Upgraded Rear Differential Options...
 

With Guru going out of business it seems, it makes it quite difficult to obtain their billet rear differential unit that bolts into the factory housing. I was just wondering if Guru was/is the only option for a complete bolt-in 600HP capable (need some headroom) unit available for the NAs. I know I could always swap to the T-II diff or even Ford 8.8, but I'm trying to keep custom axles and hubs to a minimum if possible. If not, I guess I don't have a choice... Thanks in advance guys for the information!

Still haven't seen a broken Miata 7" ring and pinion. Stick an OS Giken in a 7" and see what happens.

Thanks for the reply Andrew... I'm just afraid that a turbo 13B-RE will grenade the diff pretty quickly. Then again, I could be wrong...

Give Emilio at 949 Racing a call. His website describes the OSG diff as "uttery bombproof even with big boosted horsepower and Hoosiers", so your car might make a very good test of it. If it fails, he was wrong. If it survives, your car gets to go on the website as a testimonial.

"My name is Calton, and not even my 600 HP rotary-powered Miata was man enough to break the OS Giken."

I've built an OS Giken 7" rear diff. Used the S2K diff in a MSM pumpkin with MSM axles. I think it will be pretty strong.

What about the front differential?

Going with an ARB locker.

Thanks Joe. I might take this challenge. :D

[Gary Coleman]What you talkin' 'bout hustler?[/Gary Coleman]

Theres always the ford option, several ratios to choose from.
but youll loose the PPF which I think would be a bad thing

Loosing the PPF is in the plan anyways as this will be mated to a Ford T5 Tranny...

Install of the T5 will be similar to this only not in an FD: http://www.rx7club.com/showthread.ph...ht=dt-5+damian

well then you might consider that

Boss frog sells a bolt-in differential subframe for the ford 8.8, and it comes with hardware, bushing and whatnot.
They also sell the axles to go with it, unfortunately theyre pretty damn expensive...

Miata V8 Swap

http://www.bossfrog.biz/pics/Image4.jpg

http://www.bossfrog.biz/pics/Image6.jpg

Hmm. I hadn't considered the Ford option. Definitely indestructible, and more R&P options.

If you do go with the OSG, I'd strongly consider using the MSM axles as they are slightly larger than the stock ones.

FTP- what's a complete Ford diff with LSD typically go for?

FTB- I actually sent Boss Frog an email about that kit and such as far as the axle clearances with the Miata control arms and coilovers. I will be running tubular control arms built by John Brakke for E-Production cars and didn't want to spend that kind of money only to find out that the Ford 8.8 install would completely make them useless in my application. But by looking at their description it seems fine, but just wanted some verification.

I'm not sure on the going rate for LSD's for the Ford 8.8 either... And thanks for the tip on the MSM axles Joe. I will look into it for sure.

Im not real sure about the details regarding the boss frog kit. I talked with the owner of racing concepts here in jax about their ford differential options and it sounded like a good way to go as far as R&P options and LSD options. But apparently people have trouble dealing with that them, so idk if you want to bother asking them about their kit.

I would imagine that the tubular control arms would have more clearance all around than stock, but I guess you never know.


Ford racing has several LSDs availible for the 8.8

Ford Racing Performance Parts [Differentials]

and lots of choices for a R&P

Ford Racing Performance Parts [Ring ]

Thanks for the links and info on dealing with Boss Frog FTB. I like the CHEAP options for the Ford 8.8 so far though. :D

It's just the unfortunate part of gaining weight on the rear end to gain power up at the engine. Sigh...

The 8.8s you want for the Miata are the IRS versions found in the TBird/Cougar and Lincoln MVIII. TBird has a steel housing, the MVIII has an AL housing. Pick & pull prices are probably under $200, a lot of times it is cheaper to buy a non running car for a little more, pull the rearend and part out the motor and trans (MKVIIIs have an all aluminum Teksid block which you can sell for $150-200 bare) and then scrap the car. I believe both the Tbird and MKVIII use a carrier w/28 spline axles. 31 spline axle carriers (often found in newer Explorers) allow you to use stronger axles, but the Factory Five IRS axles (cheapest option for an 8.8 in a Miata) are currently only available in 28 spline. TBirds and MkVIIIs typically have open carriers, so you will need to swap out the open carrier to get an LSD. Eaton, Detroit Locker, Traction-Lok, Auburn etc. all make 8.8 carriers.

