Technical Which Ford 9" Posi Unit

I am getting a bare Currie 9" rear end housing with the frame mods on my '37 Buick build. I am looking at a menu of 28, 31 and 35 spline axles and options of Detroit locker, True Trac, Trac Loc and Wavetrac for posi units. I am going to running a 401 nailhead making no more than 450 hp on it's best day and 6 speed manual transmission. I will be running gears in the 3.70 -1 range as that will give me a 2100 - 2200 cruise at 70 mph in 6th gear. What is stout without being overkill?

 

True Trac if cost is not a problem.

Locker if you want some fun.

Get a spool if you really want some fun.

31 splines, either way

 

I absolutely LOVE gear type differentials. We call them True-Trac as a generic term around my part of the country.

I don't claim to fully understand what goes on inside them, (Squirrel can probably explain it) but they are super smooth, strong and always leave two nice long dark black marks. 31 spline is better from a strength standpoint mostly because the 31 spline doesn't taper down at the spline end like a 28.

-Abone.

 

31 spline for sure you will snap the 28. I speak from experience.....an embarrassing moment I would like to forget LOL!

 

What Currie says :

Truetrac and Wavetrac - Helical gear limited slip differentials (HGLSD) are the modern replacement for the classic clutch style limited slip differential. Under normal driving conditions, HGLSDs operate as a standard or open differential, allowing one wheel to spin faster or slower as necessary without the noise or harsh operation of a clutch style limited slip. When a drive wheel begins to slip, the helical gears inside the differential automatically engage, transferring torque to both rear wheels to maximize traction. When the wheels regain traction, the differential resumes normal operation. Unlike traditional clutch type limited slip differentials, HGLSDs require no special oil additive or maintenance to replace worn and tired clutches, as these units have no wearable parts, require no special maintenance and maintain their torque bias for the life of the unit.

 

Mr. Forbes has sound advice, as usual. The Tru Track is the seamless power transition you probably need. Testimonial : The Detroit Locker in mine is not a problem, no ''events'' to report even though I've been caught in some rain. The unlocking/locking is a not harsh. Car is so loud I can't say if the locker makes noise. It's like having a spool that lets go when it binds.

 

I've put over 100k miles on lockers, with powerful engines, and never ran into anything...lots of driving in rain, too. The noise depends on which unit you have, some are quiet, the racing type are loud. You can tell it's there by the subtle way it moves the back of the car when you let on and off the gas on gentle curves, and it will spin both tires and make the rear end swing around if you're not careful, but it doesn't take long to learn how to drive with one. Any limited slip can cause similar issues on wet pavement, that's what they're for!

 

Go 31 spline with a locker and you won't be sorry.

 

Ha ha ! Snap a 28 spline ! Granted 31 and 35 spline are better I really don’t think you have much worries snapping a 28 spline Axle . I would be afraid of u joint , tires hooking up , and your abality to drive it out as more concern . But if you must have a 31 spline , Detroit Locker and N case with NASCAR bearing support , it’s good for Diesel torque .

 

I snapped a 28 spline right where you would expect, just after the splines at the tapper. I have a sub 3,000 pound A Sedan with a mild SBC and a Detroit Locker. If your tire roll out is not almost exact the locker will put constant pressure on the side with a shorter diameter. This "probably" won't be any issue 99% of the time.....but the 1 time you stab the loud pedal and it hooks that tire, the axle may have some fatigue and then BANG 1 wheel peel. I have since upgraded to a 31 spline (no taper) and Detroit Locker.

As a side note I drove home from the HAMB Drags this year in a monsoon type rain. As long as you know what to expect and drive through curves at a steady speed (minimum throttle input) not on and off the go pedal it's all good. I will admit I did think about how exactly I was driving though because it can all go to sh*t real fast.

 

I've got a 9" true lock 28 spline in my dodge project and it will weight in at about 2300 pounds. 411 gears with a 200 4r tranny and Curie axles. Curie told me that I would not be breaking their axles.

 

Thanks for the advice. After a bunch of reading I think True Trac is a good way to go. I grew up when Lockers, 12 Bolts and Dana 60's were must have rear ends and True Trac seems the next generation. I like the no wear and no special maintenance claims. I appreciate all the responses.

 

The True Trac has been around over 50 years...but it didn't become popular until recently.

 

True Trac and 31 spline. The so called "soft" lockers aren't and the lighter the vehicle the more jarring the engagement. Wait 'til you take that first turn, get on it and it finally engages...... hope it points you in the same direction each time.

 

Ran a Currie 28 spline in a Super Stock dirt 1/2 mile Camaro for 5 yrs with a mini-spool and never hurt an axle. I know dirt is not pavement....just sayin.

 

I run a TruTrac, or equivalent, in everything with two rear wheels, or in the case of the wife's German missile, the front.

 

TruTrack for 20 years, smooth and quiet. I never hurt 31 spline axles with 400 hp.