I am in the process of finally completing my CD009 swap... i was going to opt for a 3.9 final gear as it was readily available, or so i thought... I used to find a 3.9 on a weekly basis for a reasonable price. I should have snatched one when i had the opportunity .

The 3.9 appears to be a good compromise with the CD009 swap but if it was a little taller i think it would have been ideal especially for higher output vehicles.
The Ford 8.8 swap is great with a whole bunch of available ratios and all but i don't want to go that route as of yet ... new driveshaft, getting a 8.8 shipped to Cyprus, and many other obstacles its just something i would like to avoid at least for now .

so in my search of a new final gear i came across Billy Johnson's that there are potentially other models that could work with the stock case. I mean the prime example is the 4.44 and the 4.77 from the Rx8 it works well with the FD diff case
https://motoiq.com/project-fd-rx7-re...ifferential/3/



i have seen some other ring and pinion setups from an MX5/miata that looks awfully similar to the ones we see on the FD
Whether it is strong enough or actually compatible, i have no idea.

Do we have anyone on the forum who has actually tried any of the different setups ? anyone who is willing to share some information ?

 

I vaguely recall TeamRX8 posting about this... Don't have time to search atm but fyi in case you want to explore.

 

I had the 4.3 rear from a Spirit R put in my Type RB (in place of the 4.1). Certainly adds some zip. Spirit R also had a different 5th gear ratio, so that might be something to retro-fit later.

Gas mileage was not affected that much.

 

@ptrhahn had tagged me in the post... definitely interesting but holy **** its overpriced lol...
https://www.rx7club.com/3rd-generati.../#post12641312

Unfortunately you did not read the post but its okay
meanwhile, George from Cyprus (carpediemsong?) had told me that he sent you a whole bunch of parts including the 3.9 ring and pinion? If you still have it i could buy it from you

 

@R-R-Rx7
Going to share this link with you and the rest of our rotary brothers: https://www.rx8club.com/series-i-tec...ar-mod-276501/
I have not fully read it yet but bookmarked it for future review.
Not sure if it helps...

 

I bought a RZ Recaro seat and the Type R Bathurst R plastics from Carpe Diem, no recollection of a 3.9 all the way from Cyprus.

Very early FD had the 3.9, and, afaik, every year's Automatics. Hence I had one from my 1992 Automatic parts car.

Its in my friends car now, which he is building out.

I think he plans to stay with it.

 

yep! in brief, the FD uses Mazda's P Type differential, which is shared with some stuff. the 3.7's linked in the post above are hard to find info on because those cars were really rare and unloved, so finding an 89 MPV would be hard, scrap value was long ago higher than its resale value! the FD and Rx8 actually go so far as to use the same part number for the ring and pinions, so the internals are compatible (cases are different though)
the FD 3.909 is a P062-27-110A, which was available new a couple years ago when i checked last. in the US the other P type diffs are the 89-98 MPV and 88-95 929's

the 1968-1991 non turbo Rx2,3,4,5,7 and 94-2005 Mx5 use an M type, which is smaller.
the NC/ND miatas use something new, and i think its in between the M and the P, but i don't really know.

 

Used to be discarded when people scavenged autos for housings after burn-out disasters - sold a couple several years ago, only one left now for own use.....and I haven't seen a 3.9 pop up for sale locally in a couple of years now

Somebody very recently was advertising having a 3.7(??) ratio in stock, either in Victoria or South Australia if you want to look at one of the usual gearcutters, I suspect it's at significantly more cost than OEM. I think Harrop also made something similar over 20 years ago for a friend, but defintely wasn't him this time.

 

I have an early 3.727 that I can sell you, or you can now buy a direct fit 3.636 from Australia as well

https://www.rx7club.com/3rd-generati.../#post12641312

It’s not cheap, but is very high quality and not really that bad for a new one offered in low quantity production. The shipped cost was about the same for the 8” Mazda Motorsport 5.125 R&P from about 10 years ago.

The 8” R&P on the FD3/RX8 is unique. Other than a few Mazda 6-cyl vehicles from 35 years ago, all of the Miata, RX7, etc use a smaller diameter R&P.

The FD3 3.909 is NLA from Mazda. If it were still available today, the cost would be close to the 3.636 one being sold from AU.
.

 

Not totally relevant to OP's needs, but people in JDM land who want a cheap 4.777 for track use apparently repurpose the ring and pinion from Mazda Bongo vans using adapter kits like this: https://item.rakuten.co.jp/syarakuin...w-qq-e-h-277s/

I question whether this would be cheaper than just using an second-gen RX-8 ring and pinion.

 

Currently on Buyee:

https://buyee.jp/item/jdirectitems/a...n_DirectSearch

 

I replaced my FD 4.11 to a 3.909 differential due to making over 400 whp along with my .806 5th gear.
This gives taller lower gears with a shorter 5th gear.
Thus their is less wheel spin in first and second gears and a closer ratio 4th to 5th..

 

That may work with the early 3.727 R&P that I have

kind of assuming that the Bongo has the same 8” diff as the early Mazdas, but I was only searching among US models not knowing what other JDM models might have it

some more info on the Bongo conversion kit that I pulled off a Supernow blog post from late 2021. Note that it doesn’t include the R&P, but is just the conversion kit:

Tsukuba Final Selection

From December 26, 2021

For the FD3S, the stock settings are 4.1 or 4.3.

The two types are due to the original tire size, 16 or 17 inches.

In this case, the gears used are 2nd, 3rd and 4th.

However, it is very difficult to control the throttle in second gear.

The number of gear changes increases

It also increases the chance of misshifting

There is also a large loss of time when changing gears.



If you change this to Bongo's 4.7 Final

You'll be able to drive in 3rd and 4th gear.

The number of shift changes is reduced compared to the stock final

This reduces the number of ups and downs by eight in one lap.

Moreover, you can basically go full throttle when exiting corners.

There's no need for throttle control.



And the most difficult jump into the final corner

Just drive in with the reverse stroke in 4th gear and there's no need to change gears.

You can turn the final corner in 4th gear.

You can then pass through the control line.



You can probably improve your time by just changing it to 4.7.

If combined with the original transmission

The strongest final in Tsukuba is 4.7.



But I don't use the 4.7 final.

It makes the car too easy to operate and makes it less appealing to drive.

The more gear changes you make, the more fun it is.

The thrill of dropping one corner in the final corner is amazing.

Another factor is the RPM when driving on the highway.

If you shorten it to 4.7, the rotation speed will jump up.

It also reduces fuel economy and makes the engine roar louder.



I've always said that 4.7 is the choice of the weak.

If you are looking for a car specifically for Tsukuba, the 4.7 is the best choice.

The FD3S that I ran the other day in 54 seconds also had a 4.7 final time with the stock transmission.

I saw the in-car footage, but I can't tell if the driver is good or bad lol

They're running so well together that it's hard to judge.



54 seconds is not an easy time to achieve.

The people who put it out are really amazing.

It has nothing to do with how you drive or what the car is like.

The faster the better.

If you want to get a good time, the best thing to do is steal it from a 54-second car.

From aerodynamics, to the feet, to the finals, to line selection

Because everything is correct.



If you're aiming for a good time, you should use the 4.7 Final.

Ah, the final replacement kit
.

 

i very recently acquired a 3.9 . but in case the 3.9 is not tall enough, what are you asking for the 3.727? pm me . i guess it doesnt hurt to have both. haha