Fitment of FDs on AT
 

Greetings,

After installing the Vortech Supercharger on an Automatic (yeah I know, it was the only one on the lot after a hurricane and I was dead set on a BRZ), I was deciding that I want to get a higher FD to stay in the higher revs after each shift. I exclusively stay in manual mode and only use AT on traffic and drive thrus.

So far I'm looking at 4.56 on a 260 HP 190 TQ BRZ.

But, besides all that I was wondering about the fitment. Does the Cusco FD fit on an AT? I noticed it's for MT on the website.

I've tried searching but, alas I can't really find much. Unless my google fu is just awful...which it probably is.

https://www.cusco.co.jp/en/news/clos...ion_gears.html

I believe you are going the wrong way with your ratio. It should be a smaller number to stretch your revs out. The Lexus IS300 diff is a direct swap pumpkin iirc M85 is the code you're looking for there. I think that's a 3.65 ratio with LSD. I don't know anything about the Cusco diffs but I know replacing the final drive gear is much more expensive than swapping pumpkins unless you can do it all yourself perhaps

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Ooh, I see.

I guess I was under the impression that a higher ratio equates to more torque at the wheels and faster acceleration...or maybe I just used the wrong words?

I guess I was wrong, eh. I'll have to read up on it again.

I'll check out the M85 then when I reeducate myself, thanks.

I took "staying in the revs longer" to mean that you wanted a taller gear that would decrease your torque and increase top speed (per transmission gear). This is common practice for turbo guys. I intend to do the same with my LS swap to mitigate my poor traction

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Ah, got you.

Yeah I just misworded then, that's my bad.

Indeed. If you want more torque then you will need a different gear set as there is no pumpkin to swap that already has an appropriate ratio. We are at the bottom of the pyramid of power

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He's definitely going the correct way for the auto, 4.56 puts you at a similiar ratio as the manual and is exactly what he's looking for. Some of the gears in the auto trans are longer and getting a 4.56 FD equates to having close to the same as the manual FR-S. I've seen N/A auto cars getting amazing results but that's not recommended in his setup.....

Hm, I could go 4.44.

But, like I said I'm not sure if these kits will fit.

I'm unsure if the AT and MT have differences.

AT can handle a lot more torque, can shift much faster than a human, and still be controlled to shift or hold gear however you want,

Any FD gear for MTs will work for ATs no different.
MT and AT have the same rear diff pre 17, post 17 MT switched to a 4.3gearing same pumpkin/lsd, which is also still a direct swap and is a much better option for FI Autos.

Final drive will not change your shift points / RPM drop after shifting. Your transmission gearing determines this not FD gearing.
A shorter (numerically higher) FD will provide more mechanical torque to the wheels at the expensive of a lower top speed per gear / rpm.

I tried 4.1, 4.56, and ultimately settled on 4.3. 4.56 was too short. 4th topped out at about 115 which came up way too quick and I always wanted a little more, as you fall on your face once you go into 5th. Also traction was much more of an issue with a 4.56 gear.

I like that idea. I get to keep it OEM basically and can switch on trade in.

Basically all I would be doing is buying the Diffrential and fluids?

Sure sounds like it. I didn't know the ratios were different between the AT and MT.

Anyone know what the ratio is for the 17+ ATs?

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All ATs any year are 4.1

Pre 17 MTs are also the exact same 4.1
17+ MTs are 4.3

The same as all previous models. The 17+ manuals were changed to 4.3.

Yup, direct swap, just get some fluid.

I picked up a 1k mile 2019 4.3FD a little over a year ago for like 600$ shipped on ebay. At that low mileage all the seals should be perfectly fine. Just make sure they have the garter spring and you lube up the axle tips and the seals to not scratch the seal on insert. Pretty easy swap.