Billet diff case?
 

Does anyone make a billet diff case for the FD?

I've never seen one. You can replace the rear cover with an RX-8 cover that has extra cooling fns. The Greddy cover has a kick out for more capacity but many people have complained about it leaking.

Gears and Gasoline just did a video where they upgraded the diff itself.


My takeaway was the diff itself needs the upgrades more than the case, The diff "ramps out" and puts pressure on the case and breaks it.

The good thing is the diff does pretty well. If you aren't making insane (over 450hp) power, doing hard drag racing launches, or doing donuts like the dummy in the video was doing, you will be OK.

Dale

I'm still trying to understand the hype behind billet. To me, I get a "carbon fiber" vibe from it....not necessary but show piece per se.

Billet allows the part to be made from a forged chunk of whatever material you are using. Then you can design the part for the best strength to weight using finite element analysis. You end up with a much stronger part, much better than the stock cast diff case that's prone to fracture

So please correct me if I'm wrong, but from my point of view I'd say the only two reasons I can think of as to why they're prone to fracture is due to age and (like the video above) mistreatment. I probably have the highest mileage 3rd gen diff on this entire forum and (knock on wood) it has been nothing but a pleasure to me with regular fluid changes.

To add to that, I definitely don't see an issue with the development of new products for our cars, but I'm primarily arguing against the case that something is inherently "wrong" with the stock diffs.

I think that, by the time you're testing the limits of the case you're testing the limits of the internals too. Usually that just means an upgrade to a Ford/Cobra 8.8 diff which is a whole new pumpkin.

If there's some sort of "stock driveline" racing rule, there are are also steel fabricated cradles and braces.

Or, if you're looking just for some billet goodness, Built2Apex does make a billet cover.

Green bros in NZ make a girdle of steel which sandwiches between the housing and cover to stiffen it up. Between that, aftermarket gears, clutch type centre and solid pinion spacer I think mine should be a fair bit more robust than standard.

Agreed. They’re not “prone to fracture” from anything resembling stock power levels and responsible use. Many, probably most, are making well over stock power and doing just fine with the stock differential.

edit: you want the whole case, not the cover

https://cimg9.ibsrv.net/gimg/www.rx7...69d323dcb.jpeg


not sure how easy it would be to retrofit and RX8 diff, but the case seems to be stronger while using the same FD internals. Also not sure on the FD, but on the RX8 the weak link is the axle stub through the wheel bearing. There was a big turbo 20B at 900-1000 whp in an RX8 chassis that never broke the diff or axles, but would shear those stub shafts off just before the wheel hub splines. The thing is though, the Ford 8.8 kits all reuse that same Mazda stub shaft on the RX8. So unless you need an R&P gear below 3.73 ratio then imo it makes no sense to swap over to the 8.8, but again not sure if the FD is the same other than the diff housing seems to be a lot weaker. Nobody seems to have that issue with the Rx8 diff housing.
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HPP 2in1 Diff Cradle / Brace

https://cimg6.ibsrv.net/gimg/www.rx7...1db4bca281.jpg
https://cimg3.ibsrv.net/gimg/www.rx7...ead51804b3.jpg
https://cimg4.ibsrv.net/gimg/www.rx7...54051a5320.jpg
HPP 2in1 Diff cradle / brace

The girdle is the obvious solution here, tried and true. It seems like this is where development has stopped, probably for a good reason.

Does anyone know if the gears break before the case, or if they break as a result of the case cracking?

As for it holding up to "normal" use, I want to be able to launch hard without worrying about cracking a case or shattering gears! Weight reduction comes with a side of responsibility I guess.

The couple I've seen in cursory inspection, the CWPs were still intact, when the cases broke from burn outs or launches. One was torsen and the other clutch pack, didn't pay much attention to that area though to see if any gear mayhem in the former.

If you wanted a diff you could throw anything at, I'd be looking at the off-road guys - 9" (and bigger!) is common place. Of course then, the rest of the driveline is suspect when abuse is piled on. Albins or possibly Harrop down here could probably make you an 8".....if you bought 5 or more! A sheet metal housing might be a more economical option for a one-off and there should be a few vendors in the US who specialize there?

