Diff ? and 6x6

[hotrdd]

Starting to think about building a new toy. Have spent the last bunch of years building stuff for my boys and everyone else and finally figured its time to build something for myself. Starting with an M1152 or M1123, but have a few questions. I'm a realist and getting older so 90% of the time this thing will be on the road and going down the highway, ideally I'd like to add some tow capacity to it by adding an extra axle and changing the gear ratios. Engine and transmission swaps are definitely happening.

1) How can I find out what Diff's are in the vehicle?
2) What is the highest (HWY) gear ratio you can get for these Diff's
3) 6x6 - Does anyone know what rear Diff. they are using in the 6x6 conversions? Any info to someone who has done a 6x6 conversion?

 

[MattNC]

I'm guessing that a lot of people have the same idea I have in my mind which is "that sounds like an incredibly difficult and complex project." The diff ratios are sometimes but not always on a tag attached to the diff housing. One of mine had the tag and another did not.

I am struggling with thinking through what diff do you use for the first rear axel that will pass through to the second, how to extend the frame and add a second independent rear suspension axel, will half shafts and inboard brakes hold up to the increased amount of towing you are looking for (what's your target? 10k, 20k?), how will it handle with these loads, etc.

 

[Mogman]

Basically the 3 speed trucks are 2.56:1. the 4 speed trucks are 2.73:1 and the turbo trucks are 3.07:1 there are exceptions of course.
So if you are going to do an engine/transmission swap anyway it is best to start with an early M998 with the 2.56 gears IMHO
I have a 92 with a GEP turbo and a 6L80 and a 93 with a 6.6L Duramax and the Allison 1000 both cruise comfortably at highway speeds, with the DuraVee cranked up into the high 400 HP range it gets kinda insane.
I do not understand the dual rear axle thing, not enough room to overload it now unless you just want to be different, I have seen a couple HMMWVs converted this way.

 

[Mogman]

The diffs are AMC 20 which are larger (8 7/8") than the GM 12 bolt so they are pretty strong, 2:56 will be lowest ratio you will find.
Not sure how adding an axle/diff will increase towing capacity, the HMMWV T cases are full time FWD so you are already sharing the load on two axles and the hubs will with the proper springs take any vertical load you want.
That brings up another plus to starting with a M998, the things ride much better than the later units that had armor..

 

[hotrdd]

I'd love to match or surpass the tow capacity of my buddies F550, hence the thought to add the additional axle. This machine isn't going to see much off road use as that just isn't my thing. Hauling the dirt bikes and trailers into the mountains and picking up equipment with my trailer will be the primary use. We have built several custom vehicles and 4x4's over the years but now all my boys are older and we really just wanted to work on something new and different "together". For me its more about spending time with my kids, but don't tell them that

I assume the Diff ratio is around 2:56 and then the portal axle adds another reduction? As far as full time 4x4 I assume it wouldn't be an issue to just add a transfer case to the mix so that can be turned off. Also we are going to be forced to lengthen the frame anyway. My smallest kid it 6'4" and they just get bigger from there, so we need to add some length and leg room.

 

[Mogman]

Yes, the gear hubs are 1.92:1 IIRC
But with the 37" tires the end result is almost identical to my one ton with 3.73 gears

 

[MattNC]

There are some builds on youtube for example just from a quick google search. It would be a cool project, way over my head!

 

[hotrdd]

I've seen the builds on YouTube and we have lots of resources to assist us in Calgary. But the one thing I can't find is what passthrough differential they are modifying or using. Is anyone aware of someone just replacing all the diff's with something that is more readily available? They seem like pretty straight forward diffs that maybe I could replace with something "Off the shelf".

 

[Mogman]

It will be a challenge, I was going to ask how you plan on driving the rear diff, there are not any other diffs set up like these that I have seen with drive flanges mounted directly to the housings. but I am sure there are others out there.
In the video above they say 9" diffs so that would be ford units, might want to ask them about it.
Of course with enough money (did not look like the guys above spared any expense) anything is possible, those things look like they would drive like crap on the road.

