Its an old question with the same (pinion) problem... Has anyone come up with a solution to lower the gear ratio in a TII diff IE from 4.10 to 3.903???

 

You want to go down from 4:10 to 3:90's?

 

The number is lower, but the ratio is higher? What are you looking to accomplish?
On a S5 at 60MPH, stock 205/55-16 tire:
4.10-2390 RPM
3.90-2275
4.30-2506.
Hardly worth the effort.
Increase tire diameter to 215/55-16 2348 or 235/50-16 2354, then there is only about 75 RPM difference.

 

i used a 3.909 Na LSD from an 88 Auto GXL. it's like having a fat girlfriend to appreciate a skinny one. i miss close gears. you're clutch will hate you on inclinded starts. still running it til i rebuild my diff.

 

Well, for a turbo car the purpose of going to a 3.9 is lengthen the power band. A 4.10 will accelerate fast but you are moving through the power band “so” fast that you are not optimizing it. Second my car is a never going to be dragged.


I used the number 3.9 bc they are common to rx7's but it could be lower (no lower than a 3.3 ratio)...


That being said does anyone have any new info on what ring and pinion can be used in the TII diff, or know of any companies that are willing to produce one on request...

I heard (unconfirmed) that some TII automatics in JP came with 3.9 but I am not sure and don’t want a radioactive diff (just kidding).

 

Honestly, anything other than a 4:10 is probably going to be a full custom job unless you source a read from Japan.

Really better off to stick with the 4:10's. A 3:3 would give you incredible top speed, but there's a reason the stocks are 4:10's. And that is because these cars don't have the low RPM torque to get moving easily.

 

I have thought about this myself. For turbo cars with significantly larger turbos than stock, it makes sense. For example you won't have to shift out of 4th at the very end of the 1/4 mile. With the stock gearing you have to shift around ~120 depending on how accurate your speedometer looks, your tire size, etc.

The FD automatic diff has a 3.9:1 ring and pinion. And I know the T2 clutch-type LSD can fit into the FD housing. See this thread: https://www.rx7club.com/3rd-generation-specific-1993-2002-16/differential-teardown-tii-lsd-install-writeup-535290/ I imagine you will have to set the backlash and all that stuff.

Find a used FD automatic diff and then pull it apart. I bet you can make it work. From the pics in that thread you can see the pinion gear once the assembly has been gutted:



and the ring gear + FD torsen LSD:



those are the 4.10:1 FD gears in the pics. There are a number of manual transmission conversions for the FD with the 3.90:1 automatic diff.

 

Mazda has 4 differential "families"

the 6" diff, T series. this is the rear diff for the 323GTX/90-93 miata. it sucks, its weak in those cars...

the 7" diff. M series. this is the common one, its used in the r100/rx2/3/4/5/non turbo rx7/94-2005 miata, etc etc anything that is RWD, 4 cylinder or Na rotary has it. honda even made a normal rotation engine and use this gear set in the S2000.

the 8" diff. P type. 8" ring gear is the v6/turbo rear end. so its T2/FD/Rx8/929.

the 2006+ miata and mazdaspeed 6 use some new funky thing, i can't find any info on it.

anyways, the FC uses the P type diff, EXCEPT that the FC has a very long nose, and very long pinion to mount it in the car. so the only bolt in gears for the FC are FROM the FC.

the NA guys have taken a "normal" gear set and the FC gear set and you cut both pinions, and you put the long part of the FC pinion onto the gear and weld it together.

 

Ok, so my new question is can you interchange just another 8 inch ring with the stock TII pinion? IE Mazda 929, Rx7 FD.

 

Just so that everyone is aware this guy is a great seller disregard that negative feed back that was given to him about the rear cargo cover, I was the one who told him to give me a week to but it from him cause I was waiting on my shop to tell me if it was a good price, I send him the payment on Friday and sure enough Saturday afternoon he send me my rear carco cover with tracking number and a picture of recite, sorry to have cause you any trouble dude.

bump for good seller.

 

Wat

 

sorry dude wrong thread...