Well, I like Stein's design. If you have a welder and a brake (or press) it'd probably be my first choice.

Looks like the new axles will be about the same either way. With the Ford setup, you gotta buy the diff assembly, install an LSD in it, and buy the mounting kit (assuming you don't fab your own).

Contrast that against buying the OSG diff and a few bucks to uprate the mounting hardware on the caps and maybe reinforce the mounting of it.

My gut tells me that the Ford option will be slightly cheaper and somewhat stronger, at the cost of more involved fab work and more weight.

Yeah I think what Joe said pretty much sums it up.

The OSG diff is supposed to be really, really good so there is that to consider as well. It is just really, really expensive. A new Torsen T2R goes for about half that, a Truetrac is much less than the T2R, and a regular Torsen T2 is even less than the Truetrac. But as stated you will have some fab work to do.

BTW if you guys like I will weigh my 8.8 Explorer IRS diff (dry) before I put it back in the car. The axles have to be considered too of course but I do not have the FFR axles to weigh. My axles are a hybrid with Ford inner CVs and Miata outer CVs.

That would be awesome... And greatly appreciated on this end.

I'm liking the idea of strengthening what is already there. I'm sure these mods could be placed into different categories of need. I'll be at 350hp, but no drag launches and no heavy track time. A few DE events here and there. Strengthening my current torsen woul dprobably be the most cost effective.

Mine will be used exclusively for the road course... So puking its guts out on the track is a no-no out of common courtesy for other drivers. Thusly why I am curious about this so much.

I agree.

I hope I didn't come across as wanting to take this thread in the direction that suits me. I was hoping we can thoroghly discuss the different levels of upgrade. Like a stage 1, stage 2, stage 3, where stage 1 is strenghten current torsen, stage 2, low budget complete swap, stage 3 is more expensive complete swap. Somthing to that effect.

Oh not at all... I believe this thread should be used for exploring all types of driving styles were the differential could be put in jeopardy. It is usually one of those things were people put in a Torsen unit and call it good until it explodes. I just want to avoid the whole exploding part. Haha! So by all means discuss away... Plus with all of the technical discussions about turbos and such, I haven't been able to find a good one with options for rear differential units yet. So this 'could' make a decent one for people to refer to...

I am really considering the OSG currently with beefed up mounting and reinforcing of the housing. Maybe I can talk Emilio into allowing me to return the blown-up unit (if it blows) for a full refund. Haha! :fingers crossed: Would definitely make a good sales pitch for him if I couldn't destroy it with a GT3582R'ed 13B-RE Miata. O_o

Both the OSG and Quaiffe LSDs are strong, but I really wonder about dropping $1,500 on a diff for a 7" rear end that still has relatively weak axles and hubs. For that amount of money, you can buy a used AL 8.8" housing, your gears of choice (2.73-4.88), a Traction Lok LSD, new FFR 28 spline axles, broached Miata hubs (or Tbird/MVIII hubs turned to fit Miata uprights) and a used Mig welder for modding the shock mounts (or building new LCAs out of square tubing). I think I'd rather roll the dice with the OBX (appropriately setup) for $300 then drop $1,500 on a "bullet proof" diff for a 7" rear end assembly that may be short lived at high torque levels.

Does the boosted 13B have a lot more low-end torque than the NA versions? I've only driven NA RX-7s and RX-8s, the torque was pretty modest but they were really impressive on top end RPM and hp. If you are not doing drag launches or hard autox starts, the 8.8 might be overkill.

The engine will probably never see below 3,000 RPM unless there is an extremely low speed corner. The torque in all rotaries is low in the lower RPM's, but like you said they do make decent torque figures in the mid-upper range. The mid-upper range is where this setup will spend 98% of its life. As for how much torque the car will be making... A similar setup made 407.20 WHP with 326.09 lb-ft @ 16 psi. Both values were pretty high in the RPM range. I will be varying my setup between 15 psi and 20 psi depending on the track and conditions so mine should be a little more and a little less depending. Oh yeah... My AFR will be tuned a bit better than his. :giggle:

http://www.dragtimes.com/images_dyno...-RX-7-Dyno.jpg

skimmed a lot of the thread, but if you guys are in need of a less expensive very high quality "quaife equivalent", someone should talk to Stephen Yeh at Mfactory (mFactory) about doing a group buy.