The Ford 8.8 Traction Lok carrier is a clutch plate style LSD and is usually available on Stang sites (Corral.net) or Ebay for $75-$100. Traction-Loks are easy and cheap to rebuild (cheap kits on Ebay) and you can put in an additional clutch plate for more lock-up". 3.27, 3.55, 3.73 and 4.10 gears are always available used at very low prices (50-100) and new gear sets start in the $150ish range.

sn - Thanks for the reply. Seems like you have done quite a bit of research on this swap in the past. I would definitely be running the aluminum housing if I were to go the Ford 8.8 route.

Stein has done the research on Miata fitment and axles; I can't take credit for that. I know enough about Ford parts in general to be dangerous. If you are set on using the AL carrier and can't find a MKVIII unit, I think the later model Explorers (version w/the 4.6L Mod Motor V8) had an AL carrier IRS with 31 spline axles and a Traction Lok (don't know if the T-Lok was standard or optional). Unfortunately, 31 spline carriers won't work with the Factory Five axles which are the cheapest option for mating the 8.8 to a Miata. You still need to swap in Ford hubs in the Miata uprights or broach the Miata hubs for the Ford axle. Haven't personally done the swap, but it is NOT a simple drop in.

Thanks again for the reply. I am personally not dead-set on the Ford 8.8 swap as I would prefer to run something in the factory 1.8L housing to make things less complicated and lighter, but based on some of the options out there the Ford 8.8 might be a better choice down the road.

7.5" Ford
 

FWIW,

The 7.5" Ford IRS is a smaller version of the 8.8 and obviously uses a 7.5" ring gear. Earlier TBirds/Cougars (2.3L turbocharged?) used it. It is stronger the 7" Miata (I believe Martin/Montster Miata may have used this as an entry level rearend); not sure if it is stronger than the 8" TT RX7. Like the 8.8", parts are cheap and readily available. The solid axle version of the 7.5" is stock in the base 4 banger Ford Ranger and seems to hold up in V8 swaps if one avoids sticky tires and drag racing launches. Not sure what your options for axles are outside of Monster Miata.

OBX Option
 

Check out the OBX carrier for the Mazda 7" (OMG, I can feel the flames coming already!). It sells on Ebay for under $300 and is basically a sound design. Like a lot of "offshore" parts, quality control is spotty so you need to take it apart, clean it, deburr it and check clearances before using it. A lot of Neon/SRT-4 owners have had good luck with the OBX at high power levels. The stock OBX belleville washers seem to be the only really weak part in the OBX, replace them and you should have a reliable unit at a reasonable price.

Once you have a strong carrier, the next weakest link in the Miata 7" seems to be the differential bearing caps. I suspect you could strengthen them considerably by replacing the cap bolts with high quality studs/nuts and adding a steel "cap strap" (this would have to be custom machined). However, at some point in time the stock 7" axles/CVs/hubs become the weak link and you start all over.

Thanks again for the tips. I will investigate on the OBX, but it does seem kind of sketchy to me though...

Has anybody ever broken a stock carrier with torque? I can't recall having seen one.

That's a good question Joe... I am just assuming that it would be too weak to handle my setup. However, I would love to have an upgraded unit that fits in the same housing to simplify cost and fabrication while giving a little bit of room for the setup to grow into it.

I've heard a couple times that the miata is so lite that you will spin the tires first before putting enough load on the diff to break it. Maybe if you are running slicks and dumping the clutch.

I think there is probably a lot of truth to this.