I made this thread more out of curiosity than actual need at this point. Cracking cases seem like a reasonably common failure, I guess this is a possible solution nobody has tried yet. With all the focus on billet engines and upgraded transmission gears and mainshaft, it's probably only a matter of time.

“Reasonably common failure”? Maybe if you’re running 300+ width tires and/or doing drag launches with a really high hp car with hard bushings. Otherwise not in my experience. As a percentage of FDs out there, a cracked differential case is pretty damn rare. The only time I’ve heard of it happening were on drag cars…something the FD was never designed to do. Hence the differential cradles, Cobra diffs, cradles etc.
“A matter of time”? Maybe…anything’s possible with new manufacturing processes and marketing to those who just got to be the first on the block with something new, even if it fixes something that’s not broken.

You're contradicting yourself. Besides, find a car where you can repeatedly beat on the drivetrain without anything breaking and I'll show you a flying pig. I wanted to say flying rotary, but they already exist.

No, it's not "reasonably common". And your solution is to a problem that inherently doesn't exist. There are cars here and there that crack them, but if it were an issue in the way you're suggesting there would've been a solution 25 years ago.

It's common enough that rear end replacements are normal for higher hp cars, and quite a few companies make diff cradles and braces. I'll be making ~450whp with an EFR. I don't understand why people would spend the money on making power just to avoid launching and end up babying the car.. There is of course a difference between use and abuse.

E30/BMW medium case diffs are damned near bulletproof. I've had ~6 of those cars over the years. The Getrag transmissions were super strong as well. Weak point is the stub axles, I watched a dude in an M coupe @700whp shear the axle launching across an intersection. Diff was fine, same diff as the 6 cylinder E30s, medium case.

Okay so lets slow down a bit and have a conversation about what's being said. I also want to note this is my opinion versus your opinion. I am by no means an engineer nor have I ever spoken to a Mazda engineer in charge of designing the FD. So lets continue.

This is not uncommon in aftermarket applications for most cars, so the FD is no exception. Manufacturers (in some cases) aren't designing these cars to run double or triple the horsepower they come with off the assembly line, especially not in the late 80's and early 90's. There is a market for upgrading these cars (the 2 of us being prime examples) so of course there will be companies that support our needs, and I greatly appreciate them! However, that example is wants versus need. We want to brace our drivetrain BECAUSE we're running high horsepower. We don't need to braces our drivetrains because its a poor design. Two very distinctly different scenarios.

I'm currently running an EFR 8374, but I've never felt the need to launch my car. I spent the money because after riding in a big single FD, this was the direction I wanted to take my build...plain and simple. I don't baby my car in any stretch of the imagination, but I treat it like I paid for it and any damage done to the drivetrain will be my responsibility to fix...and I don't plan on fixing a diff anytime soon (knock on wood). The difference between use and abuse are subjective, but I believe cracking a diff is representative of abuse to some extent...that's just my opinion.

I understand what you're saying, but how many FD diffs have you cracked? How many have you even heard of cracking? You seem to be a pretty smart individual so I'll trust you're already aware of what I'm about to say, but I'll repeat it for future readers. You do understand how energy transfer works right? When you strengthen the diff, that energy sent thru the drivetrain is still going to find a weakpoint, just like you pointed out in your BMW example. You find your billet indestructible diff and something else will sheer off under load....then we'll be right back on this forum discussing how Mazda should made x-part better.

Re the last bit, the wheel/tyre isn't bolted solid to a wall/the ground. Tyre slip/distortion and elasticity in the driveline components may mean nothing further is likely to break at the semi-slick/small drag radial grip level. I will probably see if there is anyone local in Aus that does upgraded axles otherwise look at the driveshaft shop gear. At least outside drifting you would expect a diff to fail on launch, gearbox could be a bit scarier if bits catch in the wrong place at the wrong time.


I would think for the cost a "factory" diff case seems pointless compared to a larger IRS setup unless category rules require factory gear. I don't see any point going beyond a girdle with a factory diff.

