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Old February 7th, 2010, 08:42 PM   #1
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Question TrueTrac vs. Posi.... Got any stories.. Undecided

Hey everyone, i have been looking into the truetracs and the posi for a while now and no one seems to talk much about detroit locker on here. I have a 72 Cutlass w/ 350. My rear is an open 2.73. I will be putting a 3.73, maybe a 3.90 truetrac or a posi.

I will not be taking this on the track, it will be for street riding only. Wanna have some fun.

Another question is between the 3.73 and 3.90. What kind of RPM should i expect for each gear if i take the car on the highway, short distances, (go to car show) about 65 MPH?

Thanks in advance for everyones input.....
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Old February 8th, 2010, 03:49 AM   #2
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The detroit locker are popular in off road circles. In a car they can be unforgiving and dangerous in bad weather.

The 3.73 with a th350 and stock size tires at 65 miles per hour are going to run about 3,500 rpms.
The 3.90 will run close to 4,000 rpms.

Last edited by 70 cutlass s; February 8th, 2010 at 08:59 AM.
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Old February 8th, 2010, 05:00 AM   #3
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I've had both Tru Tracs and Detroits, both work well for their designed application. The Tru Trac is more for street vs. a Detroit being for off road (drag strip or in my case 4x4). The Tru Trac is a limited slip and tends to send power the the tire with the least traction, ie off the ground, which is not a big deal on a car. A Detroit locks up and full power is distrubuted to both (or all 4) tires.

From a drivability standpoint the Tru Trac is barely noticable and you will gain traction with minimal tire wear. The Detroit, on the other hand, is very noticable. They are very quirky on the road with the rear end locking and unlocking especially around corners. Loud pops are common and normal. On slick surfaces, under power, the rear end can get away from you. You will have increased tire wear too. With that said, the Detroit is indestructable. I have had about every driveline failure there is and have not broken a Detroit.

For the street, I would use a Tru Trac.


1972 Cutlass Supreme Convertible

Last edited by we jeep inn; February 8th, 2010 at 05:02 AM.
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Old February 8th, 2010, 07:28 AM   #4
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Originally Posted by xerxes600rr View Post
Another question is between the 3.73 and 3.90. What kind of RPM should i expect for each gear if i take the car on the highway, short distances, (go to car show) about 65 miles per hour?

Thanks in advance for everyones input.....
You can use this to help figure your RPM's

David Cookman
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1999 HD Road King Classic
2012 Camaro 2SS/RS L99
2004 Silverado 2500HD 4x4 Crew Cab
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Old February 8th, 2010, 09:26 AM   #5
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limited slip

limited slip rears (posi, sure trac, trac lock, twin grip, safety track, power lock) It is all the same thing.

True trac is a limited slip used in more of a road racing application- It provides power to the wheels that need the grip...not like an open diff where the wheel with the least grip gets the power.
So when a true trac goes into a corner to turn left the wheel that gets the most grip and power is the outside or passenger wheel. If the car is going strat and you break the tires loose BOTH wheel will spin because there is no inertial affect in the rear so the worm gears are touching both side gears making both locked from side to side.
Hope this helps understand the workings.

Eaton / Auburn friction type posi are spring preloaded and vehicle axle torsion is what breaks the cone or clutch pack loose overcoming the preload spring pressure (200 300 400 or 800 pound) cause the LIMIT of SLIP in the differential unit. So in turns the traditional friction type posi has a tendency to slide out when weight is transferred in HARD cornering. where as the true trac will keep more control .

LOCKERs: ue a different mating toothed plates with again a preload spring pressure to be over come in turns and then the teeth will skip past each other one part of the plate being connected to the axle spline and the other to the carrier case which the ring gear is mount to.

Governor Lockers (factory Eaton) Uses a series of mechanical triggers run off of ramp sprockets and ramp plates with clutches to provide a locking and unlocking limited slip differential to create the best gas mileage and longest lasting clutch type unit on the market. They are strong when locked and strong when unlocked.
Only down fall is the transition from lock to unlock and vise versa. They will knock off sprocket teeth and spider gears when quick high torque loads are applied oin the transition state
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Old February 9th, 2010, 04:34 AM   #6
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I wouldn't put anything higher than a 3:73 in your car.As for the unit,a Tru-Trac would be fine,or you could even install a factory clutch style posi unit,#410408N,with the S-spring.Just make sure it is rebuilt properly.
Brian Trick
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Old February 9th, 2010, 08:10 AM   #7
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I have the True Trac installed in a 9" Moser housing and I love it. In my opinion it is the perfect street posi, just dont romp on it, while turnng in the rain or you'll do a 180!

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455, 5-speed, 3:25's
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Old February 15th, 2010, 07:03 PM   #8
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That happens with all posi

Limited slip / posi/ suregrip...ALL these are great for dry pavement driving...But for snow wet mud etc. They are just not what the factory intented it to

This is the reason the truck gov lock was only works when needed then releases again when not needed. BUT the gov locks are terrible for dry pavement performance driving. These muscle cars do not usually see SNOW RAIN and mud so I am sure anyone of the performance units will be a great choice. Get something in your price range and you'll be good. Jim
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Old February 15th, 2010, 07:03 PM
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