Rear end swap: Dana 70HD to 14 bolt

[Vintage Musclecar]

Did a search and came up empty handed, so...

I'm considering this swap for my `90 3500 Crew Cab Dually Silverado due to axle ratio availability. The current Dana has a 4.10 and the lowest numerical gear set for it is 3.54 where the 14 bolt can go down to a 3.21 gear.

Doing a little quick math shows the following (with my existing 30" tires);

60 MPH w/4.10 gears = 2755 rpm.

60 MPH w/3.54 gears = 2378 rpm--not really worth the trouble.

60 MPH w/3.21 gears = 2157 rpm--worth the trouble.

Swapping the 400 Turbo for a 4L80E is a consideration, and doing the math shows the following;

60 MPH in OD with 4.10 gears = 2066 rpm. Viable, retains the torque multiplication advantages of the 4.10's but not exactly a low buck swap. Considered a Gear Vendors OD but it pretty much falls into the same category.

What will be going under the hood will have zero problems pulling the load with a 3.21 axle (details on the engine at a later date), so that's not a concern.

As I am not intimately familiar with these HD trucks, my question is what all does this axle swap entail?

--The truck has a rear sway bar--any interchangeability issues between the housings in this regard?
--Assuming driveshaft lengths are probably different as well as U joint sizes?
--Any differences in the brakes between the 2 housings?--can the existing brakes from the 70 go onto
a 14 bolt?
--Are the leaf spring perches the same and in the same location?
--Overall widths the same?
--Any other potential issues?

Thanks.

 

[Bextreme04]

The 4L80 will be an easier solution, its a direct bolt-in swap for the TH400 and will only require moving the crossmember back about 1". You can get a core for $200-ish depending on where you are, or just get a non burnt up one from a junkyard for a little bit more. You can get good high quality upgraded parts to handle up to 800hp for about $400 and a stand-alone controller is another $400. If you do the rebuild and upgrade yourself, you can be all-in for $1000 or so. Any 14-bolt solution will probably cost you that much or more by the time you have new gears, rebuild kit, etc... plus there isn't a bolt-in 14bolt solution that I know of. I think only the C&C trucks had dually 14-bolts, which has a narrower frame and therefore different spring perch width. You'd have to grind off the old perches and weld them back on in the right place.

If you are putting that Gen VI 496 in from your previous post... you'll be close to what I'm doing. I have a 97 K2500 Suburban L29 454/4L80E that is getting swapped into my 1980 K25 Pickup. I converted it to an 0411 Corvette ECU and CNP with a 24x reluctor and will be using the ECU to control the 4L80E.

You can also run the 4L80 full manual, with no computer control.. but it will be at full line pressure the whole time.

 

[Vintage Musclecar]

The 489 is still destined for my `95 2500 Suburban. What's in the works for the Silverado will be considerably more robust.

I priced out the 4L80 parts and wound up quite a bit higher than that--I'm not a transmission builder (nor a computer guy) so that work has to be farmed out. The MSD or TCI controllers are $600/$700, good trans cores around here are often $500+, figure on another $1K to rebuild and upgrade, then the tailshaft & output shaft has to be set up for mechanical speedo provisions. Then there's the cost of a good converter. It's definitely a viable option, but I'm coming up at around $3500-ish.

If the spring perches are the biggest hurdle to swap a 14 bolt, that's not a problem at all. As far as building the 14 bolt, I can handle that myself so that saves some $. The ring & pinions are <$300.00, and an Eaton posi is about $750.

 

[Bextreme04]

The 489 is still destined for my `95 2500 Suburban. What's in the works for the Silverado will be considerably more robust.

I priced out the 4L80 parts and wound up quite a bit higher than that--I'm not a transmission builder (nor a computer guy) so that work has to be farmed out. The MSD or TCI controllers are $600/$700, good trans cores around here are often $500+, figure on another $1K to rebuild and upgrade, then the tailshaft & output shaft has to be set up for mechanical speedo provisions. Then there's the cost of a good converter. It's definitely a viable option, but I'm coming up at around $3500-ish.

