Mustang Evolution Forum banner
1 - 20 of 45 Posts

·
Registered
Joined
·
1,876 Posts
Discussion Starter · #1 ·
Going to put 373 in, replacing all bearings and such....what all is needed to upgrade to 31 spline? I know the axles, anything else need to be changed?
 

·
Registered
Joined
·
1,001 Posts
The side gears, though generally people just swap out the carrier assembly...
 

·
Registered
Joined
·
1,876 Posts
Discussion Starter · #4 ·
The side gears, though generally people just swap out the carrier assembly...
So I can do it without swapping the whole carrier? Is it cost effective to swap the gears instead of the whole carrier? Just wondering, I wasn't planning on this as of yet but I don't want to pay labor more than once to rip the rear end apart
 

·
Registered
Joined
·
1,001 Posts
So I can do it without swapping the whole carrier? Is it cost effective to swap the gears instead of the whole carrier? Just wondering, I wasn't planning on this as of yet but I don't want to pay labor more than once to rip the rear end apart
I believe the actual carrier is the same on all 8.8" axles and it is just side gears that are needed--but verifiy that before proceeding.

I have to aks why you want to "upgrade" to 31-spline axles? They (the Ford axles) are only 10% stronger than the 28-spline axles.

COT: Continuous output torque rating
MOT: Maximum output torque rating

Ford 8.8 28-spline COT: 1250 lb-ft MOT: 4600 lb-ft
Ford 8.8 31-spline COT: 1360 lb-ft MOT: 5100 lb-ft

31-spline COT = +8.8%
31-spline MOT = +10.8%


The aftermarket axle makers claim the splines on their 28-spline axles are stronger than the stock axles, one maker claims 35% stronger.

That said it is foolish to ever believe anything anyone trying to sell you something says.

Are you planning on a super-build in the future? The 28-spline axles are solid to 380 rwHP¹ or so--and that is only ceiling if you can hook up the the road VERY well...

--------------------------------------------
¹ - Axles and other power transmission components are rated in torque capacity, not HP, so some assumptions have to be made (I selected 2nd gear because it is unlikely one would get a 100% solid hook up in 1st):

If we assume 4.10:1 gears then 4600 lb-ft out would be 4600/4.10 = 1122 lb-ft in to the rear end. Now if we assume 2nd gear (2.0:1) with perfect traction that's 1122/2.0 = 561 lb-ft into the tranny. If we further stipulate that the engine's peak torque happens around 5252 rpm then that would be 561 fwHP, or 476 rwHP with a 15% powertrain loss.

Now that would be pushing the axles right up to their limit, so if we include a 0.8 design buffer that becomes 476 * 0.8 = 382 rwHP--or just about what most people here would consider to be the limit for the stock 28-spline axles.
 

·
Registered
Joined
·
1,876 Posts
Discussion Starter · #6 ·
I'm not planning a "super build" persay, but I am getting a super charger. Eventually I will rework the motor so it can handle more but others who have a supercharger on their stock motor with full bolt ons make around 400-430 on a conservative tune. This is my thing. If I'm going to need it, I want to get it done when I put my 373's in there so I don't have to pay someone a second time later to rip the axle apart.

---------- Post added at 08:53 PM ---------- Previous post was at 08:51 PM ----------

I'm not planning a "super build" persay, but I am getting a super charger. Eventually I will rework the motor so it can handle more but others who have a supercharger on their stock motor with full bolt ons make around 400-430 on a conservative tune. This is my thing. If I'm going to need it, I want to get it done when I put my 373's in there so I don't have to pay someone a second time later to rip the axle apart.
Eventually when I rebuild I will have forged internals so I can turn up the boost, trick flow heads, stage 2 cams...ect..
 

·
Registered
Joined
·
1,876 Posts
Discussion Starter · #7 ·
Ok this thread kinda died down....let me restart this... At what HP range is it best to have 31 spline rear end?
 

·
Registered
Joined
·
1,876 Posts
Discussion Starter · #8 ·
Now that would be pushing the axles right up to their limit, so if we include a 0.8 design buffer that becomes 476 * 0.8 = 382 rwHP--or just about what most people here would consider to be the limit for the stock 28-spline axles.
According to this (all other math removed for length purposes) 382 is the limit for stock 28 spline axles. A supercharger would put me between 380 and 430 depending on tune and other supporting mods. Sooooo should I upgrade the axles or not?
 

·
Registered
Joined
·
711 Posts
Just build it now. I snapped a axel and shattered the rings teeth and a few spider gears at the track last summer. All that is needed is a 31 spline diff and axels. That's it. It's simple to do since changing gears you'll already be 99% into doing it anyways. The biggest weak point in the 31 rear end I've seen is the diffs themselves, but they will hold to just about anything you'll give it. I'd suggest just sticking with the 31 trac lok frpp diff. It's cheap as hell, and rebuildable and from what I've read, they can take a beating. I'm doing the exact same thing your doing with prepping for big HP and that's exactly what I did. No need for a new carrier or tubes. Just that internal parts themselves. Obviously new bearings and such.
 

