Posi Rear Ends

[Wbulk]

Shortly I will have to order the Posi differential for my Ford 9". Over the years I have had both friction disc type and a Detroit Truetrac. The Truetrac was really purchased due to it's outstanding performance in the snow. Recently, I have heard that some guys are putting the Truetrac in their Cobras. Let me be clear I am not talking about lockers for a Cobra but just limited slip differentials. I have no experience with the performance of either of these in a light Cobra car. My experience has been on trucks.

Friction Disc or Cone Type
The friction type for the most part puts equal presure to each axle until one of the axles overcomes the force of the springs and discs. Once that force is overcome it allows the wheels to turn at different speeds for like going around corners. These have a long standing track record, but do wear out over time. This type usually needs a friction modifier added to the gear oil for smooth operation.

Detroit Truetrac
This is a gear type system that transfers traction to the wheel with the most traction. If one tire is on a wet slippery surface and starts to spin, the gears in the differential transfer the power to the wheel with the most traction. One minor shortcoming of this system is that it needs some traction or resistance on each wheel to work. So if one wheel was off the ground a light brake presure may be required get the traction to the wheel that was on the ground. This system seems to need little maintenance other than regularly changing the gear oil.

I am interested in experiences of each of these installed in a Cobra. Are there known pros and cons to a light weight vehicle? How do they effect handling?

[Karl Bebout]

I've been using a Powertrax No-Slip in my car for at least 10 years and am very satisfied with it. I do get a "click" sometimes on a turn, especially backing up, but there are no clutches to wear and it very positiely drives both rear wheels equally. Another plus is that it is very easy to install on a 9" Ford. They run about $500 and there are different units for the various applications.

[FUNFER2]

Karlos, where did you get that part ?

[Karl Bebout]

'Way back, I bought the Powertrax from Checker Auto and I don't know if they still deal with them. Summit, and I think, Jegs, curently sell them.

[RICK LAKE]

Wbulk Any chance of getting a first name? I run a detroit locker in my car. They don't make a Torsen Gleason for a dana 44 case at this time. This is the idea posi to run, has worn gears and good for 750 HP Nascar runs them on the roadracing courses. I had on in my CJ-5 jeep for many years until the case cracked. Get 31 spline axles for the rearend. Look at the top axle makers for this like Williams or Moser. Clutches are good if setup right but need fluid changes if you are running the car hard or doing burnout while going around a turn. This with wide tires is brutal on the posi setup. Side note there is no real differents between your trucks and this car except the weight per pound. On a clutch plate rearend like a 10 or 12 bolt posi, I run them tight with an extra set of discs in the carrier, It acts like a spool and will still get one tire spin a little faster than the other. When cold fluid is in the carrier it just drags the one tire aroundon a turn. Clearances are real tight to do this. Rick L. Ps check with Curries drive lines for parts also.

[Wbulk]

Rick,
Sounds like the Truetrac may be the way to go. I believe it is the newer version of the Torsen. Thanks Rick! Always good information.

Wayne,

[RICK LAKE]

WBluk Nice name Wayne.Before you put down your money BE 100% sure of the HP and torque rating of the carrier. Some are only good for 400pounds of torque and 400HP. Add a 20% increase to these numbers for the abuse.I have broken a TripleD carrier which was good for 600HP and torque back in the early 80's. My motor was only pushing about 365hp to all wheels I ran 33" tires and snapped the case in half. Quieffe sell what you are looking for but not cheap. You didn't say what kind of HP and torque you where after. You only want to build the car once and abuse it alittle. IF you are going to show off withthe car, long burnouts, go with 35 spline axles and have them rifle drilled to reduce the weight. Larger pinion bearing also for the case. Most posi carriers in todays cars will not hold up well under much abuse. I have had to repair many 02 "F" body cars and down posi rearends with the worn gear setups. They also called it a govenor posi that worked at low speed, would lock and unlock. Little gear breaks about 45% of the time from the on and off switching. These guys added the normal things, air induction system, headers with high flow exhaust, programmers, and slicker tires. This in real numbers is about 30-40 hp and that's all she wrote. Z06 vettes are breaking the posi's in them if the guys add the F1 tires and chips to the motors. Rick L.

