hey, just wondering if anyone knows about FC diffs.
i have a jap-spec Series5 rx7 and i've been trying to work out what diff i've got so i can upgrade the centre.... from what i can tell it has a 4.3 ratio (turning tailshaft counting revolutions to do a full rotation of wheel).
I always thought S5 only had a 4.1 either viscous (turbo) or open diff. But after a bit of research i found that the INFINI rx7 came with a 4.3 viscous lsd? and also a bigger 5th gear to make up for the reduction in final drive (anyone else heard this?). So now my main question is does anyone know if the 4.3 used the same diff housing as the viscous 4.1. im assuming it'd just be a different ring/pinion but want to be sure.
Also what centres are a common swap?
i've been told to get a FD torsen centre and apparently these bolt straight into the s5 housing, just need s4 output stubs or something?

 

I though the 4.3 was ONLY in the n/a housing (smaller than T2) but I might be wrong.

although I thought there are a ton of different ring/pinionsets available thru mazda for either setup.

 

actually, the Infini IV comes with the most sought after stock diff for road racers, the Torsen Diff paired with the 4.3 Final Drive, other than that, I don't know if the gear ratio in 5th is changed to compensate or to exacerbate like the GTUs had.

GTUs had the 4.3 and a .752 5th (compaired to the .697 5th that other NA's had). The 5th gear change actually increased the cruising rpm's even further; if it was used to compensate, you would see a number lower than .697.

J-spec cars got a lot more goodies than we did, especially that Torsen diff. I wonder if that diff is the same as the FD unit?

 

If you want a 4.1 gear then I am sure lots of people would trade with you. I would stay away from the FD torsen...there is a reason those guys break diffs at 400rwhp and several of them do the swap to the series 4 diff/lsd for greater strength and a (supposedly) superior LSD.

 

OF all the luck, here is one of them today: https://www.rx7club.com/forum/showthread.php?t=713661

 

Actually, that 4.3 rear end ratio is a lot more common than you think...
Japan cars do not need the top end speed, so a lot of the FC's had them.
I've heard of "base models" having the 4.3's.

The US-spec FC's needed them due to CAFE gas mileage requirements, so that's why you don't see them here.


-Ted

 

JDM '99+ FD torsen is stronger than the previous years, if you ever wondered....

 

Superior, IMHO, in its lockup characteristics, not its strength.

For drag racing, S4 TII or other clutch-types are the way to go. Even for some autocrossing if you're lifting tires, while cornering. But if you have all 4 tires planted around a road course, I'll take the torsen anyday of the week.

 

when was the last time you heard of anyone breaking an FC turbo diff, strip or otherwise? There are at least a couple of FD guys on the forum that break their diffs every year, but I honestly cant think of one single FC turbo diff that someone posted that they broke.

 

I haven't really, and FD torsen's are much more fragile, but I don't really hear many of the circuit guys breaking torsens.

At power levels under 300-350 whp, you should be safe with a Torsen, maybe even moreso with an S8 diff since they seem to be "stronger". (Hearsay)

 

FC clutch-type LSD's don't blow up - they just wear out.
Since it's worn out, it can't blow up since it's basically an open diff.
I've had my FC since 80k miles, and the LSD never worked.
The consensus is that the stock units wear out in a paltry 35k to 50k miles.

I prefer Torsens cause I like to finesse my driving, rather than use brute force.
My kick with drag racing is over, and we don't have a track anymore over here.


-Ted

 

All FC Turbo diff housings are the same regardless of year or country of sale.

S4 into S5, FD into FC or aftermarket into FC.

Correct. I put one in my S4 and it was a direct swap.

That's simply not true. Apart from the very small number of Infini models, Japanese FC's really didn't get anything that US FC's didn't also have. The exception would be digital climate control, but again that was only fitted to a small numbers of cars (and 80's climate control was crap anyway). In fact I think in general US FC's fared a bit better than Japanese ones.

It is. The Torsen centre used for the Infini IV was carried over to the FD. Or more accurately, the Infini IV used the Torsen centre Mazda had just developed for the still-in-development FD.

Plenty of FC owners are using FD centres with no problems, because they're not making 400rwhp and doing lots of hard launches. Until strength becomes an issue (and for most of us it doesn't) the Torsen is arguably a better diff for a street FC. And if you get a good deal like I did they can be cheaper than the cost to rebuild an S4 LSD.

Nobody's questioning the FC diff's strength, but that strength isn't necessarily an advantage for most FC owners, whereas the Torsen's lock-up characteristics are.

 

I have a 4.3 mazdaspeed 2 way...

I think I have fucked it though .. still locks up decent for now... but think i am going to mabye replace it soonish.

when i am doing about 7200 in third i am only doing 140-150 ish.. (i have no speedo)

 

That sounds like a 4.1?
US-spec cars do about 30mph per gear, so in 3rd would be about 90mph...which is close to 150kph?

Ever do a 5th gear top speed run?
You should top out at around 250kph at redline...
Radar gun?


-Ted

 

In m GTUs, I think I'm barely under 100 redline in third, with the 91, I think I can get to 105-110 before I have to shift.

I like my 80's digital climate control!, btw.