Need advice on rear limited slip/locker

[Nickm1024cobra]

My knowledge is in race cars, not 4wd's so pardon my ignorance.

I am finishing the build on 67. I have a fuel injected 302, 4r70w with stand alone controler, d20, and 35" tires. I am regearing to 4:56 gears. 95% pavement duty, and I may do a little trail riding as I get the 4 wheeling bug.

Next week I am taking the 3rd member out to get the 4;56 gears installed. Do you recommend I go to a different setup while it is out? I have a large amount of money in the truck at this point and don't mind spending a little more if it will be beneficial. Just not sure if i want or need a "locker". But, i also question if the stock clutch pack is crap. Let me know what ya think.

Thanks!

 

[Quick & Dirty]

The factory 9" limited slip isn't that good, especially with big tires. There is only room for a clutch pack on 1 axle shaft, due to the 3rd pinion bearing. They don't add much torque bias and wear out quickly.

An Eaton Truetrac might be a good choice for mostly street use, as it uses helical gears to achieve torque bias so they don't wear like a clutch pack. It will lose traction if a tire is lifted though. Dragging the brakes a bit will help keep it moving.

The most expensive option, an ARB air locker will allow selection between open and spool for great street and trail performance.

A ratcheting locker like the Detroit or Powertrax will work well off road, but the street manners can be harsh, and take some getting used to.

There aren't a lot of choices for the 9" as the small ID of the ring gear and the 3rd pinion bearing don't leave much room for something both strong and effective.

 

[mtxride]

For mostly street use, my vote is the TrueTrac. I just put one in. On the street, I only notice it when doing very sharp turns, like in a parking lot. Other than that it's very smooth.

I had a Detroit locker in a daily driver. It definitely took some getting used to.

 

[needabronco]

Personally I'd go with a Detroit locker in the rear. Detroit's are kinda like a 'set it and forget it' kind of locker. Once you get used to how they can be pulling out of parking lots you forget they are there until you get through a nasty trail and realize your still in 2wd.

I'd also suggest you consider going deeper in your gearing as well. On paper 4.56 gears look good, but 302's need more RPM's on the freeway to push a sideways barn door on 35"s. I run an AOD and 35"s with 4.88's and it lugs at 70 at around 2200 RPM's. It's much happier at 2500-2700 which puts me closer to 80 with soft suspension. I usually just speed up and white knuckle the expansion joints on bridges and when semi's drive by, but I'd personally be happier with say 5.13's and be able to cruise at 70.

If you don't have 31 spline axles and your buying another carrier already, now is the perfect time to upgrade.

 

[blubuckaroo]

Why not keep the rear open? With your climate and that you might only see a little trail use, what would be the point of a traction device at all?
Mine saw some pretty wicked boulders when it was open. Now it has a Yukon posi that I've been very happy with.
But now I've moved to where we get winter weather that even makes a posi questionable.

I agree with needabronco about the gears. you'll need deeper than 4.56 with true 35" tires and that .60 overdrive. I don't know if I'd go all the way down to 5.13s, though since I like an overdrive to be just that. Overdrive is for the freeway.

 

[Nickm1024cobra]

After much research I chose the 456. It has 3:50 right now and on the 50 miles of shakedown it tries to go in overdrive at about 55mph and lugs unbearable.

I have a set of 456 gears laying around. Now you guys have me reconsidering.

Can anyone link me to the correct truetrac I need? I was researching the eaton before the post. I may go 31 spline.

 

[Timmy390]

I just installed a 31 spline TrueTrac and WH 31 spline axles after my 28 spline Trac-loc exploded.

Got mine off eBay new in the box for $483

Tim

 

[mtxride]

Can anyone link me to the correct truetrac I need? I was researching the eaton before the post. I may go 31 spline.

I got mine from Bronco Graveyard. Here's the 31 spline - http://shop.broncograveyard.com/Ford-9-Inch-Detroit-Truetrac-31spline/productinfo/22255A/

 

[bronconut73]

I wish I had put the Detroit locker in years sooner.
Had it for about 4 years now.
I am so used to it now that I don't even notice it's quirkiness.
Seamless...there when I need it without switches or double footing the pedals to engage a tired old LS clutch pack.
Just stone cold reliable.


Of course I do not have to drive on ice....I am told they can be tough there...

 

[jw0747]

My set up is 4.56 gears and 35" tires and absolutely no problems for 10+ years. Full lockers have bad street driving manners. But a posi-trac or True Trac would be beneficial for driving in snow and ice. True Trac is a gear driven, posi trac made by Eaton that has no clutches to wear out.

http://www.eaton.com/Eaton/ProductsServices/Vehicle/Differentials/detroit-truetrac/index.htm

 

[Broncobowsher]

My set up is 4.56 gears and 35" tires and absolutely no problems for 10+ years.

