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Need some advise on my rear end. ;)

W

Waffles

Member
OK.

I am a rear end IDIOT. I understand little to nothing about them, and while I posted on mf.com and asked for advice, needless to say i got little.

I have a 408w mated to a built c4... i have what i believe to be the stock rear end in it, and I am ready for something that'll burn both tires...

I have heard many different ideas...

"detroit locker"

"trac-lok"

etc.

Whats the difference between posi, spool, and traction lock? Is there one? My rear is a 9" and I want some better gearing, and for t to lay it out between both tires...

I have never taken my car to the strip. I'd like to, once in a great while, but that is hardly what the car is for. It is a street car, so my main concern is about good highway gears, but that I can get some go off the line when I want it... so 4.11s are not quite what I'm looking for, I gather.

I was told to find a posi 3.25 or 3.5 3rd member with 28 splines, because as an original 302 2bbl car, i most likely don't have 31 splines.


Can anyone do some explaining to me? What gearing does it sound like I need, and what kind of ballpark am I looking at for like a posi 3.25 or so 3rd member? Are there other options.

Thanks alot.
 
"Laurie S." said:
Gee, that looks a little smaller.... :laugh:
Click to expand...


There is a guy on ebay, I'll have to send the link later, my work blocks ebay....that sells completed units in the gears of your choice for around $600 IIRC.
 
Let's explore your rearend issues..... wait, that didn't sound quite right...

Rearend options....

A "spool" is a no-no for street use. It's a permanently "locked" rearend that will not slip when the inside turning radius is less than that of the outside. The tires will "chirp" as they lose traction in a corner. Not good for daily driver street use at all, but great for the strip where the car is going straight 1/8 mile at a time.

A "posi" rearend is best described as a "semi-locked" rearend. It will unlock during cornering as the inside turning radius is less than the outside radius, but the "lock" is usually a mechanical lock and the unlocking leads to what can best be described as a ratcheting sound. This would be your "detroit locker". Some guys swear by them and some never get used to the little idiosyncracies and dump them in favor of the Trac-Loc.

A limited slip rearend is the happy medium for street use and what your situation warrants. Ford produced the limited slip "Trac-Loc" and installed it in numerous vehicles. This limited slip unit sends power to each wheel through "clutch plates" and is street friendly.

With your 408 motor, I'd be concerned about the strength of a 28 spline unit, but in theory your C4 should explode before the 8" 28 spline rear would. If you ever grenade your C4 and step up to C6, then the rearend would be your weak link.

Ultimately, your best bet is to find a 9" Lincoln Versailles rear-end. 28 Spline, Trac-Loc and Disc Brakes.
 
That sounds pretty good... thanks for all the tech advice.

Will the lincoln rear end bolt right up?

I'm not 100% positive i have 28 spline, ATM... but assumed i do since i believe its the original rear and car was only 302 2bbl to begin with
 
"Posi" is short for "positraction," simply GM's specific type of limited-slip differential. It does not have a mechanical lock (sorry dave:)). It's similar to the Trac-loc in that it uses wet clutches to lock the axles together. As you go around the corner, the opposing rotational forces cause the clutches to slip, allowing the wheels to turn at different speeds. This clutch engagement allows for a fairly smooth power transfer between wheels, making it a good choice for a street car.

Locking type rear ends use a mechanical tooth engagement to lock the differential, which tend to cause that ratcheting (and eventual BANG) sound Dave mentioned. Unlike a limited slip, the differential is either locked or unlocked; force either goes to both wheels, or only one, with no split and no in-between. This can cause interesting handling characteristics, especially in wet weather. If you hammer the gas around a corner, the differential will instantly lock and possibly send you to the side of the road. Great for the strip though, where the diff will remain locked to send full power to the wheels.

The last type of differential is a torque-sensing helical gear type. Torsen invented it, but the Eaton/Detroit True Trac is the more affordable version. It's similar to a limited slip, except it uses constantly meshed helical gears instead of clutches to send power to the wheels. The differential senses traction loss and instantly sends power to the wheel with the most traction. This results in an extremely smooth response, making handling even more predictable than a limited-slip, with the added advantage of having no clutches to burn out.