If the spring perches are the biggest hurdle to swap a 14 bolt, that's not a problem at all. As far as building the 14 bolt, I can handle that myself so that saves some $. The ring & pinions are <$300.00, and an Eaton posi is about $750.

Yeah, if you aren't going to do the work yourself its going to be way more expensive. I got my entire donor vehicle, running and driving, for $500. You also don't necessarily have to convert the 4L80 to mechanical drive. Your 90 might actually be an electronic speedo already, I sourced a '91 Suburban gauge cluster locally for $50 that had an electronic speedo for my swap. Honestly, if you are ok rebuilding a rear end... you should have no trouble rebuilding a 4L80. They actually aren't that complicated or difficult to rebuild. There's a lot of guys that will just pull them out of a junkyard for $500 and run them as-is for years behind twin-turbo LS motors with no issues. They are notoriously tough. You can find all kinds of videos only of what is involved in the rebuild and ASTG manuals are readily available.

I'm not sure what else would be different, I just know the perch width would be different. You will likely also have issues with overall width of the axle since I think the C&C 14 bolt is overall narrower.

What the engine will be is also a pretty big determining factor. If you are dropping something that powerful in it, you will likely need to be changing the converter and rebuilding your TH400 anyways... which is going to cost you pretty close to the same as a 4L80 anyways, so that should be factored in. Also, just because you will have the power to use a ridiculous gear ratio, doesn't mean you should. If you are planning on towing anything heavy, that 3.21 is going to put a huge load on the transmission. We have 3.21's in our 2011 Suburban and it is horrible towing even a 3500lb trailer. If you do go with that tall of a gearing, I would recommend you get the biggest external trans cooler you can find and also make sure you keep it plumbed through the radiator. A non-lockup TH400, with big power, pulling a heavy load, is going to be getting very hot.

 

[Craig 85]

The 14 bolt 1-ton spring perches and shock mounts are narrower than a 3/4 ton. Also the shock mounts are reversed. Here's my SRW K30.

K30

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K20

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[nvrenuf]

The frames are all the same width, it's the spring hangers that are different on 1 ton frames vs 1/2 and 3/4 tone. So spring perches are the same for SRW, DRW and C&C 1 tons.

I've seen some C&C trucks with a rear sway bar and it's my understanding that the set up is the same for a 14 bolt and D70HD. I put a sway bar from a D70HD on the 14 bolt in my old crew cab without any problems. You will find that the u-bolt cradle that goes under the axle is slightly overized for the smaller 14 bolt axle tubes but it'll work.

I'm pretty sure the brakes will interchange between the dually versions of each axle but I don't think the dually brakes will work on a SRW housing, the backing plate flange is set further back on the tube for a dually.

A HUGE hurdle is going to be axle housing width - a dually D70HD has a 74" WMS (wheel mounting surface) where square body SRW 14 bolt is 67"ish and a C&C 14 bolt is 63". After the first 3-4 years (around '93+ ?) the GMT400 used a 74" 14 bolt in the dually pick ups but the spring perches will have to be moved, this is the best / closest matching option IMO.

The pinion snout on a Dana is slightly longer than a 14 bolt but I'd bet there's enough "play" in the driveshaft slip yoke assembly to work. The u-joints will be the same, 1350 series.

An option for even higher gears is a D71(?) from an early Dodge Cummins pick up as 3.07's were moderately common.

 

[Vintage Musclecar]

Still researching all this and found the following video:

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It appears the 3.07 Dodge ratio "can" be installed in a 70HD, but it's far from a bolt together affair. Definitely not worth that kind of effort.

Still considering the later 14 bolt option as well, looking like this may be the most effective option if I can narrow down an exact year, make and model that has the correct width--not to mention finding one that already has the right guts in it.

That 3.54 gear and limited slip option in the 70HD is looking more and more plausible.