How strong is an 8 inch diff?

[Len69Coupe 33]

My Mustang currently has a 8 inch conventional 3.00:1 setup. It has a 96 5.0 with mild upgrades. So maybe 280 - 300 HP. I also have a T-5 (WC) and I never take it on the interstate, but it will cruise in 5h with ease at way to low RPM. I'm looking to add limited slip and upgrade to 3.55 or 3.73. I've been looking at different 3rd members already built to plug and play. Am I wasting my time with the 8 inch? I don't plan to add anymore power, just increase the off the line performance.

Will the 8 inch hold up? if so, any limited slip recommendations? 3.55 or 3.73?

 

[RPM 1,256]

Unless you'll be getting more power and slicks, you'll be fine. 

[Brian Conway 276]

My opinion? Traction will be your problem before the 8" gives you any trouble. Brian

[Len69Coupe 33]

Thanks guys.

 

Anyone have recommendations for a Limited Slip diff? I'd really like to go 3.73 ratio, but I'm afraid the RPM's would be to high. I googled what RPM's should my engine be at 70 and of course got all kinds of conflicting info. Is around 2500 at 70 MPH about right?

[Mach1 Driver 602]

Just saw a thread on VMF about this. Most like Truetrac, since there are no clutches, and they are very smooth. A locking diff is downright nasty going around corners. Personally if you are going to put that much cash into the diff, I'd go all the way and switch to a 9".

https://www.vintage-mustang.com/threads/best-traction-lock-differential-assembly.1228092/page-3?post_id=11016895#post-11016895

[det0326 194]

1 hour ago, Len69Coupe said:

I'd really like to go 3.73 ratio, but I'm afraid the RPM's would be to high

wasn't the t5's in the later model mustangs paired with a 373 gear. 

[jmlay 91]

There is no real answer to how many rpm at 70. In my opinion you need to determine at what rpm you engine is comfortable cruising without lugging in high gear. Use that as a guide.

[Len69Coupe 33]

OK, I ran the numbers and a 3.55 would be 2210 RPM an 3.73 would be 2323 RPM. I think I can live with that since I rarely drive 70 MPH. 

[Len69Coupe 33]

On 12/5/2024 at 10:17 AM, Mach1 Driver said:

Just saw a thread on VMF about this. Most like Truetrac, since there are no clutches, and they are very smooth. A locking diff is downright nasty going around corners. Personally if you are going to put that much cash into the diff, I'd go all the way and switch to a 9".

https://www.vintage-mustang.com/threads/best-traction-lock-differential-assembly.1228092/page-3?post_id=11016895#post-11016895

Thanks. I'll take a look at that thread.

On 12/5/2024 at 11:50 AM, det0326 said:

wasn't the t5's in the later model mustangs paired with a 373 gear. 

I'm not sure. You're probably right though.

17 hours ago, jmlay said:

There is no real answer to how many rpm at 70. In my opinion you need to determine at what rpm you engine is comfortable cruising without lugging in high gear. Use that as a guide.

Thanks.

[RPM 1,256]

On 12/5/2024 at 6:49 AM, Len69Coupe said:

Thanks guys.

 

Anyone have recommendations for a Limited Slip diff? I'd really like to go 3.73 ratio, but I'm afraid the RPM's would be to high. I googled what RPM's should my engine be at 70 and of course got all kinds of conflicting info. Is around 2500 at 70 MPH about right?

With OD I'm just below 2,500 at 80 mph.

[Len69Coupe 33]

17 hours ago, RPM said:

With OD I'm just below 2,500 at 80 mph.

That's  a reasonable RPM for 80 mph.

Thanks.

[rwcstang 210]

On 12/4/2024 at 2:49 PM, Len69Coupe said:

My Mustang currently has a 8 inch conventional 3.00:1 setup. It has a 96 5.0 with mild upgrades. So maybe 280 - 300 HP. I also have a T-5 (WC) and I never take it on the interstate, but it will cruise in 5h with ease at way to low RPM. I'm looking to add limited slip and upgrade to 3.55 or 3.73. I've been looking at different 3rd members already built to plug and play. Am I wasting my time with the 8 inch? I don't plan to add anymore power, just increase the off the line performance.

Will the 8 inch hold up? if so, any limited slip recommendations? 3.55 or 3.73?

