How to do it

Here's a list of parts I used:

38100-0C760 : 4.083:1 Ring & pinion
38140-0C00A : Pinion Front Bearing
38120-EC000 : Pinion Rear Bearing
38189-P0117 : Oil Seal
38154-0C000 : Washer adjustment
43262-01P10 : Drive Pinion Nut Lock
38165-0C020 : Differential Spacer



I had 170,000 kms on my diff so I figured I'd replace the above parts while I had everything open.

Here's an exploded view of the diff incase you're wondering what all these parts are:



If you want to swap the diff bushings, now is the time too I got a set of Whileline Bushings as my OEM's were shot.

One thing's for sure, you will need an MT differential housing. I tried to use my OEM G37 A/T one and the pinion shaft wouldn't even fit in it. I then tried an A/T from a 350Z and the pinion would not touch the ring no matter how much we moved it around. The only way to pull this off is to use an M/T diff housing. I picked up an MT diff from a 370Z and used my friend's 350Z M/T LSD. Everything fit like a glove.



Here's the tricky bit, if you're coming from an A/T G37, then your pinion flange will have three bolts in a triangular layout. The MT's and older ATs came with the round 4 bolt flange. Here's a pic of them side by side.



This means your AT G37 driveshaft flange will not bolt straight up. Since my driveshaft has been already customized, I did not mind cutting its end off and fitting a 4 bolt flange from an M/T. SharpByCoop reported you could use an older G35 driveshaft's rear half and it would bolt right up.

The Facts

You will gain 21% more torque in every rpm in every gear when going from 3.36:1 to 4.08:1! You will however lose 21% speed at every rpm. To give you an idea, my car did almost 100 mph at 3k rpm in 5th. It now does around 85 mph. Not complaining!

Butt Dyno

The car finally feels alive again. The RB25's long ratio's killed my car when mated to the 3.3 ratio. The difference was really impressive in every gear. I can only imagine what this would do on a VQ G37. This IMO is a must do mod if you're planning on staying naturally aspirated.

Impact on Fuel Economy

I have not tested it yet but many say it would cause it to drop. There are more aspects to fuel economy than just engine rpm though. Throttle position and engine efficiency come to mind. My findings may not be very relevant here though.

 

Busy but will read this ASAP. Might be my next mod!

 

Good job Sam.

 

OK... read everything. And read multiple threads in the pass. Sounds like I need to do this swap. Quick question Sam... Recall my memory but there are different ratios we can choose from? Oh! Think I'll throw a Quaife in at the same time as well. Time to save! Thanks!!!

 

Couldn't have been made possible without your help J!

There's the 3.9 ratio: 38100-0B66A and the 4.36 ratio: 38100-0C860. People on the 370Z confirmed the 4.36 will fit but Z1 Auto denied. Only one way to find out!

A Quaife LSD surely is a plus!

 

Great job. Another great mod coming out of the desert!!!!

 

@G37Sam: Instead of modifying the driveshaft of your AT to make it mate to the manual differential, it is possible to swap the AT driveshaft flange from the AT Diff to the MT one you put the 4.08 gears in. That's what I did for my 3.9 gear swap.

 

True, but by doing this you are now asking your rubber coupler to perform the job of a u-joint because the the pinion does not have a centering pin. I do not think it is wise to use the rubber coupler in this way.

 

How much do you think a mechanic would charge to do this to a 6mt?

 

Hi Sam,

Thanks for the clear info. Most important was your trials to attempt to make the auto diff work, and the realization that it's unpossible.

We covered much of the same in my 4.08 thread.

Yes, the auto 3-bolt flange CAN work. No, it's not suggested. At my 500+ hp level we felt a distortin vibration that was cured after we replaced the rubber OEM A/T coupling with the M/T 4-bolt coupling.

One edit needed: ONLY 2007-up G35/G37 rear M/T driveshaft sections will bolt up. NOT from the early ones. You only need the rear section and the mating diff flange.

I KNOW how you feel about it's spirited driving. So cool.

Thanks for the good news!

Coop

 

Good stuff Sam. I did this mod in my 350Z but it was a manual to begin with. Best mod I had ever done. Hands down.

 

I went to a taller gear on my 350Z. But I plan on racing the Tx Mile one day. (If I still have my Z.)

 

Did this mod earlier this year. By far the best thing Ive done to the car! Glad you like it

 

can anyone confirm if this final drive with part number 38100-0B66A would fit an 03 6MT? Thank you

 

Nice! Anyone know if the G37S Auto Diff with the limited slip will swap out with a regular G37 Journey diff? The entire unit be a direct bolt in replacement?