henorek

Hello there,
As I mentioned in the title, I would like to ask for advice on choosing a differential for my project. Yes, I know that the gearbox and other elements will give up first, but let's focus on the differential itself

There are three basic assumptions, it has to withstand 500hp (with a reserve), it is to be able to add a 1.5 way LSD from, for example, Cusco and it must have a gear ratio of around 3.2

1) Stock Torsen
Pros:
- pnp
- 3.3 gear ratio easy to buy
- heavy duty driveshafts are easy to buy
- I can keep PPF
Cons:
- changing the gear ratio and the driveshaft will come out very expensive
- I don't think he can handle 500hp

2) Nissan 350Z
Pros:
- very durable and easily accessible
- cheap
- 3.3 gear ratio easy to buy
- 1.5 way lsd is easily obtainable
Cons:
- looks huge, might cause clearance issues
- need a lot of fabrication to fit

3) V8 Getrag kit
Pros:
- pnp
- heavy duty
Cons:
- I have not seen 1.5way LSD for this model
- very hard to find in my country

Did I miss something? Could you please recommend some other solution?

Regards, Jarek

Formerly ZX-Tex

There are two options to implement a Ford 8.8 IRS differential. There is the 'Thunderbird' rear end, and the 'Explorer' rear end. The Ford Explorers came with an IRS for a few years back in the late 90s - early 2000s or something like that, don't quote me. It is an aluminum housing so that is an weight advantage compared to the Thunderbird; I believe those are all cast iron, could be wrong.

The Thunderbird housing is also popular, and is the go-to option for Factory Five Cobra kits. I have a friend that use to race a FF Cobra and he used the Thunderbird diff.

I pulled my Miata subframe and modified it so that the Explorer differential was a direct bolt in mount, very clean. Search for a thread on Explorer differentials posted by ZX-Tex (my old handle here) and there should be pictures.

Once you have the Ford 8.8 housing in place, there is a HUGE selection of carriers (differentials) and ring gears to choose from. Driveshaft Shop can make the axles, and there are some other options as well IIRC. This Diff will handle 500 HP and then some, as long as you are not launching in 1st gear on wrinkle wall slicks with a LS1 6000 RPM clutch dump. Even then, the relatively wimpy Miata trans would probably let go first; I am using a T56.

If you are going to run this kind of power then you should consider going to the beefier hub splines from V8 Roadsters (if they still offer them). These are what I use on my LS1 track car with 275 A7 Hoosiers. I run this car hard and have had no issues with anything in the rear. It's very solid.

Formerly ZX-Tex

Also, a looooooong time ago I had a phone conversation with Emilio at 949 Racing when I was thinking about diff swaps. IIRC he thought the stock Miata housing with a Giken diff would be able to handle high power levels. Again, this was a long time ago, so you should check with him or do more research before going that route. The advantage is the only thing you have to change is the diff carrier. The disadvantage is Giken diffs are very nice but very expensive.

emilio700

iTrader: (3)

To my knowledge, no one has ever internally broken a 7" (1.8) Miata diff. The weak links are the breakaway notch in the aluminum housing and the tripod in the halfshafts. The housing can be reinforced, a few kits on the market for that. OEM half shafts die quickly in the 300whp range.
DSS (Driveshaft Shop) axles solve the tripod wear problem. V8 conversions retaining IRS use Cadillac CTS diffs. Our 425whp time attack car used DDS shafts, and OS Giken and reinforced housing (Kpower plate)

Formerly ZX-Tex

Thanks for the clarification. There you have it, at least five basic options.

BTW the Truetrac is amazing in the rain. Last time I had a wet race I was killing everyone on corner exit throttle due to the ultra smooth transition characteristics. It had just started raining at the beginning of the race (NASA ST2 wheel-to-wheel at Hallett CCW) so the track was really slick due to grime and dust. I was late to grid so I had to start in the very back and fight through traffic on Hoosier A7s. No one was on rains IIRC; they would have overheated most likely since the track was damp, not standing water wet.

So if you are in an area where it rains a lot you might want to consider that. Emilio will have to comment on the rain characteristics of the Giken.

Erat

iTrader: (5)

Do you mean unlike this?

emilio700

iTrader: (3)

"to my knowledge"

Any more details other than the pic?
HP level. Drag or road course, etc.

Erat

iTrader: (5)

Built 1.8 with a 2560 at around 18psi. Sub 300?
Completely stock 4.1 out of my 94 r package car. 5 speed at the time too.
Lost it at the top of 1st at the drag strip. I was running either a Toyo R1R or a Star Spec tire, can't remember.

The cast iron part of the case was also cracked. I'm not sure if something flexed and caused those gears to bust. Or those gears busted and cracked the case. I also know wheel hop was most likely a huge factor due to the wrong tire being on the car.

Formerly ZX-Tex

Stock suspension bushings or poly?

Erat

iTrader: (5)

All stock. Stock r package suspension as well(TAKE MY MONEY).

I've since beefed it up a bit more and don't drag race on the wrong tire.

henorek

Thank you very much for your involvement in this thread.

The ability to stay with the factory solution is extremely valuable for me, so your post is very uplifting.
Looks like Erat broke his torsen insert, not ring and pinions which allows me to assume that these are much more durable and after paring them with OS Giken I would have nothing to worry about.
So my final setup would look like: OS Giken + 3.3 MFactory gearset + reinforced housing + MiataRoadster 1000hp Axles.
Plus, this might be a good time to mention the tires, I want to run a 275/35R15 setup (although I am still considering whether 245/40R15 will not be ok).
My gearbox ratios are 4.7143 3.1429 2.1064 1.6667 1.2847 1.0000 0.8392 0.6667

So Emilio, of course you don't have a crystal ball but looking with your experienced eye do you think a setup like this would be okay? I would not like to be in a situation where I will be afraid to use the LC for fear of grinding the differential.

emilio700

iTrader: (3)

That's a pretty common set up. Should be fine IMO.

TNTUBA

iTrader: (1)

To bump an old thread.

In 11 years of HARD launches in Pro Solo. 20+PSI of boost on the 2 step at 4400 RPM and For the last 3 years with the dog box we have been shifting at 8000+ RPM without the clutch. I have never broken a Torsen but I did break 1 ring an pinion which was 100% my fault with an instillation issue.

I am going to a OS Giken this winter and am searching around looking for info on the diff and came across this thread.

I agree with Emilio, I haven't really seen the need for the "other" diff swaps in these cars unless you are going with built V8 power levels. I'm just not sure the juice is worth the squeeze.

Arca_ex

iTrader: (2)

Another data point, I know of one car that is over the 500whp mark now with the stock torsen and 3.636 gears, reinforced case etc and it is surviving in a road course application at places like COTA and other SouthWest tracks.

I haven't seen any that have broken internally that were NOT used for either drag racing or drifting. Both of those have massively higher shock loads than you would come across on a road course. Autocross is kind of in the middle, the launches are pretty hard on it but it's still an unprepped surface so nowhere near as bad as a drag strip.