 

I put down 453rwhp 525rwtq with 3:50s with a TKX (68OD) behind my 427w stroker. At 65mph I'm around 2100-2300rpm, under 60 Im at 1800-2k rpms with ford 9 3:50's. 

if you have the 5 speed already installed, go with a ford 9 w/ 3:50s and go 31 spline trutrac, thats what I have and its a fun car to drive, good  rpm pick up for street and highway.

Otherwise, stick with the stock 8 until you upgrade. no need to invest in a ford 8 when it cost a few hundred more to go with a ford 9.
 

[Len69Coupe 33]

On 12/7/2024 at 11:20 AM, rwcstang said:

I put down 453rwhp 525rwtq with 3:50s with a TKX (68OD) behind my 427w stroker. At 65mph I'm around 2100-2300rpm, under 60 Im at 1800-2k rpms with ford 9 3:50's. 

if you have the 5 speed already installed, go with a ford 9 w/ 3:50s and go 31 spline trutrac, thats what I have and its a fun car to drive, good  rpm pick up for street and highway.

Otherwise, stick with the stock 8 until you upgrade. no need to invest in a ford 8 when it cost a few hundred more to go with a ford 9.
 

Did you start with a 9? I can get the setup I'm after for about 900-1000. Is there somewhere I can get a 9 setup for close to that?

 Thanks for the advice.

[smh00n 91]

I have 420 crank hp 347W through a TKO600 with the .82 5th gear and a 3.50 8” fitted with a truetrac and 28 spline axles.

Diff builder said no need for 9” and so far so good. Wheel hop is biggest issue. 
I was sceptical about the truetrac but it locks both wheels solid when being silly, gives me drive through 180 degree corners that drop off up hill, and acts like an open diff when driving like grandma. 
I’m not sure of exact rpm on the highway as I do little running like that but the times I have it’s been ok for revs and not screaming. 
 

If I were you I’d stick with the 8” and keep the extra cash. 

[rwcstang 210]

On 12/8/2024 at 11:27 AM, Len69Coupe said:

Did you start with a 9? I can get the setup I'm after for about 900-1000. Is there somewhere I can get a 9 setup for close to that?

 Thanks for the advice.

I was running the stock 8 when I had my old 393w stroker, I was putting down 365rwhp 400rwtq. it can take the power, just don't put a sticky tire on, unless you are racing. 

if you are that dead set on going with a ford 8 and want posi, send your old one or get a new rebuilt set here: https://8and9inchfordrears.com/ plenty of ppl used him and had no issues. just go with upgraded spline to 31 and upgrade the axles and you'll be set. 

 

The reason I say save up for a ford 9, its because thats what I did, I didn't want to pay twice if I wanted to go bigger, I rather set it and forget it. 

[Len69Coupe 33]

On 12/12/2024 at 5:39 AM, smh00n said:

I have 420 crank hp 347W through a TKO600 with the .82 5th gear and a 3.50 8” fitted with a truetrac and 28 spline axles.

Diff builder said no need for 9” and so far so good. Wheel hop is biggest issue. 
I was sceptical about the truetrac but it locks both wheels solid when being silly, gives me drive through 180 degree corners that drop off up hill, and acts like an open diff when driving like grandma. 
I’m not sure of exact rpm on the highway as I do little running like that but the times I have it’s been ok for revs and not screaming. 
 

If I were you I’d stick with the 8” and keep the extra cash. 

Thanks for letting me know your experience with the LS you used. It helps me to know which LS are good to use.

On 12/12/2024 at 7:16 PM, rwcstang said:

I was running the stock 8 when I had my old 393w stroker, I was putting down 365rwhp 400rwtq. it can take the power, just don't put a sticky tire on, unless you are racing. 

if you are that dead set on going with a ford 8 and want posi, send your old one or get a new rebuilt set here: https://8and9inchfordrears.com/ plenty of ppl used him and had no issues. just go with upgraded spline to 31 and upgrade the axles and you'll be set. 

 

The reason I say save up for a ford 9, its because thats what I did, I didn't want to pay twice if I wanted to go bigger, I rather set it and forget it. 

Thanks for the link. I think I checked that out before, but I'll look again.

I'm not really dead set on the 8. It's just my budget, and not having any plan to go with more HP, has me leaning toward the 8. I have been keeping my eye on FB marketplace for an affordable 9. There are a few there, but nothing that will fit without modifications.