I ran a group buy with him to get them going making mazda fwd g-series differentials. ours ended up being $650 and very very nice. i have literally compared side by side with a quaife, and theirs was equal if not better. the group buy went so well (almost 50 people) that they now stock them with two different spline counts to fit tons of fwd fords and mazdas.

if i remember right we needed a minimum of 20 buyers, and a sample diff and axle stubs for them to work with. it also ended up being the only drop-in diff for our application... the quaife requires that you buy an additional $200 ARP bolt kit while mfactory provids the custom bolts with the diff for $650!

PS: they can also do final drive gears...

Might be worth a shot... Thanks for the info.

Apologies if you've already posted this, but mind sharing where you got these axles and how much they were?

No problem, I do not think I have posted about it before. So basically this LS1 NA I have was converted by the previous owner using a kit from the now defunct Racing Concepts.
Racing Concepts - Products

According to a post from this company (on m.n) about thier kit, this is what the axles are:
"The axles are custom built. They use the Ford 31 spline inner cup, a custom axle shaft, and the Miata outer cup to fit the stock hub. Custom CV boots are used to properly fit the axle shaft."
That quote is in this thread
Open for business! LS1 Conversion Kits! - Page 3 - MX-5 Miata Forum

So to state it all in one place, my rear setup is:
The axles above, 31 spline
Ford 8.8" IRS all aluminum housing sourced from a 2002+ Ford Explorer
New Eaton (Detroit) Truetrac Differential http://www.eaton.com/EatonCom/Produc...trac/index.htm
3.55:1 gears (low mileage set from some other vehicle)
New bearings, races, ring bolts, seals etc.
Racing Concepts differential mounts, modified (beefed up) by me after I managed to bend one at the drag strip :giggle:

Based on my research and questioning this setup should do well for road racing duty which is what I am modifying the car for.

Some interesting conversation on essentially the same topic over at V8Miata.net: Axle and drivetrain shops? - v8 Miata Forum - Home of the v8 Miata Conversion

A company that produces custom axles: Gator Racing Axles Drag Axles Street Axles

Where I found the link to Gator: Warning my bad experience with Racing Concepts - dont end up in the same boat as me - Page 13 - MX-5 Miata Forum

All of these custom axle options are REDICULOUS in price... I could find better places to spend ~$1000 than axles. Then again, this just could be me...

That's why I used the $110 per side axles from FFR.

I fully agree Stein... My statement was directed at the post about using the custom axles above. Your plan is by far the cheapest route. However the only thing about it that bothers me is the relocation of the lower strut mount. I wonder how much this will effect suspension geometry in the rear? As my project will be for road racing where everything must be optimal to be competitive, this could be a major concern.

Because you are moving the upper and lower mounts back the same amount, geometry is preserved. The only difference is the center of the force is back further, but still within the mounting points of the control arm to subframe and control arm to upright, so should not be significant.

I agree with Stein. The a-arm geometry and the motion of the rear wheel is unchanged since this mod does not change the suspension pivot points.

Now, like Stein said, moving the shock aft increases the torque load on the lower a-arm. That is, the shock has more leverage to twist the lower a-arm, approximately about the axle. But, if you are not running Spec Miata (clearly you are not with a high-horsepower Miata) then you will probably be allowed to use poly suspension bushings, which you should. They are cheaper than stock, you want fresh bushings in the suspension, and they are much stiffer. The poly bushings make the arm mount very stiff. You might add a bit of twist into the lower arm itself but I doubt it is enough to significantly tweak the geometry. Additional binding could be an issue, but again I doubt it.

BTW by no means am I implying that this would be a problem with stock bushings. I'm just describing my take on things based on thagr's build context.

If I were doing this from scratch I would be all over Stein's method. I considered it briefly (even though my conversion axles were already fine) just for the extra outer CV strength. I had to decide for sure when I bought the 31 spline diff (carrier) since the FFR axles are 28 spline. But mine should be OK so I'll leave them as-is as I have higher ranking build priorities.

Good call on the bushings. I have pressed out my stockers and have Energy Suspension bushings ready to go in after sandblast and paint. Still need to locate and drill and tap for grease zerks. As this car will see a lot of dusty conditions (I live on a gravel road), I want to be able to grease them easily.