[Cobra #3170]

For what it is worth I have tried a Detroit Soft Lock, a Quaife, and a clutch differential in a Dana 44. This is on an independent rear so may not be applicable to your 9", the Detroit was undrivable it would lock and unlock very unpredictably and really screwed up handling, Straight line performance was great however. Under hard cornering the Detroit unit and the Quaife would both spin the inside tire (lightly loaded ) and really slow corner exit. The clutch type diff worked the best in my opinion. Ditto on the axles I have 19 spline and twisted one off with the Detroit soft loc. I am going to try to have some 31 spline stub axles made for this diff (Jag).

[Wbulk]

The car will be a 302, about 270 hp, T-5, 9" rear end with 3.5 gears and 31 spline axles. Pretty mild ! After reading Jag's comments I am thinking maybe I sould play it safe and go with a clutch type posi. It's cheaper, traditional and has a long track record.

Wayne

[RICK LAKE]

Wbulk Wayne sounds good. Go easy on the T-5 trans, they don't take alot of abuse. This is a sound drivetrain for NORMAL driving. If you get bitten by the MORE POWER BUG, you will already have a good rearend to start with. The gear ratio may change depending on what the ratios of the trans are. Alot of guys have started with 3.50 gears and gone to either 3.70-4.10 gears. With the overdrive transmission. What is 5th gear ratio in your T-5 trans? You might want to start with a 3.90 ratio. The problem is that the motor is not a big one or have a ton of torque. Torque is what moves your car not HP. You want your car to cruise easy in the power band of the motor which is about 2,250rpms. If you cruise at too low of an rpm the motor lugs and burns more gas. It also will not pass as well and you will be downshifting more than you want. Have to also remember that you are driving a barn door down the road, not a bullet. The CD of drag is not the greatest with cobras. Rick Lake Ps I run 3.31 gears with a 1-1 five speed, a ton of torque at 482 cubes. This is a nice all around setup for a cobra. First gear is too quick with a 3.15 ratio for the trans in first. Going to a 2.80. Rick L.

[RICK LAKE]

Cobra #3107 If you are staying with a Jag rearend, the up grade is to 30 spline from 27. This is with a power locker rearend. CWI sells this upgrade for the Jag rears. I bought the stub axle shafts and rebuilt my own rearend. Mark Williams also sell stub shafts if you tell him what you want. Rumor has it that Williams is the one maken and selling them to CWI after there was a problem with the ones from down under. The stub axles where twisting. Williams builds the 1000hp front shafts for the frontend racers. Complete with his new joints that are better than CV designed ones. The stubs are good for 600hp I am still running the same ones after 11 years of racing. The other thing I will recommend is a rearend support cover. This puts pressure on the carrier caps to stop any kind of walking. They are about $140.00 from LWP. You will need to drill and tap a hole for the breather that was in the old cover. If you don't the pressure from the fluid getting hot will leak or blow out a seal. If you are having a problem with the car going around turns and not freeing up 1 wheel, there is something wrong with the posi unit. The clearances may be too tight in the carrier. The locker is a little clunkey but have not had any hang ups. What weight and fluid are you using in the rearend? Full Synthesis or semi? With the locker I run a full with a lighter weight. With the clutches I run an 80/90 with 2 bottles of additive and run them a little tighter without binding the clutches to engage. I am going to a cooler for the trans and rearend this years to bring down the temps of these to parts. Rick L.

[Wbulk]

Rick, I have have not bought the gears yet but was thinking of the 3.50s, not based on overdrive fifth gear that is .68 to 1, but on 4th, that is 1-1.

With 3.5 at 60 mph 4th is 2713 rpms and 5th is 1844 rpms.
With 3.73 at 60mph 4th is 2892 rpms and 5th is 1966 rpms.
I believe that's right based on the calculation program I used.

I wasn't considering the overdrive factor as the basis for the math. I felt dropping it down to 4th was fine when the power was needed. Looking at the numbers again I may go with the 3.73s.

Wayne

[olddog]

Originally Posted by Wbulk

Rick, I have have not bought the gears yet but was thinking of the 3.50s, not based on overdrive fifth gear that is .68 to 1, but on 4th, that is 1-1.

With 3.5 at 60 mph 4th is 2713 rpms and 5th is 1844 rpms.
With 3.73 at 60mph 4th is 2892 rpms and 5th is 1966 rpms.
I believe that's right based on the calculation program I used.

I wasn't considering the overdrive factor as the basis for the math. I felt dropping it down to 4th was fine when the power was needed. Looking at the numbers again I may go with the 3.73s.