For what it's worth, I'm a mild fan of the Gears and Gasoline guys, although I'm fairly sure he was running waaaay to grabby a clutch to be doing stuff like that and driveline shock is what kills these cases or any driveline component in general. In previous video's the mention it a lot being VERY on and off and unpleasant. I was REALLY surprised though because I do stuff like that on occasion and my diff had not exploded or shown signs of cracking on 255/40/17 300tw tires for roughly 3 years. That clutch needs to go.

I'm also never not going to have a sprung disc/heavy duty organic based clutch, the slip on the clutch while mildly painful is an easier fix for me than rebuilding/locating a diff case. The springs and face should absorb(slip?) the heavy driveline shock not your poor diff case.
I did stick the HP2IN1 brace/cradle on after sticking on some 200tw tires though as a precaution. I also went drag racing a little bit for fun so weight in the rear+brace = big win to me :icon_tup: Also the red puts the fear in people :cool: I actually went faster when sticking a 100lb-ish sand bag in the rear.
Also it does doughnuts and burnouts just fine :)

I think for value/time the brace and budgeting for a new differential(not diff case) if the stock torsen spider gears say no more, is the way to go.

If you really truly wanna stick 450+ horses to the ground though your gonna need a built driveline, no if ands or buts about it. The drag racing FD crowd 100% understands that. Search around there's tons of good stuff on here about building your drive line for that.

Although I 100% get, hate the feeling babying the car, so I usually work around it.

One way around it could be just run less power in first gear(if your ECU supports it), limit boost to 8-10PSI. You just can't safely stick that much torque to the ground safely at 0mph all 2700-3100lbs of the car is going to attempt to resist that multiplied torque from the trans/diff. Stockish FD's makes like 3000 ft-lbs down to the tires in first gear, it's ALOT to resist, your basically placing putting the entire car on that baby pumpkin. While I have no doubt it could handle it, shock/dynamic load is a whole different beast to static load.

You could also run a tekton slow release one way valve to basically automatically slip the clutch for you! It'll perfectly slip it every time with the right size orifice and let the car launch without overloading the diff. Although I've shied away from that because I feel like I'll lose the ability to do a skid/slide. It's good for consistently launching the car. I might build a hot swappable clutch line with it to test it out soon.

With the boost limit in first or the valve, your not "babying" the car but your also not wrecking your driveline either and can get what is effectively max acceleration that is available out of your gear. I use boost by gear on my ECU to make it launch perfectly on most unprepped surfaces first still absolutely rockets the car out of the hole but also won't explode my relatively small FD pumpkin. I can also still do skids if I get aggressive with the throttle/clutch it's not fool proof but it's effective.

One of the money no object FDs from years past, did run a BM case with Harrop gears....they seem like a bloody good thing!

You seem to be getting a hard time, but I could understand the possible need, with the hill work you mentioned previously. I know one the guys here bringing in cars to part out, he sells cases nearly before they hit the dock and one of those blow ups I mentioned earlier, was running twins on 255s - admittedly R-comps.

Raceonly sells uprated "1000hp" axles and tripod/CVs if you felt the desire. Seems a waste of time to me to upgrade those when stuck with the diff limitations though.

The stock setup with the PPF stresses the diff case, that's a fact of the design. There are already upgraded diff internals available, like you said CV and shaft upgrades, and there are transmission main shaft and gearing upgrades available from OS Giken and Liberty Gears among others. Plenty of people here are dropping silly money on carbon bits to drop weight. A nice billet case would keep the OEM PPF with the NVH benefits, drop weight, and could be made with a larger fluid reservoir and better cooling. If it's an alloy case there's no question heat dissipation would be better. You can absolutely slap an HPI or similar brace on the diff but that's a band aid even if it's a good band aid. It would be the best OEM+ style upgrade for the application, maybe not "cost effective", but neither is FD ownership in general 😉

​​​​​​A billet cover is $700, a diff fluid pump and diff radiator is a few hundred more and adds weight. An HPI or similar brace is $400+, call it $1500 all in. A billet case wouldn't cost much more than that I'd assume, and if you're upgrading diff internals anyway the new diff building and setup costs are a wash. You even get to use the existing aftermarket axles and diff upgrades that already exist in the aftermarket, no more cross referencing what parts will fit between 2 vehicles.

