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beef up stock 8 inch diff?

8K views 52 replies 28 participants last post by  cmefly 
#1 · (Edited)
I have a 400whp/440tq(just Dynod) 347 in a 65 convertible mustang with a stock t5. I currently have the stock open diff(with some steeper gears my dad put in for towing the boat, I think 3.31?). I was thinking about putting this in:
https://www.currieenterprises.com/ce-5018
Anything else I should do? (besides look for a 9 inch which might not be needed for this hp/tq)
 
#15 ·
Has anyone bought from Alan since he took over for TJ? I don’t have experience from either but was planning on buying from TJ based on feedback here. I saw following the link today that TJ has passed away, and Alan has taken over the operation (per the eBay page).
 
#4 ·
I've read at-length articles that the 8" can be just as sturdy as a 9" unless you just had to have big-journal bearings and 31 spline, maybe with a spool for drag etc. I have pounded the $h!+ out of some 8s and never broke one, but I've broken a couple of u-joints and a trans mount.
 
#5 ·
Which T-5 do you have, the stock GT Mustang one or the Cobra T5z spec'd one? The first gear ratio for a stock GT Mustang sucks as it is way too low. Regardless with a single leg rear end traction will be an issue. I was fortunate to find an 8" Traction Lok in a bone yard years ago. It has held up to plenty of abuse. I have heard good things about the new style Detroit locker that you posted. I see Traction Lok 8" units are available for a little less. https://www.quickperformance.com/Ford-8-Inch-Trac-Lok-Posi-Unit_p_16014.html
 
#9 ·
Yup. That T-5's a goner with slicks...... or DOT "slicks"
(high HP, slicks and missed shifts are the kryptonite for that trans)
Never broken an 8" diff. I've broken leaf spring segments, u-joints, engine mounts, etc. though.
 
#10 ·
I've got a T5 behind my 408 stroker, typically flog the dogsh*t out of it, and it's held up just fine. As for your rear end, look into the Explorer swap. It's an easy was to swap in not only a double legger, but better gears AND disc brakes. Best part is, they're fairly cheap to obtain!
 
#13 ·
Interesting, what tires are you running on the rear?
 
#24 ·
Not trying to he a downer but there is no way a 347 is making 440ftlbs of torque at the wheels on pump gas. Your tuner needs to calibrate his dyno unless it is somehow calculating flywheel HP because at 15% loss that would mean your 347 is making over 518ftlbs at the flywheel to read 440 at wheels. A 408W will make 500+ftlbs but not 347. Either way an 8” and aggressive driving with sticky tires will not last behind a 347.
 
#26 ·
Unless you are running slicks (or really sticky drag radials) there are lots of benefits to the 8" rear. It's lighter so you have less unsprung weight, it steals about 4% less power than the 9", and there are tons of gears available for them. They're really pretty good! I've had both in my Cougars. To be honest, unless you could drive an 8" and 9" side by side, you probably wouldn't know the difference. I've never had a problem with my 8, and I have run it pretty hard.

If you're running nice grippy tires, and you are putting big-block torque through one with a manual transmission, an 8" is probably not the greatest choice. But for a street driven car, your limit will be the tires, not the diff. Any 'extra' power that might hurt it will just get let out in the form of smoke. If that's not okay for your application, then I'd go with a 9". An 8.8 is good if you have some welding skills; it's sort of in-between the two in every way. Stronger than an 8", but not as tough as a 9". Keep in mind, you will need C-clip eliminators if you want to run it on most tracks with an 8.8. Safety inspectors frown on cars that could spontaneously fling a rear axle and wheel into the grandstands.
 
#27 ·
Safety inspectors frown on cars that could spontaneously fling a rear axle and wheel into the grandstands.
I know this is true, but it is funny. Have you ever had an axle bearing go on you at speed? I have, twice. Both with a 9 inch in a 69 Ranchero. Freeway speeds around corners. First time, I saw the tire/axle bouncing along passing me while I was trying to get off the freeway. Second time, I knew the sounds and managed to pull off and stop before it went flying.

On the 8 and 9 inch rears, that bearing and the retaining plate are the only thing keeping the axle in place.

