8 inch rearend???
- Thread starter Woodie
- Start date
1967 and later webbed castings and 4-pinion spider gears help a lot for the 8" since the pinion is better supported and the gears don't "ride up" the pinion under a lot of power. Properly built, an 8" can take quite a bit of abuse, but not as much as a similarly built 9". Some are running well over 400HP on the street with an 8", so they can handle a fair amount of power. A 9", although much stronger, is heavier and soaks up more power due to the larger ring and pinion. This means that you have a lot more unsprung weight and will handle a bit worse than with a lighter 8". If you can build an 8", go for it! 
Daniel
Daniel
Rear ends a somewhat like transmissions. The how much HP or torque can it take argument is a little bit silly in one sense. I have seen people with the same equipment brake and not brake stuff. I've seen people with stock WC T5s not break with 400HP motors and I've seen Tremec 3650 that broke from 400HP motors. The same is true for the rear end. A lot if it has to do with the driver. If you drive the crap out of your car all the time and push it to the limit you are going to break stuff. If you are easy on your car its going to last a lot longer and you provably wont bust yer rear end.
There does of course comes a point where the reliability of the unit becomes an issue due to it being over powered by the driveline. How much power are you making (HP/Torque) and how hard do you drive ?
There does of course comes a point where the reliability of the unit becomes an issue due to it being over powered by the driveline. How much power are you making (HP/Torque) and how hard do you drive ?
The 8" can be built to take lots of abuse. BornInAFord was right about the webbed casings and 4 pinion spiders being stronger than the early non webbed casings and stock 2 pinion spiders. For even more strength go with a Detroit Locker.
Ford 8" rears come with 28 spline axles as do many 9" rears. The 9" can be easily upgraded to 31 or even 33 spline for more strength. The 9" rear is larger and therefore heavier but the weight is in the right place on early Stangs (the back).
I would suggest a 9" on a heavier car (3400 lbs or more). A big block should have a 9" because of the amount of torque it produces. A stroker 351 should use a 9" as well for the same reason. A hard launching stick car should definitely be using a 9" rear. Stick cars are MUCH harder on drivetrain components than autos.
Lighter cars and small block cars with automatic transmissions are good candidates for the 8" rear. My car included. Mine is a 66 Coupe with a 347 stroker that weighs less than 3000 lbs with me in it. It has 3.55 gears and a
C4 auto. It uses stock 28 spline axles and a Detroit locker. I also studded the pinion support for greater reliability.
Just remove the 5 (I think) stock bolts around the pinion support and replace with hardened studs and hardened washers and use nyloc nuts torqued to spec. This will ensure the pinion does not have excess movement away from the ring gear on hard launches.
I race with 235/60 15 drag radials and ran a best of 11.96 @ 112 in the quarter last week. So each rear has an application.
So you have to ask yourself what is the weight of the car is it going in? What is the hp and torque you will be making? Do you plan on drag racing it with sticky tires or slicks? Auto or manual? High (2.79-3.55) or low gearing (3.73-4.56)?
Ford 8" rears come with 28 spline axles as do many 9" rears. The 9" can be easily upgraded to 31 or even 33 spline for more strength. The 9" rear is larger and therefore heavier but the weight is in the right place on early Stangs (the back).
I would suggest a 9" on a heavier car (3400 lbs or more). A big block should have a 9" because of the amount of torque it produces. A stroker 351 should use a 9" as well for the same reason. A hard launching stick car should definitely be using a 9" rear. Stick cars are MUCH harder on drivetrain components than autos.
Lighter cars and small block cars with automatic transmissions are good candidates for the 8" rear. My car included. Mine is a 66 Coupe with a 347 stroker that weighs less than 3000 lbs with me in it. It has 3.55 gears and a
C4 auto. It uses stock 28 spline axles and a Detroit locker. I also studded the pinion support for greater reliability.
Just remove the 5 (I think) stock bolts around the pinion support and replace with hardened studs and hardened washers and use nyloc nuts torqued to spec. This will ensure the pinion does not have excess movement away from the ring gear on hard launches.
I race with 235/60 15 drag radials and ran a best of 11.96 @ 112 in the quarter last week. So each rear has an application.
So you have to ask yourself what is the weight of the car is it going in? What is the hp and torque you will be making? Do you plan on drag racing it with sticky tires or slicks? Auto or manual? High (2.79-3.55) or low gearing (3.73-4.56)?
A lot of good advice here. I would suggest you talk to the folks at company's like Currie Rear Ends as far as what is available. When you say "built" their is a lot of leway in that term. Like the saying "speed costs money, how fast do you want to go" the same applies to building your rear end. We are currently building our center section using a Ford casting, which incorporates additional webbing for support and possibly more important, additional support in the area of the pinion support area of the casting. We are using 3.55 gears with Eaton's Tru-Trac carrier and 28 spline shafts. the engine is a 347 stroker with an AOD transmission.
brianj5600
Active Member
I would not even consider an 8", I don't care who builds it. An 8.8 or a 9" will be the ticket. You will still need 31spline axles.
geostang351
Member
Woodie said:
If it is in a I6 car, the rear end is a 7 1/4" and in no way cabable to withstand what you have instore for this car.