I just finished my 8.8 IRS build. I haven't updated it yet, but I made some clearance adjustments as my exhaust wouldn't fit the way it was originally built. I have $540 total in the rear end with a free Traction Lok and 3.73's but paid $140 for labor to swap them, new FFR axles, broaching Miata hubs for the FFR axles. Oh, yeah, add $40 for new bearings.

Start here https://www.miataturbo.net/forum/t40520/#post510010 for the 8.8 swap.

Well, that's sort of my approach here.

People have reported breaking Torsens. Ok, so we have the OXG diff to sole that problem. The vendor says it'll survive a direct nuclear attack. Maybe you could even wrangle a mini-sponsorship out of the deal if you are, in fact, racing competitively.

The 7" R&P gears, while perhaps not totally indestructible, do seem pretty stout. There aren't a lot of documented failures. sn95 has a pretty good idea about replacing the cap hardware. Reducing flex in this area would have the effect not merely of protecting the cap and bearing assembly itself, but of better maintaining tolerances between the ring and pinion during periods of extreme loading and thus decreasing the potential for damage to them.

(T1-IRS Volkswagen transaxles in particular were notorious for killing their R&P gears, not because the gears themselves were weak, but because of case flex that allowed the gears to move apart under load. Billet side-covers are now an almost universally accepted part of any high-power tranny build for these cars.)

I'm not sure if we're using the term "carrier" in the same context. To me, the carrier is the big lump of cast iron that the diff sits in. I cannot even imagine one failing.

If we're talking about the aluminum cover, there might be some gain to be had in strengthening the wings by bolting a set of steel beams across them, particularly in the area where they are notched (think of FM frame rails for your diff) but that's about the only failure mode I can think of for one. If you're really paranoid, you could fab a little cradle to more directly mount the diff to the rear subframe.

Axles / hubs / CVs... Yeah, they could be a problem. Hard to know. Get the MSM units (make sure your diff matches them) and see what happens. Worst-case scenario is that you break a set of cheap stock axles, and then you can either replace them with another set of cheap stock axles, or sell your über-diff to someone making "only" 400 HP or so for what I imagine would be close to $break-even.



Edit: Stein posted while I was typing. That's a pretty impressive build.

Joe,

I've seen a couple of pictures of 7" "pumpkins" (the carrier assembly that the diff mounts into) where the carrier cap assembly had been broken off...I suspect studs and properly fitted "cap straps" would help this. I have also heard of people grenading the 7" Torsen with drag launches on DRs or slicks. I have not heard of anyone grenading a properly prepped OBX (cleaned, deburred, properly assembled & torqued with new bolts & Belleville washers). There is a guy on one of the Honda sites that claims to have over 25K miles on his OBX at 400+whp. The OBX isn't as well made or as nicely finished as a Quaiffe but you can buy 4 OBXs for the price of 1 Quaiffe.

I don't like the idea of using tires/lack of traction as the "fuse" to protect a weak rearend. That always seems to backfire when you are far from home.

+1 Stein has a great build going and the thread on mt.n is very, very good.
+1 on going with the Ford diff. Strong, full aluminum housings are available, and parts are cheap, including ring and pinion sets in a huge range of ratios.

FWIW mine in my LS1 Miata is an all-aluminum housing from an IRS Ford Explorer. I just had an Eaton Truetrac diff installed which I bought NEW with shipping for $410. This is a Torsen-like diff (no clutches) with a good torque ratio well liked by the road race and the street crowd. Torsens are available too, as well as several clutch-type posi units. Mine is a 31 tooth instead of a 28 tooth but only because that is what came with the car which was already converted.

Since you are using a T-5 anyway, I would get a Ford 8.8 IRS with 28 tooth splines and follow Stein's build thread.

And just for the record this is being built for race racing/time trials... So no drag launches or 0 MPH clutch-dumps need apply. I might give an upgraded 1.8L unit a shot and see if it dies. If so, just upgrade from there... I will talk to Emilio about the OSG in the near future, especially if it is claimed to be able to survive a direct nuclear attack and all...

