FD modifications may eventually lead to a focus on the rear end/rear gears. and of course "8.8" emerges as settled science.

maybe.

before taking the XXXXX$ plunge consider that most of the rear end failures (broken diff cases) have occurred due to the fragile spider gears. break the gears, break the case. i don't recall stripped pinions. that's why i run an S6 T2 plate type LSD. out w the spiders, in w the plates. happy OE rear gears.

while i am currently running 600+ i have never done a first gear launch so my trouble free usage should not be necessarily supportive of my suspicion.... that our gears may be fine but our LSD def needs to be changed in many apps.

another very important link to a trouble free driveline is the clutch material. the new ACT Mod Twin ORGANIC sprung double disc is waay easier on the rear gears than any cerametallic disc and that includes the Exedy double.

if i was building another FD i would keep the rear gears, swap out the torsen/spiders and run the ACT organic double.... and save 5+K. my bet is i would have no rear gear problems.
actually, i am building FDs, just the motors. the last 5 motors i have built have left w ACT Mod Twins.



this is just my opinion and i would be very interested in feedback... caveat: i am not saying the 8.8 isn't stronger than our OE gears, rather that OE gears should be fine for most usage with a change in the LSD.

unlike the trans... take the 8.8 money and spend it on a different trans.

(i initially posted this in the Suspension Section but didn't receive any on point comments)

 

In Australia/Nz where alternative IRS rear end/gear combinations aren't as cheap or easy to source:

Solid pinion spacer rather than crush tube.
Aftermarket centre.
heavier rear cover or brace that clamps rear of housing and nose or girdle like Green Brothers sell.
Solid bushes not oil filled.
diff brace.
decent coilovers.

Reducing wheel hop seems to help.


I think running a spool, semis and clutch banging Mike Whiddett had factory diff casings as a consumable part though.

 

" running a spool, semis and clutch banging"....with an OE gearset and Torsen spiders would lead to rear end failure, but for the other 95% of us that aren't at all concerned w 60 foot times the following should do the job at 600 rwhp:

friendly LSD
organic clutch
aftermarket diff and front lower link bushings
diff brace
aftermarket coil overs (BTW, love my MCA Pro Sport Aus coil overs)

way cheaper and not needing to butcher your subframe

 

I will add that adding a late model RX-8 rear cover is a relatively common and lowish-cost mod that (in theory) adds a bit more heat capacity to the stock diff.
Though I'm not sure how effective that little heat sink is compared to the Greddy rear cover, which greatly increasses the oil capacity.

 

Does that translate to what MCA Reds are here?

I'm happy with mine, better ride quality than the Jap spec Ohlins(?) it had in it and much better body control.

 

"Pro Sports" are in the same neighborhood as the Reds but down a click or two so as to be a bit more street friendly. i totally love them. after a lot of research.... suspension is always my number FD focus... i was sold on them because they are literally designed and built by RACERS. racers who WIN routinely on road courses in Australia. their site says it best and there is a lot of tech content on the site:

"who we are

MCA Suspension is an Australian family owned and operated company founded in 1991 with a strong and successful back ground in highly competitive motorsports. Our founder Murray Coote has been an Australian Gravel Rally Champion and led the MCA World Time Attack team to multiple World Time Attack Challenge outright wins.

Years of testing and development from Street Use to Time Attack racing has led to our latest valving style, Fusion V3, which offers less compromise and more versatility than ever before. All of our products are designed and assembled by hand in Australia. We are not your usual suspension brand, we are not just suspension specialists, but are also highly successful car builders and drivers too. The approach to our products is quite different to the norm, but is very practical and logical. We put the time and energy into the parts that really matter. We also strive to offer the best customer service and warranty support in the industry."

i am a North American dealer for MCA, but more importantly, a proponent of their product.

 

Ric Shaw fitted mine when I was living in Sydney, he used/s MCA on his Nurberg 24hr cars, Targa Tas/SA/West cars, I think the Aus RX8 series cars too.

I was certainly impressed at the ride/control compromise compared to off the shelf Korean and Japanese stuff I have ridden in other rx7s and silvias, you don't pay much more for a significantly more refined product.

 

Hey Howard, I really enjoy your posts and that you're one of the few people USING their car and constantly providing feedback, data, and insight to the community.

I wrote a few articles on the differential topic:

When to upgrade the differential:
https://motoiq.com/project-fd-rx7-re...-differential/

Building the Ford 8.8 differential with a Detroit Truetrac LSD and WPC:
https://motoiq.com/project-fd-rx7-re...c-lsd-and-wpc/

Installing the Ronin Ford 8.8 IRS Mount Kit:
https://motoiq.com/project-fd-rx7-re...irs-mount-kit/

The conclusion from the first article is the least expensive way to reliably handle 400whp is to use a Turbo II LSD & use a differential brace and 600whp is the Ronin Explorer 8.8" diff kit.

