When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
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.
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
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.
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.
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.
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.
.
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.
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!
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.
Originally Posted by mr2peak
Cracking cases seem like a reasonably common failure, I guess this is a possible solution nobody has tried yet.
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.
Originally Posted by mr2peak
It's common enough that rear end replacements are normal for higher hp cars, and quite a few companies make diff cradles and braces.
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.
Originally Posted by mr2peak
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.
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.
Originally Posted by mr2peak
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.
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.
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 Also the red puts the fear in people 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.
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.
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.
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.
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.
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
12-09-21, 09:09 AM
Also the red puts the fear in people 
I actually went faster when sticking a 100lb-ish sand bag in the rear.
