8 Inch or 9 Inch?

Help Support 7173Mustangs.com:

Galucha

Well-known member
Joined
Jul 2, 2015
Messages
179
Reaction score
90
Location
Chattanooga, TN
My Car
1971 Coupe 302
So I'm working my way back on the car's suspension. I just finished installing StreetorTrack's front coilover conversion system with their 1 1/8" sway bar, now I'm looking at doing the 3 link conversion they offer. Right now I have a pretty much stock 8 inch open rear end, and while SoT's 3 link kit works with an 8 inch, it involves welding some brackets to the housing. So if I leave the 8 inch in, I'm committing to keeping it on the car. 

My question is, are the weight savings and cost of upgrading the 8 inch worth it versus swapping in a 9 inch, or even an explorer 8.8? From what I've read there's somewhere between a 20-30lb unsprung weight difference with an 8 inch vs a 9. And since my goal with the car is to have a good handling track/street car, would that be an appreciable difference?

I've priced out what it would cost to make the 8 inch stronger/better, and it came out to be about $1,370. 

  • Axles: Dutchman Custom 4130 Steel Axle Pair: $480
  • Center Section: Quick Performance 3.90 Posi-Trac Center Section: $890

Would love to hear some thoughts on other folks' experience with the SoT 3 link system as well.

 
Joined
Jan 7, 2015
Messages
8,032
Reaction score
491
Location
Western North Carolina
My Car
Multiple Mustangs!
I myself do not like a spicer type 8.8 rear. The clunk clunk every time you go around a turn and just not as strong. They have never made a heavy truck with a spicer type rear. How many true race cars do you see with them. NONE. Tires are what breaks the rear. I do not think I have seen a post on here about anyone breaking a rear or axle. I broke mine in 73 under warranty 3 times. They put the wrong one in the car at the factory. After right one not again. There again I only drove 12,000 miles. I broke transmissions, clutches, drive shafts, rears, axles all the time with built engine in my hot rod. If you do not break something you are not running very good. |
Run the 8" with the stock stuff if you break it move on. Don't fix if not broke.
You can get 35 spline 9" you could not break with 1,500 hp. if you want over kill.

 
Joined
Jan 1, 2012
Messages
8,173
Reaction score
931
Location
Springfield, OR
My Car
1971 Mustang Sportroof M code
I had a '98 Mercury Mountaineer that had the 8.8" rear end. drove it on a lot of winding mountain roads, never hear a clunking noise, and that was when I could still hear fairly well, and if there would have been a clunk my wife would have let me know, she is still very sensitive to noises.

An 8.8" has 3.25" diameter axle tubes, a 9" has 3.0". 8.8" 31 spline axles are 1.29" diameter, 9" 28 spline are 1.20", 9" 31 spline are 1.33". The pinion shaft on 8.8" axles is bigger than on 9" axles.

A 31 spline 8.8" axle is stronger than a 28 spline 9" axle.

Shortcomings of an 8.8" axle are C clips, which means if an axle breaks the axle and wheel/tire will come out, C clip eliminators are available, not cheap. The other shortcoming is that that axle tubes are just plug welded to the center section, and should be welded for the full circumference for additional strength.

If I were building a drag racer I would definitely use a 9 inch rear end, a lot of aftermarket support and they can be made very strong. GM guys use them for the strength and because GM 10 and 12 bolt also also have C clips and with E.T.s below a certain point (maybe 11 seconds?) they can't be used, same as the 8.8" axles, unless they have the C clip eliminators.

I had a long wheelbase F100 that had a Spicer 60 rear axle and 4 different Jeeps with Spicers, never had a problem with them, and I'm not known for treating vehicles gently, especially pickups and 4x4s, meant to be used and enjoyed.

 
Joined
May 15, 2011
Messages
3,436
Reaction score
274
Location
PA
My Car
1971 Mach 1 Mustang
I agree Don. I have been running my 8.8 for over 10 years in my mustang with no issues and no clunking sounds at all.  When I had my 9 “ in my bronco it clunked a lot with the Detroit locker in it. But my 8.8 has a factory posi with a clutch set up that is quiet and works well. I definitely don’t baby my car all the time.  Take a look at my video in my signature.  That 8.8 holds up to 600 hp everyday. If I had big sticky tires I would probably start braking stuff if I was at the track. But in its current set up this car will blow the tires off even at 45mph if you stomp on it. You do need to weld the axle tubes which is easy. 8.8” rears came in f150s, Broncos, expeditions, mustangs, and rangers. Although the only ones to easily fit our cars are from the explorers.  If my car would of came with a 9” I probably would of stuck with it, but my car had an 8” so I went with the 8.8” You can buy an 8.8”  with 3.73 gears, posi, and disc brakes for less than a gear change in an 8”or 9”   It’s not for everyone but it is a very good cheap viable option. 

 
Last edited by a moderator:
Joined
Jan 1, 2012
Messages
8,173
Reaction score
931
Location
Springfield, OR
My Car
1971 Mustang Sportroof M code
I currently have a 9" with 3.73s and a Detroit locker, and yes it rattles some. I like it, but I know when I get it back on the road my wife is going to want to drive it, and I'm concerned about her being able to handle the power and the idiosyncrasies of the locker. That, plus having disk brakes on the rear may push me in the direction of the 8.8". If a good deal on one comes my way I'll probably get it, and sell the 9" to some Camaro jockey.