·
Registered
Joined
·
407 Posts
Cliffyk mentioned 28 spline are generally good for up to 380 HP but that torque was the actual measurement to judge by.
 

·
Registered
Joined
·
1,220 Posts
I put in 31 spline axles and differential with 4.10's in my mustang and have launched it at 6k with slicks like 100 times and I haven't broke anything. I know people with stock 28 that have broken there axles so many times and they don't have slicks. Get at least the moser 31 spline axles and any Posi differential I got mine from a ford f150 and it fit perfectly. I know people without slicks that have broken so many 28 spline axles at the drag strip I didn't want to worry about it. You should get 33 spline moser axles which have a lifetime warranty if your axles ever break you get new free ones.
 

·
Registered
Joined
·
711 Posts
I was gonna do the 33 spline but the options on diffs are so limited. Especially in the price region. You'd never worry about breaking the axles though. Guess if you really wanted, spools are available everywhere and cheap as dirt.

---------- Post added at 12:20 PM ---------- Previous post was at 12:19 PM ----------

Did you do any work to the tranny or still kickin the stock 3650
 

·
Registered
Joined
·
1,876 Posts
Discussion Starter · #14 ·
Just build it now. I snapped a axel and shattered the rings teeth and a few spider gears at the track last summer. All that is needed is a 31 spline diff and axels. That's it. It's simple to do since changing gears you'll already be 99% into doing it anyways. The biggest weak point in the 31 rear end I've seen is the diffs themselves, but they will hold to just about anything you'll give it. I'd suggest just sticking with the 31 trac lok frpp diff. It's cheap as hell, and rebuildable and from what I've read, they can take a beating. I'm doing the exact same thing your doing with prepping for big HP and that's exactly what I did. No need for a new carrier or tubes. Just that internal parts themselves. Obviously new bearings and such.
Your post about that is what got me thinking..
 

·
Registered
Joined
·
1,220 Posts
I was gonna do the 33 spline but the options on diffs are so limited. Especially in the price region. You'd never worry about breaking the axles though. Guess if you really wanted, spools are available everywhere and cheap as dirt.

---------- Post added at 12:20 PM ---------- Previous post was at 12:19 PM ----------

Did you do any work to the tranny or still kickin the stock 3650
I did fix my synchros cause they were grinding really bad but other than that I got a Valero clutch.
 

·
Registered
Joined
·
1,001 Posts
Just build it now. I snapped a axel and shattered the rings teeth and a few spider gears at the track last summer. All that is needed is a 31 spline diff and axels. That's it. It's simple to do since changing gears you'll already be 99% into doing it anyways. The biggest weak point in the 31 rear end I've seen is the diffs themselves, but they will hold to just about anything you'll give it. I'd suggest just sticking with the 31 trac lok frpp diff. It's cheap as hell, and rebuildable and from what I've read, they can take a beating. I'm doing the exact same thing your doing with prepping for big HP and that's exactly what I did. No need for a new carrier or tubes. Just that internal parts themselves. Obviously new bearings and such.
I am curious as to how many hard launches before it broke? What gear ratio? I'd like to run some numbers and see what sort of output torque is was carrying when it let go.

Do you have HP and torque number for your engine?
 

·
Registered
Joined
·
1,876 Posts
Discussion Starter · #17 ·
I am curious as to how many hard launches before it broke? What gear ratio? I'd like to run some numbers and see what sort of output torque is was carrying when it let go.

Do you have HP and torque number for your engine?
Cliffy, I just wanna say I applaud your math ability..
 

·
Premium Member
Joined
·
19,890 Posts
Just go 31 spline. You can pick up a cobra diff for under $200 now. Pair of strange axles and be done with it. One less thing to worry about.
 

·
Registered
Joined
·
1,876 Posts
Discussion Starter · #19 ·
Just go 31 spline. You can pick up a cobra diff for under $200 now. Pair of strange axles and be done with it. One less thing to worry about.
Didn't know cobra diffs were 31 spline....that's what I was thinking when I started it. SC will put me at 400ish and I knew totalblackout had blew up his axle so I figured just do it while its open
 

·
Registered
Joined
·
711 Posts
No I sure don't. It's just a full bolt on, cammed, clutch car. I had 3.90 gears, spec stage 3+ clutch, and 315 555R radials. That was the first time it had been to the track and I launched it at 5k and BOOM.

---------- Post added at 07:39 PM ---------- Previous post was at 07:36 PM ----------

Yea you might as well jut do it now lol spend the cash and get good axles too.

---------- Post added at 07:42 PM ---------- Previous post was at 07:39 PM ----------

I did fix my synchros cause they were grinding really bad but other than that I got a Valero clutch.
Where did you get your synchros. I didn't wanna spend 800+ on the D&D upgrade kit when it ultimately won't hold the final power I wanna put out.
 
1 - 20 of 45 Posts
Top