But what overdrive are you running? Big factor you forget to include.

OK, so you have 4.56 gears laying around. Go ahead and try them. But from someone who has run a 351 on only 33s with 4.88 gears (and my fair share of 3.50 and 4.11 gears without overdrive with 302 and 351s) I'm going to say that the 4.56s are not going to be optimal. You are going to find yourself digging deep into the throttle (and have to keep the OD shift point set to a fairly deep throttle setting) to keep it from hunting. Probably fine on the flat, but in and out of OD going up grades. New stuff (in the past 10 years or so) take that kind of shifting in stride. The engine and transmission are matched, power is cut during the shift. The way the shift is pulled off it feels smooth. The clutches are not taking the abuse. But even though the 4R70W is modern, it is still old school and is giving the clutches a beating with that heavy throttle shifting. Hunting in and out of OD is murder on these old school overdrives. That is why they have had such a bad reputation for so long. If you can get it to hold a gear most of the time and only force a downshift on a few hills it will be so much happier and it will drive better as well.

 

[jw0747]

But what overdrive are you running? Big factor you forget to include.

Don't have overdrive and don't want it due to very minimal street driving. On my pickup when it is shifting in and out searching for overdrive I pull it out of overdrive and into drive (1:1) until conditions allow no more searching.

 

[Broncobowsher]

Don't have overdrive and don't want it due to very minimal street driving. On my pickup when it is shifting in and out searching for overdrive I pull it out of overdrive and into drive (1:1) until conditions allow no more searching.

Well now that we know that we can put your comment into prospective. Since the original post is based on an overdrive transmission. Without an overdrive, 4.56 and 35s powered by a 302 is just fine. But add that overdrive and now those 4.56s are going to try and cruise the highway like a set of 3.21s. That's even more tall legged than the base 3.50 gears.

 

[ntsqd]

IF you want something other than an open diff I'd look at the Tru-Trac. If you want something other than an open diff AND you you want add some bling I'd look at the ARB.

 

[blubuckaroo]

Well now that we know that we can put your comment into prospective. Since the original post is based on an overdrive transmission. Without an overdrive, 4.56 and 35s powered by a 302 is just fine. But add that overdrive and now those 4.56s are going to try and cruise the highway like a set of 3.21s. That's even more tall legged than the base 3.50 gears.

I thought that was the idea of overdrive.
I never liked cruising at 60 with the original 3.50s and 27" tires. If I remember, it was over 2600 RPM @ 60. That made the RPM @ 70 way too high for me.

Like I've said before...
Overdrive isn't for normal streets. It's sure a blessing on the freeway though!

 

[Broncobowsher]

Overdrive lets you run low gears on the street and the trail, and still get down the highway.

Don't mistake the Bronco for what a Mustang can get away with. A mustang can cruise down the highway just off idle because it is aerodynamic (a Bronco is anything but) and has low rolling resistance (Broncos are not known for there easy rolling). You need some RPM to make the HP needed to push the br(onco/ick) at highway speeds. 2600 RPM on stock tires may be a little much, but add the drag of being lifted and rolling 35s and 2600 will start feeling about right.

 

[blubuckaroo]

Overdrive lets you run low gears on the street and the trail, and still get down the highway.

Don't mistake the Bronco for what a Mustang can get away with. A mustang can cruise down the highway just off idle because it is aerodynamic (a Bronco is anything but) and has low rolling resistance (Broncos are not known for there easy rolling). You need some RPM to make the HP needed to push the br(onco/ick) at highway speeds. 2600 RPM on stock tires may be a little much, but add the drag of being lifted and rolling 35s and 2600 will start feeling about right.

I'll agree that the top speed for mileage in a Bronco is around 60 MPH. However, I can't drive like that here.

Mine's happy with 2400 RPM @ 70 MPH.

It's a matter of where you've built your power curve. Many builds get carbs and cams that won't work in this range because of the "Mustang influence." With proper part selection and tuning, it works great.

Also, this same low end torque is the best for trails too.

 

[jw0747]

Didn't he ask about a traction device?

 

[ntsqd]

He did.

4.11's, no OD (NP435), 37's and 35's both seem to be OK behind a 351W. Drove it on the fwy with the 37's and felt the lack of OD. Lead a group thru Calico with the 35's and never felt over-geared. Rarely used first gear when in low range, just too low most of the time.
I'd think 4.56's behind a 302 would be alright. Seems like we get lost in trying to build the ultimate when good enough is good enough.