 
Joined
Jan 7, 2015
Messages
8,032
Reaction score
491
Location
Western North Carolina
My Car
Multiple Mustangs!
If you go out and jack the vehicle up and pull back and forth on the wheel tire you will feel the play and hear the axle moving into and hitting the pin in the spider gears. There is no way to build them without the play. If they did the expansion of the axle from heat would bind up the rear. My X complained from the week we got the mustang about the clunk clunk. They do make 35 spline 9" my friend has one for sale a Detroit locker also. He took out of his car because the locker is so noisy in tight turns. Here is a link to video on youtube of what I am referring to They all have this end play. This is the reason Kenny Brown states to not use them on track because it pushes the piston back in the caliper everytime you go around a turn causing the race drive to have to pump the pedal to get full brake after each turn. I know you guys are not racing on track so does not mean anything to you but does do someone in competition. The end play is in any spicer type rear has to be or could not assemble them. 9" does not have since no C clip and has sealed bearing and axle bolted in. I have had to put the offset bearings in 8.8 after the bearing wore into axle shaft. 9" the bearing does not turn on axle so never wears out. Can replace bearing if worn.

https://www.reddit.com/r/Jeep/comments/3i1piw

 
Joined
Sep 22, 2019
Messages
306
Reaction score
91
Location
USA
My Car
1972 Mach1 (wife's)
This topic is relevant to future plans for my wife's Mach 1. It too currently has an 8". I have been unsuccessful at locating a 9" that fits the 71-73 Mustang. Locally.

I do like the cheaper option of upgrading the 8". I haven't checked the local pick a part yards for an 8.8" out of an explorer. So I don't know the cost.

But I have priced out an aftermarket 9". That is going to come in around $4k. 

It comes down to intended use I think. And engine power level. For my wife's Mach 1 it will never see a road course. It might do a couple passes at the local drag strip on "Ford Fun Days" or some such event. And the h.p. goal is 400 at the flywheel. So in our example the smart money says upgrade the 8". I don't like that option. Before paint and bodywork I envisioned (fantasized) bulletproof drivetrain and track worthy suspension.

I look forward to more "talk" on the subject!

 

Hemikiller

Well-known member
Staff member
Moderator
Joined
Jul 12, 2010
Messages
3,662
Reaction score
926
Location
Killingworth, CT
My Car
71 Mach 1
65 coupe
Those "crate" rear axles are a huge ripoff. 

An 8" or 9" center section will cost about the same. The axles and the brakes from the 8" will bolt right onto a 28 spline 9" rear if you wanted to keep it low budget. Bare housings can be had for a fair price. Maybe @Motorcity Mustang or @OMS would have one. WCCC has some listed. 

https://secure.cougarpartscatalog.com/d1zz-4010-b.html?sessionthemeid=26

or you can buy a new housing and axle package from Quick for about $800 with your choice of options. 

https://www.quickperformance.com/QP-Bare-9-Inch-Housing-Axle-Package_p_15999.html

Alan from https://www.ford8and9inchrears.com/ has a great reputation and sells on eBay as well. 

https://www.ebay.com/itm/123719471884?epid=5013339111&hash=item1cce41530c:g:SOgAAOSw4f5YCX1T

 

Galucha

Well-known member
Joined
Jul 2, 2015
Messages
179
Reaction score
90
Location
Chattanooga, TN
My Car
1971 Coupe 302
This topic is relevant to future plans for my wife's Mach 1. It too currently has an 8". I have been unsuccessful at locating a 9" that fits the 71-73 Mustang. Locally.

I do like the cheaper option of upgrading the 8". I haven't checked the local pick a part yards for an 8.8" out of an explorer. So I don't know the cost.

But I have priced out an aftermarket 9". That is going to come in around $4k. 

It comes down to intended use I think. And engine power level. For my wife's Mach 1 it will never see a road course. It might do a couple passes at the local drag strip on "Ford Fun Days" or some such event. And the h.p. goal is 400 at the flywheel. So in our example the smart money says upgrade the 8". I don't like that option. Before paint and bodywork I envisioned (fantasized) bulletproof drivetrain and track worthy suspension.

I look forward to more "talk" on the subject!
I've had a chance to do some more research into the topic, and it seems that dollar-for-dollar, a 9 inch is generally going to be stronger than an 8 inch.

However, in theory, you can put the money into an 8 inch to make it very stout and handle a lot of power - it's just going to cost a good bit more than finding a 9 inch and doing some basic upgrades to get the same level of durability.

I think for your 400hp goal, especially since you already have it, upgrading the 8 inch makes the most sense, and I will likely be going the same route.

There's definitely a lot of variables at play, and I think weight and parasitic draw shouldnt be overlooked, since they seem to be some of the more prominent reasons as to why an 8 inch would be good.

 
Joined
Sep 22, 2019
Messages
306
Reaction score
91
Location
USA
My Car
1972 Mach1 (wife's)
Great thoughts Galucha.

I just "wanted" a 9 inch! But that darn bodyshop bill was higher than anticipated....

More an economic reason for us to stay with the 8 inch we already have. But yes, parasitic drag, unsprung weight; factors to be considered undoubtedly.

We're not there yet, so maybe we'll match 5 numbers on the powerball soon...

 
Top