Agreed, and it sounds like essentially a similar failure mode to the one I described with the VW trannies; displacement of the differential mounting points due to side-load imposed by the hypoid configuration of the gears. And your idea of beefing up the hardware sounds like a cheap and easy fix here.


Well, the exact phrase used is "uttery bombproof", which I interpret to include strategic nuclear weapons. I'm not sure whether it would survive the very small bomb created by Hactar for the Silastic Armourfiends, but then, there wouldn't be a universe left for you to complain about it in if that were ever the case.


While I like the Ford 8.8 idea, it gets expensive when you consider the axles. Which, ironically, are probably the weak link in the chain of a Miata-based configuration.

Decisions...

THIS! I would much rather spend the money saved on not buying the expensive axles to adding additional aero tweaks or suspension. Plus it would be my guess that overall the Miata unit with axles would be much lighter than the the Ford 8.8 unit with axles...

Also, glad I'm not the only one benefiting from this topic.

Anyone found a cheaper solution for Axels when using the Ford options? I remember someone had.

Thanks, Marc

I haven't but I'm listening... :D

Edit: Based on a little looking around, Stein used the factory T-Bird axles and had the Miata hubs broached to work. Something like ~$250 or so...

Ok, it's crazy idea time.

Where do the stock axles typically break? Is it: A) the CV joint, B) the main axle shaft, or C) the stub axle and / or wheel hub?

If it's A or B, then consider the following:

You take a pair of complete MSM axles, and disassemble them. Specifically, you dismantle the CV joints themselves to the point where you're left with just a set of stub axles that look similar to the ones that go into the differential on a '90-'94 car. Only we have two each for the diff side and also the wheel side:

http://www.miata.net/garage/diffguide/28.jpg

Now, you start looking around for any sort of axle whose end-to-end length (at the mounting planes) is similar to that of the space you need to fill between your two stubs. Maybe it's from some Japanese SUV, maybe it's from an Evo or an STi, who knows...

Moving up in the world, custom-length VW / Porsche style axles are very popular with the sandrail crowd, and if anybody knows a thing or two about big shock loads, it's those guys. The idea here would be to buy a set of 930/934-style CVs, adapt them to fit your newly created stub axles, and then purchase the correct length axle shaft to go between them. Total cost for all the purchased parts would be maybe $400-$600, plus whatever it takes to mount the CVs on your stubs.

VW Parts: Axles
http://www.miata.net/garage/diffguide/28.jpg
VW Transaxles & Parts Drive Axles - Chirco Performance & Restoration

Don't just limit yourself to CV-style axles either. Lots of big-power cars use U-joints. Corvettes used them for the longest time, and pretty much any ole' driveshaft shop could adjust their length to fit your car, and whip up a set of end-plates for 'em. Hell, they could probably fab 'em from scratch for not too much bread. Here's what a $120 'Vette axle looks like:

http://www.ecklers.com/images/vette/41998.jpg

'Course, these don't have much flex in 'em. Might be a good thing for the axles. Probably a bad thing for the gears if you do a lot of clutch-dropping.


Just thinkin' is all...




EDIT: Yeah, basically the same idea you just wrote, only mine took a lot longer to type.

Seems like a do-able idea if going that route... I wonder what the weight difference between the Miata unit vs the Ford 8.8 unit is?

You did read Stein's thread right? That is the whole point of it; the axles he sourced from FFR ARE cheap. You have to do some other modifications which are documented in the thread. There is also a long thread on M.N on the same subject. Long, but informative
http://forum.miata.net/vb/showthread.php?t=352424

I just read the whole thing... Looks like he had to relocate his rear lower shock mount points to run these axles. This is something I want to avoid.

Yeah, pretty hot shit.

I just can't quite put down the idea of retaining the stock hubs and diff housing. Not sure why this is so appealing to me, what with it being more expensive and all...

Simplicity perhaps? Or the idea of, if it won't break why replace it mentality? I'm in the same boat BTW...

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.