.27s i would be in heaven for 95% of the east coast wheeling.

I'm also running spring over and if I had to do it again, i would go spring under to make the jeep more streetable. Its not awful now, but its not great on the street either. BUT, offroad my jeep out articulates just about every spring under cj and alot of yj's too.

 

Now I'm re thinking the Holbrook long leafs as soon as I can afford them. Any opinions on frame boxing of a cj6 frame? or should I just add some additional strapping? I've been told by a few people they are weak frames and boxing is necessary.

 

Hmmm. If you have a CJ-6, And you go sprung over, and do something to widen your stance, You could be very, very capable off road. But it has to be done without shortcuts. If you don't have the time, don't do it. 33" tires would look a little small on it also.
I would not recommend SOA for short wheelbase vehicles, and wouldn't be surprised to see those people return to SUA.
^
With all your other low gearing, 3.73 gears in the diffs are OK. And actually the pinion gear is larger than say a 4:88 pinion gear would be and of course should be stronger.

 

I have the fabrication skills and time to do whatever I decide. I have the springs made already for a spring over. I made my own using two sets of stock wrangler rear leaves. I cut the eyes off of one main leaf and installed it under the main leaf in a new pack. I then took the second leaf from a pack and placed it on top of the main leaf with eyes. So now there are 7 leaves, in hopes it will be a wrapless springpack. I just can't decided what I want to do. I think a perfect cj6 has enough clearance for 35" tires minimum and maybe larger. But you have to have lift or clearance to get that. Also larger tires usually require larger running gear or axles to hold up well to off road abuse. I guess I could build the stock rear 44 with full floater and then get a front 44 housing made up with the correct caster, camber and crossover steering. I was also thinking if I go this route most of my money will be spent on axles, suspension stuff I can finish because I already have the springs. I keep waffling back and forth. I guess I should just go spring over with what I've got.

 

Most of the people I wheel with like the axles to be about wrangler/scout width. It seems like the old stuff is a little too narrow, and full truck size starts to get a little wide. You could possibly play with the offset of your wheels to make the width more appropriate. I narrowed my own ford front axle to scout width , and moved the pumpkin to the passenger side. Wasn't hard, just needed a welder and a grinder. Set the caster to where I wanted it. I did pay Moser to cut my axle shafts the correct length.
Another thing I did was to keep my leaf springs under the frame rails, not outboard them, it really gives it a lot of articulation.
Also, Even though my rear leaves are fairly stiff, I made a traction bar. You can really feel the difference with it attached.

 

Here's an old pic of my CJ-6 project but gives you an idea what it could look like with a spring over and 36" swamper radials. That is not a stock frame so the body sits up a little higher than what it should on your stock frame.
This is a spring over with 4" lift Scout 2 springs with several leaves removed. It's also stretched to 106" wheel base using long CJ fenders and pushing the front axle forward a couple of inches.

 

My question is -- What do you need the overdrive for?

I run the 5.38's in my JEEP and doing the math and all the final drive ratio equates to running 4.09 gears. The tack would drop from 3k at 50 to around 2500 or so.

3.73 is Hiway gearing. Or so I was told.

 

I guess I don't need the overdrive? I do have the overdrive. Would it be nice to be able to use the overdrive if you had it? Originally I was going to modify a tuxedo park edition. I was talked out of that because it was all original and way to clean. Now I am modifying a cj6. Which I like the wheelbase much better.

 

By the way, the '6 in my pics above are with Scout 2 width axles and 8" wide wheels with 2 3/4" back spacing so the tires stick out further than with standard backspacing 8" wide wheels.