In my advice above, I made the assumption you already had an 8". If you are buying, go with a 9" as it is usually comparable in price (for the housing), and withstands most anything you will throw at it. Something has to give with as much power as you are hoping to make--in the driveline, the transmission, u-joints, drive shaft, pumpkin, axles and tires (well, and to some extent the springs) are the limiters. Since you are going with a Tremec and likely will have "sticky" tires, the 8" will break.
Free advice: make sure in your build you budget $$$ for chassis work, too. Your springs will need to be at least 4 1/2 leafs or wheel hop will grenade your gears (unless you have rear support like under-riders or watts link or panhard). You might consider stiffening your unibody with export brace + monte carlo bar and a four to 6 point roll cage or at least sub-frame connectors as the unibody isn't designed to support that much power and will crack in all sorts of places. Also, make sure your brakes, steering, and alignment are able to handle that type of abuse.
Have fun! 
Daniel
Free advice: make sure in your build you budget $$$ for chassis work, too. Your springs will need to be at least 4 1/2 leafs or wheel hop will grenade your gears (unless you have rear support like under-riders or watts link or panhard). You might consider stiffening your unibody with export brace + monte carlo bar and a four to 6 point roll cage or at least sub-frame connectors as the unibody isn't designed to support that much power and will crack in all sorts of places. Also, make sure your brakes, steering, and alignment are able to handle that type of abuse.
Have fun! Daniel
ultrastang
Founding Member
There's all sorts of assumptions about what is and what is not, in regards to the differences between Ford 8- & 9-inch rear ends, but the primary assumption is that the 9-inch rear is drastically heavier than an 8-inch. In reality, a '57-'59 Ford small bearing drum brake 9-inch rear assembly is only 24 lbs. heavier than a comporably equipped '65/'66 Mustang 8-inch rear.
That equates to only 12 lbs. per side heavier, which to me, isn't what I would consider a significant margin. If the rear end was 50 or 75 lbs., over all heavier, then that to me would be a significant amount.
I don't know of any Ford-produced 8-inch differentials that came with 4-pinions (spider gears). There are aftermarket 8-inch differentials (such as from Curries) that have 4-pinion setups, but I've never seen any 8-inch diffs from the factory with them installed.
The 9-inch, on the other hand, could have either 2- or 4-pinion setups.
Here are some rear end/rear end parts weight comparrisons:
FORD REAR END WEIGHT COMPARISON 8
8-inch Rear End Assembly Weight : 170 lbs.
1. Housing: 36 lbs.
2. Axles: Medium Journal, 28-spline -15 lbs. ea.
3. Brakes: Small 10" x 1¾" Drum Assemblies - 22 lbs. ea.
4. 3rd Member: Reinforced Revision 8-inch Case - 60 lbs.
9-inch Rear End Assembly Weight: 194 lbs.
1. Housing: 38 lbs.
2. Axles: Large Journal, 28-spline - 18 Lbs. ea.
3. Brakes: Massive 11-1/32 x 2½" Drum Assemblies - 26 lbs. ea.
4. 3rd Member: Conventional Diff. - 68 lbs.
a. w/Traction-Lok Diff. - 71 lbs.
(24 lbs. Difference Between Drum Brake 8" & Drum Brake 9" Rear End Assembly).
Versailles Disc Brake Rear End Assembly Weight: 215 lbs.
1. Housing: 39 lbs.
2. Caliper: w/Pads, Anchor Plate & Flexible Brake Hose - 24 lbs. ea.
3. Axles: 20 lbs. ea.
4. Rotors: 10 lbs. ea.
5. 3rd member: Conventional Diff. - 68 lbs.
(45 lbs. Difference Between Drum Brake 8" & Disc Brake 9" Versailles).
Wheel/Tire Comparison:
1. Stock '66 Mustang GT Styled Steel Wheel w/ P195/70R-14 Tire - 46 lbs. ea.
2. Weld Draglite Forged Aluminum Wheel (15" x 8") w/ P245/60-15 Tire - 38 lbs. ea.
Miscellaneous:
1. 8" Drum Brake Assembly: (10" x 1¾" Drums): 22 lbs. ea.
2. 9" Drum Brake Assembly (11-1/32" x 2½" Drums): 26 lbs. ea.
3. Lincoln Versailles Disc Brake Assembly: (10½" Rotors) 30 lbs. ea.
4. Lincoln Mk VII Rear Disc Brake Rotor/Caliper Assembly Weight: (11¼" Rotors) 22 lbs.ea.
5. Ford Explorer Rear Disc Brake Rotor/Caliper Assembly Weight: (11¼" Rotors) 25 lbs. ea.
www.ultrastang.com
That equates to only 12 lbs. per side heavier, which to me, isn't what I would consider a significant margin. If the rear end was 50 or 75 lbs., over all heavier, then that to me would be a significant amount.