Wayne

Wayne it depends on the cam your going to run. I have a 1:1 4th and 0.63 5th with 3.5 rear ratio. My engine doesn't smooth out until about 1800 rpm (cam is 236/242 duration Think 108 SLA). It's a 5.0 stroked to 347 cid. The pipes are too loud at 2600 rpm and it lugs too bad in 5th. Honestly this combo sucks in my opinion. I will eventually change it to a 0.8'ish ratio for 5th. A milder cam would help, but the huge step in 5th I do not like. If you get the chance, go with a higher ratio 5th gear. You won't be sorry.

With a mild engine I would go lower in the rear - 3.73 would be the highest I would consider.

[Cobra #3170]

Originally Posted by RICK LAKE

Cobra #3107 If you are staying with a Jag rearend, the up grade is to 30 spline from 27. This is with a power locker rearend. CWI sells this upgrade for the Jag rears. I bought the stub axle shafts and rebuilt my own rearend. Mark Williams also sell stub shafts if you tell him what you want. Rumor has it that Williams is the one maken and selling them to CWI after there was a problem with the ones from down under. The stub axles where twisting. Williams builds the 1000hp front shafts for the frontend racers. Complete with his new joints that are better than CV designed ones. The stubs are good for 600hp I am still running the same ones after 11 years of racing. The other thing I will recommend is a rearend support cover. This puts pressure on the carrier caps to stop any kind of walking. They are about $140.00 from LWP. You will need to drill and tap a hole for the breather that was in the old cover. If you don't the pressure from the fluid getting hot will leak or blow out a seal. If you are having a problem with the car going around turns and not freeing up 1 wheel, there is something wrong with the posi unit. The clearances may be too tight in the carrier. The locker is a little clunkey but have not had any hang ups. What weight and fluid are you using in the rearend? Full Synthesis or semi? With the locker I run a full with a lighter weight. With the clutches I run an 80/90 with 2 bottles of additive and run them a little tighter without binding the clutches to engage. I am going to a cooler for the trans and rearend this years to bring down the temps of these to parts. Rick L.

Rick, Thanks for the info, I am very happy with my stock locker but would like to upgrade the stub axles and spiders to 30 spline. Does Mark Williams make them for a Jag diff with a 4 bolt flange so I can use my stock Half shafts? I put Corvette drag race joints in them (no zerks) and they have held up well. How do I get hold of him, do you have a number. The Detroit soft loc was just to rough in lock and unloc it upset the cars balance every time it cycled so I took it out. I use conventional 80/90 gear oil with GM friction modifier in my clutch unit and it works fine. The Quaife unit spins the inside tire instead of driving off a corner like it should. Other people have had the same problem. I have been told that the original design did not do this but they are no longer available. Getting the axles seals in the housing on a Jag is a real nightmare but I guess I can do it again if I get 30 spline shafts. I believe the original rear in the car was a 10 spline stub unit but it is so rare I do not use it anymore. The original race cars also ran an oil cooler so I think that is a good move on your part.

[YerDugliness]

I just bought a roadster replica with a 5.0 HO (converted from EFI to Edelbrock Performer and Thunder Series carb) and the WC T-5 (0.68 OD). Mine has the Ford 9" with some sort of limited slip (reported to be 3.70 ratio), nobody seems to know which one as the car has changed hands many times since it was built.

I have no idea of horsepower but I do know that it cruises at 70 MPH at about 2100 RPMs. That is at the bottom of the power band but with the light weight of the car it does accellerate well when passing without downshifting to 4th. The car has 295-60/15's on it, I'm thinking about replacing them with 295-50/15's just to get the cruising RPM closer to that 2250 RPM target.

As to the OP's question, every now and then I do get a bit of chatter from the differential.....it was unnerving at first but now that I've gotten use to it, it bothers me less. I thought something was caught up in between the pinion gear and the ring gear the first time I experienced it. So far neither of my rear wheels needs to "slide" or "spin" to get around a corner, so it must not be a true locker.....

Dugly

[Wbulk]

We'll in the end I went with the 3.50s and the Truetrac for my nine inch. I have used the Truetrac before with good results. I really went back and forth between the 3.50s and 3.70s. I think the thing that swayed me to the 3.50s was a survey I read with owners that had 302s. The vast majority had 3.54s in their 8.8s. Additionally some who had 3.73s with the T-5 complained that you might as well start out in second gear. In the end it's all a trade off between the high and low speed driving. I also called FFR and they recommend the 3.54s for their customers with stock or slightly modified engines. They are selling the 3.73s in their complete kit only because of availablity of the 3.54s for the new axles.
Anyway if I don't like the 3.50s I will change them out. Setting them up is what's fun for me.