As far as what you should be using, it depends on how you're going to drive it. If it's a daily driver and you plan on seeing adverse weather fairly often (ie. heavy rain and snow), keep an open diff in it. Any type of differential that sends power to both wheels is going to make driving in adverse weather difficult and dangerous. An open diff will keep you on the road.

If you plan on driving it hard on the street and won't see adverse weather all that often, go with a True-Trac. With a price competitive with an Auburn limited slip (superior to Posi and Trac-loc), it's hard to go wrong.

I wouldn't recommend a locker unless it's a weekend-only toy. Lockers are fun, but not so good in the rain, and they can get annoying on the street. They also tend to eat up tires faster.

Gear ratio is going to depend on tire size and cam selection, as well as ideal cruising speed. 3.25's are good highway gears for shorter tires. I have 3.25's with 24" tires and I can cruise all day at 70mph @ 3200rpm. If you have taller tires, don't need to cruise that fast, or don't see that much highway use, 3.50's would probably be better for you.

I'd probably recomend looking for a 9" for that car for the future. The 8" with a good differential will probably hold ok under normal driving conditions (and like Dave said, the C4 will likely go first), but I'd rather have a 9" behind a stroker windsor.
 
Without calling me stupid.... I think I have a 9inch...

how can I tell for sure?

I just found a guy with a complete, rebuilt 3rd member. Posi 31 spline 3.50 gearing located a couple hours from me for $300 obo... said he owns a rear end shop and would be willing to either set this thing up for 28 splines for $50, or custom make me 31 spline axles if I wanna go that route/

Anyone's thoughts? I found a versaille rear end here locally but the guy doesnt know if its pos and doesnt know the gearing and wants $600....


Thanks for opinions!
 
IIRC, if you can fit a box-end wrench over all of the bolts, it's an 8". If not, it's a 9".

I can tell by looking at it, as the 8" housing is flatter on the top and bottom, while the 9" is more round.

$300 for a drop in 31-spline posi 3rd member is a steal, whether it's 8" or 9". I'm used to seeing full units for >$500.
 
how can I tell for sure?
Click to expand...

It's doubtful that you do, but someone could have swapped one in previous to you owning the car. Read through that first link I put up, it explains the differences.

I just found a guy with a complete, rebuilt 3rd member. Posi 31 spline 3.50 gearing located a couple hours from me for $300 obo... said he owns a rear end shop and would be willing to either set this thing up for 28 splines for $50, or custom make me 31 spline axles if I wanna go that route/
Click to expand...

That's a real good price for a 31 spline unit, but is it a Trac-Loc? One of the obstacles in finding these items is the cost associated with shipping. These pieces are heavy. I bought a 28 slpine Trac-Loc with 3.50 gears on it for $202 on fleabay and paid $100 just for shipping.... only three states away.
 
Well he's within driving distance and said If I tow the car he'll put it on his rack and help me swap it out for free.

So... you're saying I should move on this, ESPECIALLY if it's trac-loc?
 
Also he said that he'd custom make 31 spline axles for me, but needs to know all the specs... what specs do i need to provide hinm? just length? How do I know that without removing an axle? Thanks for all the info so far guys, I don't feel quite as stupid when talking to people about t now, haha.
 
Well he's within driving distance and said If I tow the car he'll put it on his rack and help me swap it out for free.

So... you're saying I should move on this, ESPECIALLY if it's trac-loc?
Click to expand...

If it's a Trac-Loc, the deal sounds like it's getting better and better.

The rear axle specs he'd probably need would be axle length (each side), bearing size and wheel stud pattern/size.

What did you do with the 9" out of the ThunderBird?
 
Well then, maybe you're not an "idiot" afterall..... ;)

Seriously Waff, I know you're not an "idiot". The point I was trying to make is some of the comments over there come from way out of left field. You've seen what I'm talking about.
 
It is Trac-Loc, he said.

Would I be best off just having him set it up for 28 spline instead of me having to worry about getting new axles cut?
 
Yeah I know what you're talking about, but that comment was pointed specifically at me... not at MF.

Yeah and he paid so much cause they had like 4 sponsors so he was building a serious car.

Asshole busted his driiveshaft anyway within like the first 3 cars to die. what a waste.
 
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