3.90 GR86 Diff in a 2013 BRZ - Good combo for boost? - Page 2 - Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community

RT-BRZ · 06-03-2025, 06:25 PM

I actually did the swap last Friday evening. I ran a local autocross event on a smaller lot and the difference was noticeable. The car felt a little closer to the way it used to feel though getting tail happy was still a minor issue, it wasn't something unmanageable like it mostly was with the 4.30. I am going to try to run Mid-Ohio at the beginning of July so I'll know for sure then.

Koba · 07-24-2025, 07:33 PM

Quote:

Originally Posted by DocWalt

I did 3.73s on mine, no regrets.

Please share more info... very interested in a 3:73 idea.

DocWalt · 07-25-2025, 10:41 AM

Quote:

Originally Posted by Koba

Please share more info... very interested in a 3:73 idea.

I bought a diff out of a ~2014 IS250 RWD and installed my OS Giken LSD into it, pretty simple swap!

DarkPira7e · 07-27-2025, 09:08 AM

I second the 3.73. it's called the m85, I have sold a couple on here- you need to remove the axle stubs, and most of them could do with a new breather (easy to find and cheap)

DocWalt · 07-27-2025, 10:15 PM

Quote:

Originally Posted by DarkPira7e

I second the 3.73. it's called the m85, I have sold a couple on here- you need to remove the axle stubs, and most of them could do with a new breather (easy to find and cheap)

Oh, interesting. Mine was Y32 I believe and had no stub axles to pull.

I definitely second the tip on the breather, mine was clogged and smashed.

Stonehorsw · 07-28-2025, 02:17 PM

Deleted
Forum not letting me upload the calculations

B T · 08-13-2025, 06:52 PM

Used 3.7 direct swap with a good diff seemed almost impossible to find, just switched the ring and pinion in mine to 3.7, still no traction until 70

07-27-2025, 09:08 AM	#18
DarkPira7e Rust bucket enthusiast Join Date: Feb 2017 Drives: 2013 Turbo Firestorm FRS Location: Vermont Posts: 4,024 Thanks: 3,289 Thanked 4,177 Times in 2,098 Posts Mentioned: 33 Post(s) Tagged: 0 Thread(s)	I second the 3.73. it's called the m85, I have sold a couple on here- you need to remove the axle stubs, and most of them could do with a new breather (easy to find and cheap) __________________ Turbo FR-S Build - Build Thread JDL EL Recirc manifold, Boostlab BL58x Turbo w/ T51R, 17x9 ARC-8, IAG block

07-28-2025, 02:17 PM	#20
Stonehorsw Senior Member Join Date: Jul 2020 Drives: 18 Brz (Sold) / 22 BRZ Location: Michigan Posts: 670 Thanks: 1,122 Thanked 445 Times in 261 Posts Mentioned: 1 Post(s) Tagged: 0 Thread(s)	Calcs Deleted Forum not letting me upload the calculations

08-13-2025, 06:52 PM	#21
B T Senior Member Join Date: May 2015 Drives: 15 BRZ, 2003 MR2, '70 Elky, 06 TBSS Location: Mobile, Alabama Posts: 536 Thanks: 209 Thanked 229 Times in 158 Posts Mentioned: 7 Post(s) Tagged: 0 Thread(s)	Used 3.7 direct swap with a good diff seemed almost impossible to find, just switched the ring and pinion in mine to 3.7, still no traction until 70 __________________ IG YouTube

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
GR86/BRZ Diff Mounts	Jianlun	GR86 General Topics (2nd Gen 2022+ Toyota 86)	16	06-17-2022 01:25 PM
Which is better? Reaplcing rear diff bushings? or installing rear diff brace?	Frs30	Engine, Exhaust, Transmission	22	04-13-2020 11:23 AM
Difference between Whiteline Diff Outrigger Bushings and Diff Cradle kits?	mistople	Engine, Exhaust, Transmission	10	09-21-2018 07:01 PM
Diff Mount Inserts install - Diff won't lower???	ChRZ	Suspension \| Chassis \| Brakes -- Sponsored by 949 Racing	4	07-17-2017 01:07 AM
Swap AT Diff with MT Diff	sydious	Engine, Exhaust, Transmission	2	03-17-2016 03:59 PM

06-03-2025, 06:25 PM	#15
RT-BRZ Senior Member Join Date: Jan 2021 Drives: 2013 Subaru BRZ Location: Columbus, OH Posts: 466 Thanks: 560 Thanked 290 Times in 180 Posts Mentioned: 5 Post(s) Tagged: 0 Thread(s)	I actually did the swap last Friday evening. I ran a local autocross event on a smaller lot and the difference was noticeable. The car felt a little closer to the way it used to feel though getting tail happy was still a minor issue, it wasn't something unmanageable like it mostly was with the 4.30. I am going to try to run Mid-Ohio at the beginning of July so I'll know for sure then.

User Name		Remember Me?
Password