Help! Do I upgrade to posi and 31 splined axles?

[Robster]

I am upgrading to 4:56 gears in my 74 bronco and my builder is recommending posi and 31 splined axels.
Im building a weekend driver with 35" tires and a 300 hp 302. I don't off-road. Is it worth the extra 850 to upgrade both axels. He recommends the posi only in the rear. Thank you

 

[Skiddy]

limited slip (posi is chebby term:-X) not really needed, upgrade axles with 35's and you will be fine. one thing you can always add a traction device later

 

[Robster]

If I have to replace my carrier to accept the new 31 splined axels it's not much more to add limited slip or at least I am told. Didn't some Broncos come factory with limited slip? Mines a 74. How can I tell? Thanks

 

[Apogee]

With 35's and 300 HP, I would definitely do the upgrade. It's cheaper to do it all now if you can afford it than to come back later and swap out the carrier for a limited slip. I'd personally probably opt for a Detroit Tru-Trac with the 31-spline axles...a lot of folks here run them in their EB's and even more of my muscle car friends and customers run them in their pro-touring applications with great results on the street up to about 500 HP. Much better than a friction-type limited slip which requires friction modifiers and maintenance IMHO.

 

[SavageBurro]

Definitely do 31 splines and l.s. now in my opinion.

 

[blubuckaroo]

My differential builder recommended 31 spline axles with tires over 33".

 

[STalon89]

If you are doing gears, I would definitely do the upgrade to 31 and posi now. You wont want to get back into the rear end again later if you regret it.

 

[RODRIG3911]

If I have to replace my carrier to accept the new 31 splined axels it's not much more to add limited slip or at least I am told. Didn't some Broncos come factory with limited slip? Mines a 74. How can I tell? Thanks

Yes some come with factory limited slip, but if you want 31 spline axles you'd need another carrier because your factory carrier would be 28 spline reguardless.. so you'd loose your factory limited slip when upgrading if you had one and upgraded to 31 spline with a 31 spline open carrier (Unless you op'ed for a locker of some sort).... You could use the 28 spline axles and probably be ok if you took it easy...

I love my Detroit Truetrac and 31 spline axles, but I 4x4

 

[Broncobowsher]

Many a failed 28 spline axles with lesser rigs.

I would do 31 spline upgrade. Don't need any exotic race parts, the most basic and generic 31 spline axles will do. Cut down and resplined factory 31 spline axles will even do.

Now the differential will need to be changed to get 31 spline axles as well. There would be a whole lot of head scratching going on if you put an open diff at this point. The factory limited slip wasn't that great of a design new and getting all the rebuild parts and 31 spline parts will get you an expensive but still stock limited slip. Best to just put that money toward a good aftermarket unit. Lots of choices. The tru-trak is a real good street (and off-road as well) limited slip. Any of the helical gear based limited slips are a good choice. I avoid plate and cone clutch types limited slips as much as possible, on the street they are often considered the worst of both an open diff and a full on locker.

 

[Timmy390]

As was stated to upgrade to 31 spline you have to change the center section so you're "all in" at this point anyway.

I grenaded the stock T-loc with a mild 351W and 32's.

I went with a 31 spline axles and Detroit/Eaton Truetrac couldn't be happier.

Tim

 

[DirtDonk]

If you were doing it all yourself, then just changing out the side gears of the stock differential to 31 spline versions would be very inexpensive.
If your junkyard powers are still high and you have a good selection of '70's and early '80's vintage F150's with 9" rears, you would likely be able to find a factory 31 spline limited-slip diff on the cheap there as well.
Or if you got really lucky even an old T-bird or similar large Ford car with a large engine might have a 31 spline L.S. as well. Not many Boss 429 9" car parts in the junkyards these days though!

Next up is a factory 2-pinion track lock (I'm never sure about the correct name and spelling) is available brand new for around $250 maybe? I have not checked lately, but someone was going through this in the last couple of days and we were discussing options. A Ford Racing 4-pinion version is around $450.
At that point though, you might as well spend a few extra bucks and get the Truetrac.

Just like some of the others, I too am a big fan of the Truetrac unit. Probably the best all-around limited-slip for a 9" out there, but also more expensive.

Agree too that if you have a fairly light right foot, you could likely make your 28 spline axles last. But with 35's it's a fine balancing act. Going with a limited slip actually makes 31 splines a virtual requirement. Not the other way around.
More broken 28-spline axles with limited-slips and lockers than without it seems sometimes.

Good luck.

Paul

 

[bronconut73]

Do the 31 spline axles and Tru-Trac.

You won't regret it.

With a stock tranny 4.56 with 35's would be perfect off-road. Might have a little high rpm on the highway.

I went with 4.11's with 35's and stock tranny but I am running a low rpm Windsor, so it works well off road as well.

 

[Scoop]

Is your 74 a big bearing axle? Makes the swap to 31 splines easier. I recently upgraded to 31 spline and went with a Currie big bearing axle housing.

Then you might as well do a disk brake upgrade . . .