Russ
 
#30 ·
I have to second (or third or fourth) the notion that the 8 inch will grenade if you hook up at least if you use the early 65-66 carrier. I grenaded mine with a 306 with only about 375 hp to the flywheel on street tires at the drags. I did have fairly large (255/60/15) tires and homemade Caltrac type bars. I dead hooked and shattered the carrier and ruined a detroit locker. Everyone who talks about the strength of the 8 seem to be talking about the later one with the upgraded carrier or are using an automatic. My 2 cents is that if you have a 65-66 it is not worth upgrading 8 inch. Sure you could find a later carrier but by the time you add a posi and gears you have money spent that could go towards a 9 inch. You also still have 28 spline axles which are not that strong (Broke one of those too).

If you can weld the best option would be an 8.8 out of an explorer. Otherwise pony up and have a 9 inch made which is what I did.
 
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#32 · (Edited)
Dyno session was done at a reputable shop-EPC Racing here in Spokane. 440 torque was max at 4,000 rpm and 408hp was max at 6,000rpm. I guess the dyno could have been wrong but I tend to doubt it. He even said people think he has a stingy dyno! it is a new engine and the first session so no way to know for sure yet. Some specs are:AFR 185 heads, custom Bullet Racing cam(.588/.585, 230/235, 109) quickfuel 750 race carb, Air Gap. Doug’s tri y headers. Magnaflow 2.5in exhaust. I think I will go with a third member assembled in a later case with TruTrac by Alan off eBay. I contacted him and he says it should work good my setup. Also I just picked up a Z spec T5. I can’t decide between 3.8 and 4.1 for gears.
 
#33 ·
You should replace the axles with aftermarket ones. Sticky tires and stock axles are a bad combo. By the time you replace the eight inch third member and axles for just a little more money you would have a nine inch rear end and 31 spline axles. I would not use an eight inch rear at that power level with sticky tires. Do it once and do it right.
 
#36 ·
I won’t be running slicks. Currently I have MT E.T. Street SS radials-235/60r15 and get some spin when hitting it hard in 1st. They are WAY better than my BFG 215’s I had on there before. Do you still think I need aftermarket axels? If so what kind?
Yes, you need aftermarket axles. The stock axles will twist or break. The ford 8 inch and 9 inch small bearing 28 spline axles are interchangeable.
https://www.moserengineering.com/1964-66-mustang-ds-and-ps-spline-28.item
 
#37 · (Edited)
#39 ·
My take on this? If you're going to 1) add a posi 2) change gears 3) put beefier axles in.. You might as well just go to a 9in. You're gonna come out close in cost and then not worry about scattering all your new parts all over the ground when it does break.

I'm one to talk.. I put 3.55's a +67 centre and trak lok in my 8.. then had to replace axles. Now I'm fixing a bent housing. Lesson learned!
 
#41 · (Edited)
The later style 8" third member well built is usually good up to about 450HP. The stock axles are not. Also all 8" third members that I have seen use the small 1310 u joint and if you go that route make sure to use the solid style. If you upgrade to the 9" you can use the stronger 1330 or 1350 u joint on the third member. I have the small bearing 9" housing in all my cars with no issues and i use a solid 1330 u joint up front with the big solid 1350 on the rear. You can use the stock 8" rear end backing plates/brakes with this setup.
 
#42 · (Edited)
I am apprehensive to go that route because I don't like the fact that the 9 inch weighs more and robs power- I would be totally in without a doubt if I was making 500+hp, but I plan on keeping this beast n/a and looking as stock as possible. I am kind of pushing the limit of everything right now. I have a well built 347 with forged internals but in a stock blocK (all be it a Mexican block but I know that doesn't help much). I have a WC t5 now but just bought a T5z with I know is only a bit better for toughness.
 
#43 ·
I am apprehensive to go that route because I don't like the fact that the 9 inch weighs more and robs power- I would be totally in without a doubt if I was making 500+hp, but I plan on keeping this beast n/a and looking as stock as possible. I am kind of pushing the limit of everything right now. I have a well built 347 with forged internals but in a stock blocK (all be it a Mexican block but I know that doesn't help much). I have a WC t5 now but just bought a T5z with I know is only a bit better for toughness. It's a convertible with no roll bar so I will have to ease off at the track or figure something else out like maybe get one or something I don't know. The new tires (MT 235/60r15 street ET ss) have made a huge difference. The car hooks up now and I can feel the HP. It's fun.
Then buy the 8" third member, replace the axles with the Moser or call Quick performance to get a price on the house brand axles they carry and make sure to install Solid u joints in your drive shaft. I hope you are not using the stock drive shaft as it will not be safe in your car and will reach critical speed easily.you will need at a minimum a 3" Aluminium driveshaft with the solid 1310 u joints.
 
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