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Educate me 7.5 guys


Daniel Black

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On my '87 302 swap I still use a 7.5 diff. It has 3.73 gears and factory limited slip. The problem is the clutches have gone out so only a one tire burnout, even in the wet now. Could it just be the nearly 35 year old clutches or is the V8 the problem? The 302s been in for almost a year but the clutches have been going for about 6 months. Mustangs had 5.0s and 7.5 diffs, did they have LS problems? I'm just trying to decide if it's worth putting clutches in. The rear is in great shape with new seals, bearings and a fresh fluid change with additive in the past year. I'd really like to keep it since I know it's condition. The elusive '90-92 8.8s are really hard to find and when I find one it's 10 states away. Lockers for the 7.5 are outrageous so that's out of the question and my rear tires already hang past the fenders so I'd have to find new wheels to use the newer 8.8.
 


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So put clutches in it. Those things are only good about 100K anyway.
 

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Yeah. Rebuild the clutch pack. You said it yourself. It's 35 years old. They're clutches. They wear out.
 

Daniel Black

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Excellent! I just didn't want to spend the money if they were gonna go right back out again. Thanks.
 

Daniel Black

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Well, new clutches and the same problem. One tire in 1st gear but both tires spin from 2nd on. Is that just how limited slip works? Maybe that's where the "slip" part comes from?
 

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The main advantage of a limited-slip differential is demonstrated by considering the case of a standard (or "open") differential in off-roading or snow situations where one wheel begins to slip. In such a case with a standard differential, the slipping or non-contacting wheel will receive the majority of the power (in the form of low-torque, high rpm rotation), while the contacting wheel will remain stationary with respect to the ground. The torque transmitted by an open differential will always be equal at both wheels; if one tire is on a slippery surface, the supplied torque will easily overcome the available traction at a very low number. For example, the right tire might begin to spin as soon as 70 N⋅m (50 lb⋅ft) of torque is placed on it, since it is on an icy surface. Since the same amount of torque is always felt at both wheels, regardless of the speed at which they are turning, this means that the wheel with traction cannot receive more than 70 Nm of torque either, which is far less than is required to move the vehicle. Meanwhile, the tire on the slippery surface will simply spin, absorbing all of the actual power output (which is a function of torque provided over time), even though both wheels are provided the same (very low) amount of torque. In this situation, a limited-slip differential prevents excessive power from being allocated to one wheel, and so keeps both wheels in powered rotation, ensuring that the traction will not be limited to the wheel which can handle the minimum amount of power.
 

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Ranger 8.8 is $60 at any UPAP yard. Will handle much more torque than your 7.5.

Only way you'll get an actual lock on both wheels is a locking diff of some kind.

Drop in locker has the street manners of spoons in a garbage disposal. I daily drive one. Takes a lot of getting used to, and your rear tires will HATE you. But, I havent needed 4wd since installing it.

Detroit locker is the strongest locking option out there, but $$$

Auburn cup and cone is awesome but has a lifespan.

Look into an Eaton truetrac or torsen torque biasing. Same thing in theory, just different approaches. At some point my LockRite will be getting pulled in favor of the Eaton (I wanted the torsen, but they refuse to answer their phones or email).
 

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When you replaced the clutches did you check the setup or just throw new clutches in there? Mustangs used the 7.5 behind 302's through the 85 model year when they made 225 hp, if you have a lot of power and/or sticky tires I'd consider an upgrade. I have an Auburn Pro in my Mustang and would recommend a clutch type limited slip or a torsen instead. The Auburn sheds fine metal particles in the oil so the fluid needs to be changed frequently and it destroys the bearings even then. I can put one tire on dirt, the other on pavement and spin both but it's treacherous in the rain.
 

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8.8 will do you good.
 

Daniel Black

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When you replaced the clutches did you check the setup or just throw new clutches in there? Mustangs used the 7.5 behind 302's through the 85 model year when they made 225 hp, if you have a lot of power and/or sticky tires I'd consider an upgrade. I have an Auburn Pro in my Mustang and would recommend a clutch type limited slip or a torsen instead. The Auburn sheds fine metal particles in the oil so the fluid needs to be changed frequently and it destroys the bearings even then. I can put one tire on dirt, the other on pavement and spin both but it's treacherous in the rain.
I honestly didn't know there was anything to check. I soaked the new ones and put them in the same order the old ones came out. I'd read something about adding one of old clutch plates to each side but it just didn't seem like it was going to fit back together. It's highly unlikely I have 225hp but the rear started going down hill after I put 275/60s on the back. Rangers are so light in the rear I wouldn't think it would be an issue. Maybe it's just a coincidence with my transmission gearing and the rear gear ratio that makes it slip in 1st? Even in turning hard left it'll only spin the left back tire. Even an open diff will usually spin both turning left.
 

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I honestly didn't know there was anything to check. I soaked the new ones and put them in the same order the old ones came out. I'd read something about adding one of old clutch plates to each side but it just didn't seem like it was going to fit back together. It's highly unlikely I have 225hp but the rear started going down hill after I put 275/60s on the back. Rangers are so light in the rear I wouldn't think it would be an issue. Maybe it's just a coincidence with my transmission gearing and the rear gear ratio that makes it slip in 1st? Even in turning hard left it'll only spin the left back tire. Even an open diff will usually spin both turning left.
sounds like a too much power for the available traction.
since they are so light its real easy for the driveshaft torque to lift the pass rear wheel.
for a quickie test try adding 300-400 lbs over the rear axle, play sand works great then give it to kids afterwards.


(it's a slow day, I just felt the need to post something)
 

19Walt93

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I honestly didn't know there was anything to check. I soaked the new ones and put them in the same order the old ones came out. I'd read something about adding one of old clutch plates to each side but it just didn't seem like it was going to fit back together. It's highly unlikely I have 225hp but the rear started going down hill after I put 275/60s on the back. Rangers are so light in the rear I wouldn't think it would be an issue. Maybe it's just a coincidence with my transmission gearing and the rear gear ratio that makes it slip in 1st? Even in turning hard left it'll only spin the left back tire. Even an open diff will usually spin both turning left.
I can probably look it up or you can google the procedure, the clearance needs to be measured to determine the right selective shim. Too loose equals open rear end.
 

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I would rearrange the pack and add more if there's room... From the factory they stacked like clutches (friction to friction and steel to steel), stagger them and put the right parts in the right place (IE don't put a friction against the carrier, not a moving surface) to get the maximum contact area.

Stick as much in that will fit which is probably one more plate per side and change the order and it will probably act better... I've had good luck with the Expo 8.8's I've done that to, my '00 Explorer is very predictable in it's behavior, works great for a $0 fix (I had the parts, just restacked junkyard stuff)
 

Daniel Black

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I can probably look it up or you can google the procedure, the clearance needs to be measured to determine the right selective shim. Too loose equals open rear end.
I should've thought of that. I've run into wear on an open 8.8 diff in my F150 where the cups had welded to the spider gears and ate into the carrier causing a ton of slack.
 

Daniel Black

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I
I would rearrange the pack and add more if there's room... From the factory they stacked like clutches (friction to friction and steel to steel), stagger them and put the right parts in the right place (IE don't put a friction against the carrier, not a moving surface) to get the maximum contact area.

Stick as much in that will fit which is probably one more plate per side and change the order and it will probably act better... I've had good luck with the Expo 8.8's I've done that to, my '00 Explorer is very predictable in it's behavior, works great for a $0 fix (I had the parts, just restacked junkyard stuff)
I saw lots of different ways to stack the clutches on Google but I chickened out and just put them back the way they came.
 


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