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Discussion Starter #1
Hello everyone!

After dropping a new high HP/Torque motor in my 1970 Cougar last summer, I'm looking to have the rear end upgraded as well. Currently, the car has 28-spline axles and a 3.0 open differential. I bought a set of 3.7 gears which I've been waiting to have installed until I had the cash put aside. Also, I'd like to install some type of traction control differential. So, the questions:

1) Will the 28 spline shafts hold up to 500 HP/ft lb or do I need to upgrade to 31 spline shafts?
2) Are the gear sets the same between 28-spline axles and 31-spline axles or do the gears have to be changed as well if I upgrade to 31 spline axles? I've seen reference to "28 spline pinion gears" and don't know it that has to be changed if I go to 31 spline axles. I hope not! :)
2) Any suggestions for what type of differential to get? The car is street driven/cruised, I just want to be able to get power to both wheels when I mash the gas rather than a one tire fire for those burnout contests and traffic light theatrics.
Thanks!
 

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28s are perfectly fine for 500hp on the street. If you're drag racing with a stickshift and using slicks, no.

Your carrier (the unit the ring gear bolts to) would have to sized for the larger diameter 31-spline axles. The hole in the carrier that the axle goes through to get to the splines in the spider gears is sized for the axle size. Most factory carriers that are used for 28s are not bored big enough for 31s. Some are, but most are not. Anyway, when you upgrade your carrier to a posi of some kind or other, you'll need to know what size axles you'll be running. For a street car, a Tru-Track and 28s will be plenty. Traction-loc clutches wear out over time. The tru-track uses meshing helix-cut gears that don't wear out.

BTW, it is not the horsepower you need to worry about, it is the torque. Remember that torque is the resistance to twisting motion - like when you tighten a bolt or nut. That is the force you're putting on the axles when you try to accelerate.
 

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Discussion Starter #3
Awesome Milo, thank you for the quick, informative reply! I just didnt want to waste money on a differential for 28-spline axles only to find out they wont hold up to the power and have to buy new axles, gears, and differential again.
 

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3.70's might be a touch low for floating down the highway---might try 3.25 and a low gear set in the trans---i think they are 10 % lower 1st gearand 5% lower in 2nd---a good choise for a torque motor---the 3.70's woud be perfect for an o'drive type trans
doctordesoto
 

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I recently did this upgrade on my 69 Cougar and I went with 3.55 gears and a truetrac locking rearend. Don't forget to change out the speedometer gear in the transmission. If not your speedometer will read slow, I got my gear from Mustangs Unlimited but WCCC has them as well.
 

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A torque monster engine means you need less mechanical advantage in the axle ratio to move the car. So, theoretically, you could leave the 3:1 and it'd be fine.
But, the 3.7's will make it giddiup a bit more.

I always suggest a detroit locker for a car with that level of power. I like them. They just work.
 

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I find a Traction Lock is good for a couple thousand miles behind a 500 HP engine. After that the clutches are gone. Don't waste your time.

I switched to Detroit Lockers 20 years ago. Haven't had a problem since. Detroit Lockers are utterly silent and smooth. You won't know anything is back there. You will never have to replace a worn out or broken differential again.
 

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While we're on the subject. How about putting a 31 spline center section and 31 spline axels in a stock 69 rear that came with 28 spline axels?
 

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It goes right in. The housing doesn't care how many axle splines there are.

While we're on the subject. How about putting a 31 spline center section and 31 spline axels in a stock 69 rear that came with 28 spline axels?
 

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Thanks Royce. It may be in the not to distant future....

The bearing on the wheel end is the same size? That was my major concern..
 

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Just my two cents, but with 500 horsepower on tap, eventually you're going to want to bolt on some sticky tires and see what she'll do on the strip. When you do, you'll appreciate the peace of mind that comes with having 31 spline axles. If you can swing the $, making the upgrade at the same time you're purchasing a new carrier anyway is definitely the way to go.

BTW, while Detroit Lockers work extremely well and are hard to break (but not impossible, ask me how I know) they do clunk and bump a bit in tight/low-speed corning. The newer models aren't as bad as they used to be, but they're still nowhere near as smooth and silent as a clutch-type unit. I don't find it particularly objectionable, but it can be a bit unnerving if you aren't expecting it.
 

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Have you ever been inside or driven a Detroit Locker equipped car? They are utterly silent, at least the ones made in the past 30 - 35 years.

