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?