Either way, I have my answer. Nobody has done it yet. Cases crack, enough that it's a known problem, enough of a problem that there are a multitude of braces available.

These are the type of mods that put Eagle and Singer above the rest. It's a focus is the best bits, maintaining the intended direction of design for the car, hyper focusing and refining. We have been blessed with a fairly light diff and a light transmission, those aren't things I'd want to give up in a hurry.

And yes you would need the rest of the driveline upgraded properly. Boost by gear is a great option. If I could pick a driveline faluire point, I'd want the CV axle to be weak point, easy to replace, inexpensive and no fluids on the ground.

Mazda engineers did a great job, the whole driveline seems to have a similar limit. Exceeding that limit is our goal as mad hobbiest/scientist/engineers.

Front Billet 7075 Diff case
​​​​R35 GTR billet diff case
https://halo-performance.com/product...erential-case/
X3/Defender
Also found Dana locker cases, RZR 4x4 cases, etc. This isn't a totally new idea by any means, it just wasn't worth spending the money on a $10k FD but that's changing quickly

congrat’s on sucking so many people into falling for the tire kicker trap.

thought it was kind of odd that Sikky offers a Winters quick change differential rear subframe for the RX8, but only a diff brace for the FD3 LSX swap reusing the Mazda diff, maybe the FD doesn’t have sufficient space back there :dunno:

We're here to talk about 7s, it's a genuine question. I'm not coming to your house to look at diffs, you're choosing to click on the thread from the comfort of your couch!

A proper quick change rear would be the best. Space is tight with the fuel filter, tank and spare wheel well. But then you need axles, maybe a different driveshaft, etc. It's a ballooning project either way.

So, I guess it’s “reasonably common” if you have a lot of power and like to do “launches”. Should’ve said so in the beginning. I’ve known more than a few that made similar power as you and tracked their cars. But never cracked a diff or needed differential cases. But then they drove like adults on the street and “launches” out of the paddock weren’t part of the track day.
In full disclosure though I think some had a brace to control movement.

going back to this, recently had a peek at a 4 rotor car with a Winters. He had to cut up the subframe, and the stock tank definitely wouldn’t have fit

my Giken diff is going in next month, I’m on the lookout for a decent brace, does anyone make one that isn’t hard mounted?

For $1,500 you can have a bulletproof Ford 8.8" diff and not have to worry about the diff or PPF breaking ever again.

$950 - Ronin Ford 8.8" IRS mount.
$250 - Ford 8.8" Aluminum differential with 3.73 gears.

If you care about how your car handles and puts down power, a good FD LSD ranges from $1,000-1,500.
By comparison, you can get an Detroit TrueTrac LSD for the Ford 8.8" for $660, add a $200 Ring and Pinion of any gear ratio you want, and still be $100-600 ahead.

RHDJapan has the OS Giken for ~$940 with the current exchange rate.

Ronin 8.8, you need the kit with axels, so it's actually $1,450. Subframe needs to be removed to weld on the diff mount, good on you if you have a welder and the skills to get it done. Let's say it's $300 to get it welded in on the bare removed subframe. You also need a driveshaft, either custom or have yours modified and balanced, we will say your local guy can do it for $500. You also need an LSD, we are comparing a new motorsports center to a junkyard diff, but let's say it's $200 for the diff and $150 for new plates/clutches and you're building it yourself. You also need the CV joints, let's say you got lucky and they are free with the diff. You also need 4.10 gears to make it comparable, so $100 for used ring and pinion.

That's a $2700 diff install, vs ~$1,000 for the Giken. Unless you need the extra strength it's a bit of a waste. Unsure what the total weight difference is.

If your exhaust passes really close to the diff, do you need to thermally isolate them? It certainly doesn’t need an extra heat source right next to it…

Or am I just being paranoid?

Billet parts look cool. There’s usually little or no performance benefit over mass produced cast parts, but I think the fact that they’re often one-off or low lot production adds a certain mystique. They get used on highly customized high performance builds, so they’re cool by association.