Taken from the first article:

Average Power Limits of FD RX-7 Components**:

300-400whp – Stock Torsen LSD.

400whp – Power Plant Frame (PPF) and Transmission Output Shaft.

400-550whp – Stock Inner Axle Stub and Axle Bar Shaft (400tq limit).

450whp – OEM FD Iron Diff Housing.

450-550whp – Turbo II S4 (86-88) Clutch-Type LSD.

550whp – Braced/Cradled FD Iron Diff Housing (with clutch-type LSD).

600whp – Stock Axle Outer CV Joint.

600-700whp – Limit of 2003-2004 Mustang Cobra “Terminator” Aluminum Diff Housing (31-spline, diff mounting ears fail).

600-1,000whp – OEM Mazda Subframe Distorts.

800whp – OEM Wheel Hub.

800-900whp – Limit of 2002-2005 Explorer Aluminum Diff Housing (Single Ear).

800-1,000whp – Driveshaft Shop “Direct-Fit” Axles (with OEM Wheel Hubs).

800-1,100whp – OEM Subframe with Ronin Diff Mount starts to bend (requiring Ronin Subframe Supports to brace it).

900whp – Braced/Cradled FD Iron Diff Housing (with Spool and upgraded axles) – on borrowed time.

1,000whp – Braced/Cradled FD Iron Diff Housing (with Spool, upgraded axles, and a custom rear cover that retains the main caps) – on borrowed time.

1,400whp – Driveshaft Shop “Pro-Level” Axles.

1,500whp – Limit of 2007-2010 Explorer Aluminum Diff Housing (Dual Ears).

You make great points about the clutch and usage of the car. For your half-mile use case, not launching the car from a standing start, and not having that much time under load in 2nd or 3rd gear (relative to continuous lapping on a road course) it's great to see your setup being quite reliable. It would be interesting to see how your power level and setup lasts on a drag strip or constant road course use which would trigger a failure point much sooner.


COST:

Turbo II LSDs are getting hard to find (i'm struggling to find them for sale anymore) where they use to be easy to find around $300. Now, the most readily available option are $1,295 ATS $1,450 OSG, or $1,675 ATS Carbon clutch type LSDs.

Upgrading factory diff housing:
$1,295 - ATS clutch type LSD
$330 - HPP Diff Cradle
$560 - Sikky diff brace
$83.75 - OEM pinion bearing
$153.5 - OEM side carrier bearing
$15.75 - OEM Diff pinion sleeve
$28.5 - OEM axle seals
= $2,467

This setup will be limited by the inner and outer axle CVs or axle shafts in the sub 600whp range. Upgrading these can easily add thousands of dollars more if/when they fail, positioning people in a tough spot to either accept that they threw away $2,500 and then go with a Ford 8.8, or keep throwing money down this rabbit hole.

Ford 8.8" Diff
$1,450 - Ronin Complete Kit:
$275 - Ford 8.8" rebuild kit
$541 - DSS driveshaft
$265 - Ford 8.8" explorer diff housing
$675 - Detroit Truetrac LSD
=$3,206

SELL OEM Diff: - $550
SELL OEM PPF: -$180
SELL OEM Axles: -$300
SELL OEM Driveshaft: -$100
= $2,076 - ACTUAL COST OF FORD 8.8" DIFF SWAP (after selling OEM parts) to reliably handle 600whp

OPTIONS:
$769 - Ronin Grande Kit - axles (1,000whp capable)
OR
$1,800 - DSS "Direct-fit" axles (1,000whp capable) - easiest installation.

This setup has one of my favorite LSDs, and will be reliable at 600whp for a few hundred dollars less than upgrading to a clutch type LSD and bracing the stock diff. If you want a bolt-in axle solution, a $1,800 premium for something that you never have to think about again (at 1,000whp) was worth it to me, or you can spend $769 for the Ronin "Grande" axles if you want to save a grand and do a little more work.

I hope the above figures and breakdown helps you and others see what I did, and have a better idea of the different routes to make the best decision for themselves.