I have owned many Detroit Locker equipped Cougars with 500+ horsepower. I just got through driving the GT-E from Cincinnati to Virginia with the DL. You can't tell anything is back there. Totally silent, smooth, quiet, works perfectly.

Just my two cents, but with 500 horsepower on tap, eventually you're going to want to bolt on some sticky tires and see what she'll do on the strip. When you do, you'll appreciate the peace of mind that comes with having 31 spline axles. If you can swing the $, making the upgrade at the same time you're purchasing a new carrier anyway is definitely the way to go.

BTW, while Detroit Lockers work extremely well and are hard to break (but not impossible, ask me how I know) they do clunk and bump a bit in tight/low-speed corning. The newer models aren't as bad as they used to be, but they're still nowhere near as smooth and silent as a clutch-type unit. I don't find it particularly objectionable, but it can be a bit unnerving if you aren't expecting it.
 

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You need the proper bearing for 31 spline axle in a Cougar / Mustang / Fairlane / Comet. It is the same OD but a different bearing than would be used with the 28 spline axle because the axle shaft is bigger OD. Just get the right bearing with your new axle. It is a factory part for 31 spline equipped 1968 - 70 Cougar / Mustang / Fairlane/ Comet. Any auto part store sells them for about $60 each.

Thanks Royce. It may be in the not to distant future....

The bearing on the wheel end is the same size? That was my major concern..
 

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Just my two cents, but with 500 horsepower on tap, eventually you're going to want to bolt on some sticky tires and see what she'll do on the strip. When you do, you'll appreciate the peace of mind that comes with having 31 spline axles. If you can swing the $, making the upgrade at the same time you're purchasing a new carrier anyway is definitely the way to go.

BTW, while Detroit Lockers work extremely well and are hard to break (but not impossible, ask me how I know) they do clunk and bump a bit in tight/low-speed corning. The newer models aren't as bad as they used to be, but they're still nowhere near as smooth and silent as a clutch-type unit. I don't find it particularly objectionable, but it can be a bit unnerving if you aren't expecting it.
I like the Detroit True Trac for street use. Helical gear driven (no clutches) limited slip.
 

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Sure. I've had a 31-spline D.L. in my '69 for 7 or 8 years now. Its regularly driven, including several long trips. It was a new unit when installed, and it has clunked and popped during parking lot maneuvering since day one. Not nearly as bad or often as the D.L. unit I had for several years back in the 80s, but you can hear and feel it working in slow tight turns.


Have you ever been inside or driven a Detroit Locker equipped car? They are utterly silent, at least the ones made in the past 30 - 35 years.

I have owned many Detroit Locker equipped Cougars with 500+ horsepower. I just got through driving the GT-E from Cincinnati to Virginia with the DL. You can't tell anything is back there. Totally silent, smooth, quiet, works perfectly.
 

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From the Eaton (manufacturer of Detroit Locker differentials) website:

When I step on and off the gas pedal, I hear the unit make a “clunking” noise. Why?
Because Detroit Lockers have “backlash” or “slack” between the drive and driven teeth and you will hear this in everyday use going through corners and when going from drive mode to coast mode. Also with the vehicle on the ground and the transmission in neutral you will have 1\4 to 1\3rd of a turn of lash in the driveshaft, this is completely normal.


Can I run a Detroit Locker on the street?

Many people do use the Detroit Locker during everyday street use. The Detroit Locker is a very aggressive differential, so you will hear it and feel it everyday on the street. Eaton makes many applications for trucks that spend most of their lives on the road. (In the late 60’s, some Ford cars had the Detroit Locker as a factory option.)



http://www.eaton.com/EatonCom/ProductsServices/PerformanceProducts/Products/Differentials/FAQ/index.htm
 

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Well done Jay.
 

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Discussion Starter #19
Wow, thanks for all the replies and advice folks!

At this point in time, I've decided on the detroit locker style differential, keeping the 28 spline axles already in the car. I decided on the 3.7 gears because the car isnt really a highway cruiser, more of a fun street car and I wanted the acceleration to impress my friends! :) Sounds like the cheapest option and those axles should hold up to the occassional "acceleration contest" on the street.
 

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Discussion Starter #20
Besides, if you can hear the DL clunking, then either your exhaust or your stereo isnt loud enough!
 
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