Casting a large part is difficult and expensive and is weaker for the weight. You either make a ton of them and get costs way down, or you end up paying a foundry a lot of money for just a few, at which point it's better to just make a billet part as needed. Think you can sell 500 cases? Might make sense to go cast if you don't care about the weight penalty. If you want just a few, then billet is the way forwards.

The whole premise of this thread is false. In an adult-driven FD, even with significant power mods the stock differential is fine.

If you prefer drag launches, burnouts and general hooning then fine, get the differential of your dreams. But I doubt being “billet” will keep things from breaking for very long.

Of course a properly designed billet case will stop the case from breaking, why would you suggest otherwise? That's its entire point of existing.

What does adult driven mean to you? Adults don't go to drag strips or drift events or track days or do burnouts? Sure if you drive calmly on stock tires and spend your Sundays wax on wax off, you'll never have a case issue. No idea why that makes this thread a false premise but hey you do you and I hope your case lasts forever

I would wager that it is probably a good use of your resources to do so if you are experiencing issues with respect to differential temperatures. There was a thread somewhere on here where someone was overheating gear oil and subsequently having issues related to it on their road race first gen. I believe they ended up having to plumb a cooler and gear oil pump and create some heat shielding for the affected area. I would imagine this would be even more beneficial for a turbocharged vehicle given the relationship between forced induction and exhaust gas temperatures.

With this thread having originated as a query regarding the existence of a billet differential case option and then transpiring into a discussion of the merit of such a development, I believe this to be more productive than spending tens of thousands of dollars on "rare jdm and OEM + parts" that at times can be as overpriced as they are under engineered in the most extreme cases. We often correlate exclusivity with the value of a particular thing. I could go spend $10k on some nearly 30-year-old seats that got yanked out of a wrecked Spirit R and nobody here would bat an eye, but someone wants to explore the merit of alternative equipment designs and material selection and he's out of his mind. :confused:

We also know that the OE diff case can be a mode of failure that has been demonstrated with non-trivial frequency when subjected to the operating conditions given by the OP.
I think the last bit of information above may not have been fully understood by some. A common counterargument would be that the conditions that the drivetrain is being subjected to were not within the scope of the OE design constraints. They are correct.

Now that we have established that we wish to use equipment in a manner that it was not intended to be used, we must ask ourselves what needs to be done to make it work for what OP wishes to do. It was identified earlier on in the thread that billet/subtractive manufactured products tend to be more robust than their cast counterparts...

The general recommendation for differential reinforcement would be a girdle or a brace, but these options are outside the scope of this discussion.

We are now brought to a common conundrum faced by hobbyists such as ourselves, who may wish to solve an issue in a particular way, but the infrastructure/industry for it is not present.

The recent influx of consumer-grade 3d scanners and 3d scanning services may prove to be beneficial... Combine that with a CAD program of your choice (and someone who is capable of making designs if you aren't) and you now have a method of achieving what you wish to do.
Of course, this would require some work on your end and would likely cost more than all of the other commercially available methods of differential reinforcement or replacement. BUT you would get exactly what you want, which may very well be priceless. :nod:

Apologies for being so verbose and philosophical/theoretical, but I felt it beneficial to explain the why behind my reasoning in an effort to help others understand how I reached my conclusion(s) :lol:

I think for moderate abuse a rebuild stock diff with solid pinion spacer, LSD, case girdle (cheap, strengthens housing and increases oil volume and surface area) and diff to floor brace will cover most people's needs. Unless you are trying to fit in a racing class rule set I think a billet case is pissing in the wind, like spending over 3K for a billet steel s4/5 bellhousing, then a gearset, if you are going that deep get a bigger/ better item to start with.

The ronin option is quite costly. But you do also get the axles upgraded and it opens up the door for many types of gear ratios. It's definitely not needed for everybody, especially since the stock diff works great and most cars don't have enough torque to brake them on a 2 rotor unless you are really launching hard on a drag strip. It's a really nice upgrade for those that have a lot more torque than stock though.

Th stock rx7 drive line is plenty tough, or upgradeable. Pretty sure it's been upgraded to run 9's since way back when.
The reason most folks move to 8.8 is for the selection of R&P gears.