 

"Ric Shaw fitted mine when I was living in Sydney, he used/s MCA on his Nurberg 24hr cars, Targa Tas/SA/West cars, I think the Aus RX8 series cars too."

my first year of road racing was open wheel and i learned that you can get killed really easily when you touch wheels... especially back when fuel cells were absent etc. OTOH, fendered race cars were the exact opposite. if you were up for a bit of contact, no problems as to mortality generally.

an entirely different race.

i also learned that it makes little difference as to absolute speed if you are into actual RACING. what matters is that there are a whole lot of relatively equal cars to race with. you are literally racing for inches.

like this
:

or this



and every RX8 you see is running MCA coil overs as they are a co-sponsor the series.




here's another Ric Shaw prepared car running in the Bathhurst 6 hour... note the hood decoration



another Ric Shaw prepped car on MCAs with the crew... serious road racing here MCAs onboard



this is one of MCA's actual (house) cars. Time Attack. i thought F1 was big on aero: over one thousand hp.



i haven't scratched the surface here w re to MCA but note that you have other solid options rather than "O" or Taiwan junk.

take a tour:

https://mcasuspension.com/

learn a few things:



.



Billj747, thanks for your info. it is a big help to the thread and i am slightly sorry for the MCA detour..

i view the FD strictly as a dual purpose road racing potential supercar. (weight and suspension dynamics make it potentially so). most realize that on a road course, given our realistic tire size options and safety concerns (lack of a fuel cell 6 point cage) 450 rwhp is the number.. this is for a dual purpose configuration. a properly setup 450 FD is capable of running w just about any dual purpose competition. i know this from personal experience. i bought my FD in '99 and the same rear gears are onboard today. i did run four Ferrari Club weekends at Brainerd where turn one was 160 mph and no need to lift until turn two. i beat every car i encountered. probably an hour and a half of track time daily for each weekend. eventually my attention turned to the Texas Mile. since a 550 rwhp LS Fd ran 196 i figure 575/600 will easily get it done. once i accomplish this i will swap my G40-1150 for a smaller Garrett. i do have DriveShaft Shop 300 M half shafts but only because i don't want to go sideways at Speed. i bet i have 500 third gear pulls since 2013. all is well in the back of my car.

i am guessing it all depends on how you use your car. my suspicion is that the stock rear gears will work for most.

 

My FD is also a dual purpose street/road race car. I've driven a lot of supercars at Buttonwillow raceway in CA and set a few records there, and often the fastest lap of a given supercar at that track. My goals for the car is to turn lap times in the same ballpark as modern supercars. It will be interesting to see how close I can get the car to GT3RS and McLaren 600LT lap times once I start stacking aero on the car. It will probably need well into the 500whp range to achieve this.

When you made 500 3rd gear pulls, what power level were you at and with what modifications? (Turbo II LSD, diff brace, PPF, 300M half shafts, etc...)?

Since you have 300M half shafts you're well down the rabbit hole and probably have everything you need to keep the stock diff reliable. The issue seems to be more of the weak iron casing than the gear itself. I don't believe you can get 300M half shafts anymore and from my research a year or two ago, the only real options is the insanely expensive $3,568 DSS "Pro" axles for the stock diff housing. At this price there is absolutely no reason for someone new who wants to make 5-600whp to go down the path of reinforcing the stock diff and to just swap to the Ford 8.8" Explorer housing and Ronin kit.


After all of this research and going down the process of the Ford 8.8" swap, I have come to the following conclusions:

• The 400-450whp range (and mid-high 300lb-ft of torque), starts to become the risky zone for the stock FD RX-7’s drivetrain that doesn’t does more than just play around on the street.
• Staying below 365whp (310lb-ft of torque) seems to be a good rule of thumb to maintain reliability of the stock trans and diff when driving the car aggressively.
• The Ronin Ford Explorer 8.8” Standard Differential Kit is reliably capable of a far greater 600-800whp (conservatively) and consistently runs 8’s in the ¼-mile. They also offer a “Grande” upgrade kit that replaces the FD outer CVs with Explorer CVs to further improve durability and power potential to over 1,000whp.

Therefore:

• As soon as you consider spending $1,295+ on a clutch type differential, get a Ford 8.8" diff.
• As soon as you want a reliable 400-450whp+ when beating on the car at the track, drag strip, drifting, launching it; get a Ford 8.8" diff (unless you can find a Turbo II LSD)
• As soon as you want a reliable 500whp+ when beating on the car, get a Ford 8.8" diff

*You can have a 'reliable' setup if you're not launching the car, clutch-kicking it, have a good clutch that's not aggressive, drive the car with good technique, or just drive on the street.

 

good info Billj747.

"once I start stacking aero on the car"

aero is a gamechanger and of course requires a totally diff suspension setup. and it can get pretty crazy w cars looking more like airplanes. it certainly changes lap times. i can understand it as applied to time attack but don't warm up to it in actual racing. that said, i am currently focused on nose lift at the end of the mile. too much lift and you become an airplane. given my just under 700 front corner weights take off speed is a bit lower than the 1000+ pound corner on a Mustang/Camaro.

once up to speed, Indy cars can drive on the ceiling w 3X downforce to weight. i remember watching the trees on both sides of the 60 foot wide Road America mid straight bend as an Indy passed.