I don't know of any Ford-produced 8-inch differentials that came with 4-pinions (spider gears). There are aftermarket 8-inch differentials (such as from Curries) that have 4-pinion setups, but I've never seen any 8-inch diffs from the factory with them installed.
The 9-inch, on the other hand, could have either 2- or 4-pinion setups.
Here are some rear end/rear end parts weight comparrisons:
FORD REAR END WEIGHT COMPARISON 8
8-inch Rear End Assembly Weight : 170 lbs.
1. Housing: 36 lbs.
2. Axles: Medium Journal, 28-spline -15 lbs. ea.
3. Brakes: Small 10" x 1¾" Drum Assemblies - 22 lbs. ea.
4. 3rd Member: Reinforced Revision 8-inch Case - 60 lbs.
9-inch Rear End Assembly Weight: 194 lbs.
1. Housing: 38 lbs.
2. Axles: Large Journal, 28-spline - 18 Lbs. ea.
3. Brakes: Massive 11-1/32 x 2½" Drum Assemblies - 26 lbs. ea.
4. 3rd Member: Conventional Diff. - 68 lbs.
a. w/Traction-Lok Diff. - 71 lbs.
(24 lbs. Difference Between Drum Brake 8" & Drum Brake 9" Rear End Assembly).
Versailles Disc Brake Rear End Assembly Weight: 215 lbs.
1. Housing: 39 lbs.
2. Caliper: w/Pads, Anchor Plate & Flexible Brake Hose - 24 lbs. ea.
3. Axles: 20 lbs. ea.
4. Rotors: 10 lbs. ea.
5. 3rd member: Conventional Diff. - 68 lbs.
(45 lbs. Difference Between Drum Brake 8" & Disc Brake 9" Versailles).
Wheel/Tire Comparison:
1. Stock '66 Mustang GT Styled Steel Wheel w/ P195/70R-14 Tire - 46 lbs. ea.
2. Weld Draglite Forged Aluminum Wheel (15" x 8") w/ P245/60-15 Tire - 38 lbs. ea.
Miscellaneous:
1. 8" Drum Brake Assembly: (10" x 1¾" Drums): 22 lbs. ea.
2. 9" Drum Brake Assembly (11-1/32" x 2½" Drums): 26 lbs. ea.
3. Lincoln Versailles Disc Brake Assembly: (10½" Rotors) 30 lbs. ea.
4. Lincoln Mk VII Rear Disc Brake Rotor/Caliper Assembly Weight: (11¼" Rotors) 22 lbs.ea.
5. Ford Explorer Rear Disc Brake Rotor/Caliper Assembly Weight: (11¼" Rotors) 25 lbs. ea.
www.ultrastang.com
iskwezm
10 Year Member
mine has lasted so far and i would assume its AT LEAST 450+ RWHP, but i havent launched on slicks yet either.tick tick tick tick................could be a timebomb
blackjackbender said:
Yes. Here is an article about a set of aftermarket axles being tested in an 11 second 5 speed '67 Mustang.
http://www.fordmuscle.com/archives/2003/03/project67/index.shtml
stangkid88
New Member
http://cgi.ebay.com/ebaymotors/Ford-8-Eight-Inch-Rear-Trac-Loc-Posi-Stronger-than-9_W0QQcmdZViewItemQQcategoryZ33731QQihZ013QQitemZ230035179063QQrdZ1QQsspagenameZWDVW
This seller on eBay garuantees his built 8 inch rears to 550hp. He seems very knowledgable and backs up his work. The units go for about $750, but with the engine you're planning, I think you made a good decision to spring for a 9"
This seller on eBay garuantees his built 8 inch rears to 550hp. He seems very knowledgable and backs up his work. The units go for about $750, but with the engine you're planning, I think you made a good decision to spring for a 9"
brianj5600
Active Member
Stock cases will not last. I broke the caps on an 8" case with a 306 with 351w heads and 175hp of nos. It lasted about 10' at the track. Don't throw money at an 8".
My falcon went an 8.03 @ 86.42 mph in 90 degree heat going through @ 5000 in 2nd gear.
I got tired of the "questionable" posi floating around on EBAY so I put in a $50 mosier mini spool and I would never go back to a posi in that car ever.
But like everyone has already stated: 8" are good for autos, light cars and under 400-450fwhp.
I got tired of the "questionable" posi floating around on EBAY so I put in a $50 mosier mini spool and I would never go back to a posi in that car ever.
But like everyone has already stated: 8" are good for autos, light cars and under 400-450fwhp.
It all depends on what you plan to use the car for. I've seen some high power cars with 8" rear ends that have held up just fine. Those guys don't drag race their car,have never and never will use slicks either. If you ever plan on racing with some sticky tire get a 9" rear end and do it right. It's always cheaper do it once 
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