 

[hotrdd]

Well what I have discovered so far is that the guys building the 6x6 Humvees are using the Ford 9" differentials. It kind of makes sense, lots of aftermarket suppliers and super strong. It would also be easier to replacement parts or get modifications done. The "fun" part will be trying to calculate all of the different transmission, differential, portal axle and tire size ratios to get the final drive ratio I want for towing and hwy driving.

If we are going to use ford 9" diffs I may not need a complete frame to cut up for the extra axle. I may get away with some swing arms, portal axles and a few other bits. We will need to start taking things apart to see what is there and how its put together.

 

[Mogman]

Well what I have discovered so far is that the guys building the 6x6 Humvees are using the Ford 9" differentials. It kind of makes sense, lots of aftermarket suppliers and super strong. It would also be easier to replacement parts or get modifications done. The "fun" part will be trying to calculate all of the different transmission, differential, portal axle and tire size ratios to get the final drive ratio I want for towing and hwy driving.

If we are going to use ford 9" diffs I may not need a complete frame to cut up for the extra axle. I may get away with some swing arms, portal axles and a few other bits. We will need to start taking things apart to see what is there and how its put together.

You may have trouble getting 2.56 gears for the 9" and before anyone gets the idea they need to go to 9" just for the strength alone the 8 7/8 AMC 20s are almost as strong as a 9", certainly not worth the effort unless you wanted to do something strange like tandem axles.
If it helps as I have said before with the 2.56 gears with the reductions and the 37" tires is almost identical to my 1 ton with 3.73 gears and LT265/70R17 tires, running only about 50 RPM higher at 75 MPH (around 2100RPM @ 75MPH) both trucks have identical 2008 LMM engine/trans/transfer.
My HMMWV with the 6L80 is running 2300 RPM @ 75 MPH since the top gear in the 6L80 has a higher ratio than the Allison 1000

 

[hotrdd]

You may have trouble getting 2.56 gears for the 9" and before anyone gets the idea they need to go to 9" just for the strength alone the 8 7/8 AMC 20s are almost as strong as a 9", certainly not worth the effort unless you wanted to do something strange like tandem axles.
If it helps as I have said before with the 2.56 gears with the reductions and the 37" tires is almost identical to my 1 ton with 3.73 gears and LT265/70R17 tires, running only about 50 RPM higher at 75 MPH (around 2100RPM @ 75MPH) both trucks have identical 2008 LMM engine/trans/transfer.
My HMMWV with the 6L80 is running 2300 RPM @ 75 MPH since the top gear in the 6L80 has a higher ratio than the Allison 1000

Agreed, the issue is not the current Diff's. There are only two reason I would consider going to the Ford 9" differential.

1) Its easier to get replacement parts and repairs.
2) If I am going to do a tandem rear axle I need a passthrough Diff to drive both axles. I can't get a passthrough AMC 20.

 

[Mogman]

If you find a pass through 9" post the link here, I did a cursory search and came up with nothing.
Of course there are other ways, I think some transfer cases can be set up for dual rear output IIRC they were the early Dana units.

 

[patracy]

Well what I have discovered so far is that the guys building the 6x6 Humvees are using the Ford 9" differentials. It kind of makes sense, lots of aftermarket suppliers and super strong. It would also be easier to replacement parts or get modifications done. The "fun" part will be trying to calculate all of the different transmission, differential, portal axle and tire size ratios to get the final drive ratio I want for towing and hwy driving.

If we are going to use ford 9" diffs I may not need a complete frame to cut up for the extra axle. I may get away with some swing arms, portal axles and a few other bits. We will need to start taking things apart to see what is there and how its put together.

The person making those 9" centersections is @steve6x6x6

 

[Mullaney]

If you find a pass through 9" post the link here, I did a cursory search and came up with nothing.
Of course there are other ways, I think some transfer cases can be set up for dual rear output IIRC they were the early Dana units.

.
WWII vintage International 6x6 trucks had two output shafts on the transfer case. Both axles were driven by completely separate drive shafts. Both rear axles were interchangeable with the rear axle driven through a bolted on "Pillar Block Bearing" that was mounted on the drivers side of the axle. The rear rear axle had a flat plate with 4 bolt holes if it got promoted to a forward axle position.

Wonder if that is a possibility for @hotrdd ?