"When you made 500 3rd gear pulls, what power level were you at and with what modifications?"

initially (2013) 575 w a GT4094R including a 5th gear (Tremec T56) run on the dyno to 8630 205 mph.

2018-2020 low to mid 500s w my EFR9180.

600 to 670 w the Garrett G40-1150. roll on throttle to about 8400. peak torque 6200-6500. 430. this is nowhere near the full potential of this magnificent turbo. power was generated at 23 psi. i will be lowering the boost a bit going forward. i will say that i am pretty careful as to gear changes and my Mcleod RST organic is very friendly to the drivetrain. Magnum F swapped in along w the G40-1150 2021.

 

This will go a long way to ease abrupt engagement breakage:

https://tiltonracing.com/product/flow-control-valve/

I’d argue that an aftermarket clutch-type LSD is still needed regardless for anything more than general street use.
.

 

That could be an interesting solution to reduce the drivetrain shock from aggressive clutches.

I would say for the cost of an aftermarket clutch-type LSD in the stock iron differential housing, it makes more financial sense to go with the much stronger, aluminum Ford 8.8" differential.

For the Ford 8.8" diff, the Eaton / Detroit Truetrac torsen LSD is my go-to recommendation and one of my favorite LSDs on the market for the street or track. It's characteristics that I loved in Mustangs works perfectly in the FD. It's also half the price of a typical aftermarket clutch-type LSD or a Wavetrac (which i'm about to test in my NSX and replacing my OS Giken with).

 

here's the dyno chart 205 mph/330 kph in 5th gear. the graph covers 11 seconds in fifth around 575. i was in the pass seat holding the logging laptop and hoping the straps would hold and there wouldn't be a 5th gear burnout into the concrete wall directly in front. this is w the first original CPR low drag turbo manifold and my build motor. all happy after the run including me.

 

I’m an OSG fan myself, but as long as a drive wheel never lifts or bounces off the pavement then your preference is likely fine.

 

I use to be a big OSG fan and we had one in the FXMD NSX Time Attack car that I set many records with around the country. They're great diffs but i've done a lot of work to the one in my personal NSX and it will still occasionally grab/clunk/chatter despite extensive efforts to make it less aggressive for the street.

Your comments are actual problems for many Quaife torsen differentials (which I had those issues in a 350Z time attack car) and the Torsen T2 differentials. However, the concern about forward drive issues when the drive wheels become unloaded or 'bouncing off the pavement' is not a practical or realistic issue for the Torsen T2R or Eaton / Detroit Truetrac torsen differentials.

Unless it's a dedicated racecar with commitments to the time and effort of trying various ramp angles, clutch pack configurations, and preload, I would choose the Truetrac over the OSG in the Ford 8.8" differential. Off-the-shelf, I would put the Truetrac in a Ford 8.8 over an OSG almost everytime. I'm looking forward to testing the Wavetrac to see if that enters the chat and eclipses the OSG in those regards as well.

 

well I knew you’d pop up based on past conversations between us on this but in all transparency I haven’t used that particular LSD. Just going by what people who have told me. I think here and in the other thread we’re basically agreeing though; street vs. race.
.

 

I wouldn't put it as black and white as 'street vs race'.

A great real-world example was Round 1 of the IMSA IMPC championship at Daytona. The differential of the car I was racing was rebuilt and the configuration apparently changed which drastically threw off the handling, balance, and predictability of the car. Preload of the clutch plates were different and based off of the handling of the car, the ramp angles could have been installed backwards by accident. Regardless, we wasted many sessions and tens of thousands of dollars of on-track time chasing an issue that was solved by putting in a different differential that behaved normally.

So even in pro-racing, clutch-type differentials can be a very powerful tuning tool, but it can also take a lot of time (or waste a lot of time) if a mistake was made, or along the path of learning and optimizing the setup. Meanwhile, the open-differential behavior of the Eaton Detroit Truetrac Torsen LSD would not have the same mid-corner understeer-inducing characteristics of the preload that is in the clutch type differential. Since there's nothing to adjust and because the bias ratio and power-down ability of the Truetrac is so good, we wouldn't have lost or wasted this time.

After daily driving the FD with the Truetrac for the past two days, I couldn't be happier with the power-down characteristics, or the natural and smooth break-away characteristics of the chassis as the car transitions into oversteer. I have a hard time seeing how someone with actual first-hand experience would criticize this LSD.

 

All I said was that it’d spin the wheel if lifted or bounced off the pavement. Will defer to your direct experience though.

 

What do you think is a good preload level for an FD?
I guestimated my break-away torque to be somewhere around 9 kgf/m with a torque wrench.