Is the 3.9 Rear Gears from an Auto a direct bolt-on into a manual tranny FD?
 

Hi Guys,
I am trying to find out if the 3.90 rear gears that came in an auto FD are a direct swap with the 4.10s that came in a manual FD.

Thanks,
Jason

The only thing different as far as installation goes: the auto pinion flange has a different hole pattern than the manual trans diff flange.

Two solutions:
(1) drill matching new holes.
(2) install 4.1 flange to the 3.9 diff. (This is what I did)

Thank you!

I just hope you know that the 3.9 will give you better MPG lol , But will make it so that you have very long gears and be out of the powerband after shifts when driving hard.

Unless you have some crazy work done to the car.

There isn't much difference between 4:10 and 3.90.
I have 3:90 gears in my FD because of LS3 swap and different trans gears. I also have a TII diff. Its supposedly stronger then the FD torsion diff.
All fits in the stock FD housing.
Its still just an 8 inch ring gear and has limited fluid capacity so it tends to overheat the fluid.

You can get a Greddy diff cover that allows it to hold more fluid as well as having a bung for
A cooler for the diff fluid. A good idea if racing the car on a road course.
If you don't the fluid will puke out of the breather.

Or you can do a Ford 8.8" swap and you have many gear choices, holds more fluid and is stronger.

Hey guys,
My RX7 is LSx swapped with a 4l60e tranny. It is trapping at 123mph in the 1/4mile which is pretty much the top of 3rd gear. I have done some upgrades which will give me a trap speed of about 135mph so I'm looking for a bit more gear so I dont run out of gear in 3rd since shifting into 4th gear at wide open throttle is a very bad idea. Even with the 3.9 gears I am going to have to rev to almost 7k RPM. :blush:

Seems it should work out for you then. Trapping 135mph?
That is going to be alot of power for that rear and ring gear.

I trap 120+ and am getting close to top of 4th gear with my T56.

An auto trans is probably easier on the rear but that much power may kill it even with an auto.

high 9s to mid 10s?? have a big ass turbo on that LS?

Given that I use street tires and have an auto tranny I think it will be fine "knock on wood". :lol:

On my old rock hard tires it ran 11.6@123mph having to let off the throttle a couple of times so I didnt hit the wall. This was with the following...

LS1
PRC stage 2.5 5.3L heads
TSP 228R cam
3200 RPM Vigilant Stall converter
LS6 Intake
Larger Injectors

I have since installed a Nitrous Express wet kit, RPM level 5 transmission and some Nitto NT05 tires. Because I do not launch hard to try and save the rear end I think this will net me about a 10.8@13X mph.

Here are some pics of the car if anyone is interested...

https://www.rx7club.com/rx-7-audio-v...rx-7-a-925858/

Its not usually a matter of just unbolting crown and pinion gears and bolting an alternate set into the carrier. The new ring and pinion needs to be shimmed appropriately to get the proper tooth contact from pinion to ring gear.

Failing to shim and set the side clearance could result in a noisy and whiney diff. Shimming and side clearancing is usually a specialized area that differential and transmission rebuild shops are trained to perform.

Refer this link for an idea of how to correctly set up a rear end, or perform a ratio change. This is fairly constant across most differentials.

http://www.differentials.com/technic...n-instructions

Im taking it to a diff shop to get done. I just wanted to make I didn't have to buy any extra parts for this swap that the shop would not have.

Or you could just look here and see how to remove, disassemble, setup, reassemble and install the diff in an FD.

https://www.rx7club.com/3rd-generati...riteup-897359/

umm ....auto turbo tailshaft flange is identical to that of the manual turbo tailshaft flange
the auto shaft is 50 mm shorter than the manual one,, and swaps straight in and shakes like hell

the NA is different to the turbo

Thought they were talking about third gens here :-P

ah, yes.. didnt spot that bit did i?
but i would be surprised the auto has a different flange to the manual
.. as traditionally the flange pattern change has always been there to denote a change in U/J,, and since its the 8 5 inch diff ...auto or manual . . it already has the big uni

it does have a different flange....

How do you get the speedo reading correctly again?

He has an LS swap. The Sine wave is different coming from GM ECU then a Mazda ECU but being they are both electric then you just use a converter box in line to reduce the Sine wave for calibration. Usually calibration is done using GPS and adjustments made to the converter to give an accurate readout.
A popular converter used in LSx swaps is known as a product called a "Dakota Digital".

Another option is using a GPS speedometer like sold by Speedhut gauges.
This can also be used in a Mazda powered FD. A search can find threads relating to those.

Its also possible that a speedo adjustment is not needed in an mazda powered FD even with a gear change because speedo is not cable driven like a Fox Body mustang for instance. Those were analog and required a gear on the speedo cable changed that fit into the borg Warner trans. It was a small nylon gear on those cars.

The FD may not need such a thing. Others may chime in on that.

As ZoomZoom wrote I am using a Dakota Digital box to adjust due to the engine swap.

PS, I also ended up finding and buying a complete 3.9 rear Diff so I am just going to swap the Flange from my old one to the 3.9 and bolt everything in. :)