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...