^Sheesh, another another Sunday “wax on, wax off” guy who obviously doesn’t know how common the differential case fails.

and you don’t seem to recognize that this is a time suck thread on a pointless topic

Yes, and for $1,500 on the stock diff, you still have (and are limited by) the stock axles.

For an apples: apples comparison:

$1,450 - Ronin Ford 8.8" IRS mount and axles
$300 - Welding (optional cost)
$250 - Ford 8.8" Aluminum differential with 3.73 gears.
$500 - driveshaft
$600 - Detroit TrueTrac LSD
$200 - gear ratio (optional cost)
= $2,800 - 3,300 for an amazing LSD that doesn't make noise, with a rear end that you can't break. (A 3.73 which is more ideal at higher power levels that will break a stock FD diff.)

$700 - Billet cover
$400 - HMI Brace
$1,000 - OS Giken
$540 - Upgraded axle bar (driveshaft shop)
= $2,640 for a chattering LSD that will still break far before the above option.

Where did the $700 for billet cover come in? You can just run the stock cover and a diff girdle like the one shown above. I don't think you need the OS-G LSD either. You can run a t2 LSD, if you don't like the viscous unit on the FD.

There's plenty of LS-swap guys running the stock FD diff setup with no issues...

Unless you want to run 9's on drag slicks, or really need a weird ratio, there's no strong reason to use the 8.8. Don't get me wrong. It's a great option. It's just not needed for 99% of FDs.

This is all you need to protect a stock diff case and also the PPF.
Plus your grandmother can launch the FD like a bandit....no wheel hop.

I'm not sure where mr2peak came up with that number.

The FD has a torsen LSD and it's nowhere near as good as an aftermarket diff. But now that i'm getting older, the 'popping' of aggressive clutch type differentials is no longer acceptable to me and even the OS-G is not a great option anymore. I would be far more interested in the new Wavetrac LSD in an FD, but that will likely also be $1,000-1,500 for the diff alone when a TrueTrac Torsen is $600 for a Ford 8.8".

I don't really see a need or market for a billet FD RX-7 differential case (which will still be limited by the smaller 8" ring & pinion) when you can be all-in with famously strong Ford 8.8" from an explorer and the Ronin kit for $1,700 in parts and have very inexpensive options of LSDs ($600), gear ratios ($200), etc...

I didn't come up with $700 for the cover, that was Billj747 in post #43, not sure why he's pointing at me? However, that is what they cost. The cheaper options, like the Greddy, are sand cast.

This board exists for us to talk about these cars, if you think talking about FDs is a time suck pointless thing, go away. You don't even have an FD, you being in the 3rd gen section is pointless.


OS-G super lock shouldn't chatter, but it's also one of the most expensive options. The KAZZ is known to chatter, less plate clutch pairs with a shorter life, but plenty good for most people.

Sure, the 8.8 is a great option if you don't mind cutting up your subframe, welding, and moving on from the stock style parts. Some of us, for better or worse, want to keep the FD as true as possible to its origin. I'd argue people looking for an upgraded FD trans with a billet main shaft vs swapping to a CD009 or similar, might want to stick with the OEM style housing along with a true motorsports clutch type diff for reasons other than pure rationality. Modding cars isn't really rational anyway, try justifying why you own an old rotary without resorting to "because it's just so cool".

Billet diff housings exists for other platforms. I guess those guys are all idiots for not just swapping in an 8.8?

Should we all just LS swap? Hell, just go buy a Tesla and get over-the-air mods..

@tomsn16 , how many of those girdles do you think you have sold? And how much do they weigh? I'm guessing you've sold quite a few due to the FD case failures, and you're far from the only vendor to offer girdles or braces (not an attack, it looks like a great part. We are lucky to have options such as yourself).

Billet case would lose weight, gain strength, and aluminum will carry away heat better than the stock steel case, increasing diff life. Designed properly, you might be able to skip running a diff pump and cooler. That's more weight savings and less complexity. I also realize the 8.8 diff case is already aluminum.

We have supplied 265 HPP 2in1's since 2012.
No known user diff case failures.
Weight is 8 pounds

I am having problems with leaks, mostly.. which I think could be related to heat?