Installed 3.91 rear diff from Lexus is300- amazing
 

I just swapped out my stock 4.10 diff for the 3.91 harvested from a Lexus is300. The difference was profound.

I have a Ptuning turbo kit with e85 and it dyno'd at 430whp. Fast as hell. I have the Clutchmaster RX300 clutch. I love the car but I've never really been happy with how choppy it felt. Not terribly smooth engaging 1st gear, and when I'd lift off the gas there was a noticeable deceleration. I'd also find myself wishing there was a 7th gear, as I'd be at 3000 rpm at 70mph.

After doing some research I learned the diff swap from the Lexus was pretty much a bolt on. I figured I would give it a shot. The result : Awesome in every respect. The caveat is- this only applies to boosted 86's not NA. With the new diff the car is slicker than snot in first gear, super easy to drive in town, I can stay in each gear longer so the turbo spools up better, and at 76mph I'm at 3100 RPM. For me, the is the missing link. The car just glides, super smooth delivery of power, and wicked fast without the nervous and choppy feel of the lower gear ratio ( 4.10)

I'm a happy dude.

I never looked back when I made the swap.

Can we go even lower to say 3.7? Although a lower FD I think theoretically puts more stress on the trans/gears.

3.73 is possible but harder to come across then the 3.91

Throw this out there. But a vendor on the forums does offer such a service and the price after rebuild, knowing its going to hold together is well worth it.

Link to the site is below.
http://www.spencerfabricationcompany...rive-upgrades/

I bought my diff from Spencer. He's a good guy, and very knowledgable. I did however get the impression he is scaling back his operation...

I haven't done this swap! Just making that clear first! But I decided to go with a meatier tire. Add about an in to the circumference. This in effect lowers the final drive too. So I suggest to anyone who doesn't want to do the swap or at least see how it feels? This could be a good option. Just pick up a cheap used set of tires. Also let's say you already did the swap and want it lower even still just add a meatier set of tires! As for me I will say the ride and take off is a lot smoother and not so jarring.

Why does this only apply to boosted 86s?

For boost, you generally want longer gear ratios in order to build up boost.

For NA, you want shorter gearing for quick acceleration and keeping the RPMs high between shifts.

My cars just going in to the shop to get boosted and built engine to put down 400+.

I was wondering how I was going to manage first gear with that much power. With the stock gear set and the power you had was 1st gear even usable ?

What speed does first gear top out at with your new gear ratio? compared to stock?

Cheers and thanks for your thoughts and experience.

It's my understanding that the FRS, IS300 and MKIII supra diffs are all interchangable. The supra diffs have a 3.73 option as well as a 3.91

Yeah, i have an is300 3.73 final drive in mine.

Only the guts of the MK3 supra can be swapped into a BRZ or IS300 case then mounted in the car. The MK3 case uses different style mounts.

Higher power levels mean that your first gear or two end very quickly so you have to shift much sooner, and the potential to break the tires loose is a lot greater. I've driven some boosted or v8 swapped cars that originally came NA with really high rear end ratios where this happens, and it can be quite annoying. The usual fix is swapping in a diff with a lower ratio.