You can see more details in related posts, but if I put a double transfer case behind a mildly-built 390 (longer-duration cam, bored, 4bbl at most) and a good C-6, would the 3.98:1 low-low range overpower my Dana 44 front and 9inch rear axels? I'd likely keep stock axel ratios, as I won't be running big tires or rock crawling a lot, so I think stock ratio's are about 3.5:1 (estimation). Given the 2.46:1 1st gear, at 400 ft-lbs, that would be 13,707 ft-lbs of torque coming out of the t-case. Worse case scenrio, if I was pulling a in low-low and 2 wheel drive, all of that (minus 3% at each gear transfer) power goes to one axel. Assuming I'd never be in low 2wd, that's still over 6800 ft-lbs going to each axel.

I doubt, very seriously, that 1) I'd get 400 ft-lbs out of such a mildly built 390, and 2) I'd ever be at max torque in first gear pulling a tree stump out of the woods. But, I want to make sure the rest of my truck could take it before spending $1000 on the kit (already have the t-cases).

Thanks,

Matt

 

I have made a few launches stalling at 3200rpm with slicks and I have stock 28 spline axles. A guy I know used to dump the clutch at 5,000 with a 390 and stock axles and slicks. Neither of us have broke an axle. Both of us are putting out over 450 ft/lbs. He had 4.56 gears and I have 3.70. I know nothing of the Dana 44 but I still think you should be okay!

 

I think you will be okay. It is bouncing under throttle with hugh tires that usually break things off-road, UJs that are not lubed, or having one wheel off the ground and applying throttle and spinning it like crazy. I think 33s would be okay.

I have been using my 1971 for logging stuff and so far the Dana 44/9 inch has held up. By accident, I had both axles and three wheels flinging dirt trying to pull a stump under 1/2 throttle and nothing broke. My truck was already squatting from the load the winch/stump put on it.

With your setup and normal sized tires, I think the tires will break traction long before anything breaks. But, if you put on 35/36 tires and air them down, then I think breakage would be possible, especially in the front with throttle.

 

A little known trick to keep axles alive is tire width. I read about a guy with an older bronco with dana 30 front and 28 spline 9" in back with 33's. He said he killed axles left and right with 12.50 x 33's and 35's. He changed over to super swampers 9.00x33's and never had anymore problems. He was using a mild 302 with twin transfercases and rock crawling so I think he would probably put as much stress on axles as you might ever do.

I don't really think you will have any problems. If you start breaking things you can always upgrade to aftermarket shafts cheaper than you can swap in bigger axles. Of course you don't get the bigger brakes, but you don't get all the headaches of the swapping either.

 

If you are running the 31 spline 9" you should be OK, I have snapped and spun too many of the 28 splines. The D44 should be OK, as long as you are not locked up front. If you put a locker in, you will began to break the u joints and axles. It also really depends on what tires you are running. It doesn't matter how much torque you put out if you can't put in on the ground, by spinning.