Fitment of FDs on AT

[Devil 86]

Originally Posted by 86TOYO2k17

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.

Was this on an auto or manual? I've been thinking about swapping to a 4.8 but am too scared that 5th gear in an auto would be too long and would fall flat on its face going down long straights.

[Ashikabi]

Originally Posted by Devil 86

Was this on an auto or manual? I've been thinking about swapping to a 4.8 but am too scared that 5th gear in an auto would be too long and would fall flat on its face going down long straights.

4.8 is gonna give you hella torque and shit gas mileage. Definitely have traction problems

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[anticubus]

Originally Posted by Devil 86

Was this on an auto or manual? I've been thinking about swapping to a 4.8 but am too scared that 5th gear in an auto would be too long and would fall flat on its face going down long straights.

4.8 would be fun if you're ok with all the extra shifting, 4.5 or 4.6 is definitely useable too. Each gear would be shorter but produce more torque at the wheels, so 5th would be faster, but you may have to use 6th on a long straight which is where you'd hit a brick wall on acceleration.

The 4.3 ratio is a solid upgrade across the board for ATs. When you're cruising the 4.1 and 4.3 FDs are equally as efficient even though one sits a little higher in the revs. The only real downside is an extra shift before you can hit 60mph, but ratios are usually optimized for 0-60 times (Note the 4.1FD + 2nd tops out right at 60MPH, that's not a coincidence, that's for the reviews.)

The real deciding factor is where you want to compromise. AutoX tends to drift to longer ratios and an extended rev limit so you can work the course in 1st and 2nd. Track days are going to be course dependent, some might benefit from longer gears if the turns spiral out and normally require a shift that might upset the balance, others might benefit more from shorter gears if you're never getting past 3rd or 4th already. Street driving is entirely up to the driver. If I had an AT I'd definitely be looking at a 4.3, maybe even a 4.5 now that the term "Daily Driver" is a very loose description of how I use the 86.

[86TOYO2k17]

Originally Posted by Devil 86

Was this on an auto or manual? I've been thinking about swapping to a 4.8 but am too scared that 5th gear in an auto would be too long and would fall flat on its face going down long straights.

With an auto. You would need like a 5.3FD to make 5th useable.

A 4.88 i think puts 4th redline around 106mph
A 4.56 was around 114
A 4.3 was around 120.
Stock 4.1 I think was 128.
Can just find gear ratios online and calculate if you want exact can’t remember exact off top of head.

I also had a 25” tire vs stock 24.6 so 2nd with a 4.3 and 7500 limiter i could hit 60 in 2nd before fuel cut.

Choosing a FD is highly use specific. For a fun DD/street/canyon carver backroader, NA up to about 240whp 4.56 is good FI 240-300whp 4.3 is good. At 300whp+ 4.1 is good. Personally I liked to Hwy pulls and a 4.88 makes 4th too short. Who wants to only be able to pull to 105mph.

For track or autox FD ratio there is a very specific way to figure out best ratio.

[new2subaru]

Choose you FD based on your top speed. Here's a MT 4.3 vs AT 5.29 5th is close to the same and 5th is usable in the AT. Some check all the numbers for accuracy.


http://www.teammfactory.com/calculat...200/7200/1/1/2

[bfrank1972]

I don't really understand why anything higher than stock is needed on a Vortrch equipped car - I am Vortech supercharged and my first gear is pretty unusable if I floor it. Actually I sort of get it, it gives the perception that your car is faster overall because it pulls harder in each gear, but really it doesn't make all that much of a difference to acceleration over time. Especially if you have a hard time putting the power down.

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[new2subaru]

There was one member with FI and a 4.88. Can't recall the username atm.

[whataboutbob]

My AT with a Cusco 4.556 and the EB SC at about 300WHP was a hoot.

[Ashikabi]

Originally Posted by bfrank1972

I don't really understand why anything higher than stock is needed on a Vortrch equipped car - I am Vortech supercharged and my first gear is pretty unusable if I floor it. Actually I sort of get it, it gives the perception that your car is faster overall because it pulls harder in each gear, but really it doesn't make all that much of a difference to acceleration over time. Especially if you have a hard time putting the power down.

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It does indeed multiply your torque and therefore allows faster acceleration. But it tops out at a lower speed. It's a trade off.

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[Devil 86]

My FRS is a dedicated track car. The fastest ive gone on track is 126 mph down the back straight of watkins glen and thats reclining 4th gear. I've never used 5th and 6th gear on track and don't mind extra shifting if that means staying in the power band exiting corners. I guess I should consider something shorter than a 4.8. I plan to stay N/A

[bfrank1972]

Originally Posted by Ashikabi

It does indeed multiply your torque and therefore allows faster acceleration. But it tops out at a lower speed. It's a trade off.

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Yup indeed it does, it will make you faster in a given gear, but overall it doesn't actually make you that much faster believe it or not!

Consider a rolling drag race between a car with a 4.1 and a 4.8 FD, starting at the same speed in say 2nd gear, both cars are in the power band (above 4000).

The lower geared car would instantly start pulling ahead due to the mechanical advantage of the 4.8, but will have to shift sooner to 3rd gear while the 4.1 car gets to stay in 2nd. For that time before the 4.1 car has to shift into 3rd, the 4.1 car has the mechanical advantage and will just barely catch the 4.8 car before the 4.1 car has to shift to 3rd.

Mathematically that's how it works (we are disregarding shift times here, infinitely fast shifts). So while a 4.8 car will pull ahead, the higher geared car will always just catch up before having to shift - the 4.8 car is faster but not as fast as you think, or not as fast as it might feel in a single gear.

Clear advantages of a lower geared car:

1) from a standing start on a car that can put the power down, a lower geared car will get you up into your power band quicker, which is a true advantage the higher geared car won't recoup

2) if you are gearing for a very specific application where you want to work in a set of gears, i.e. someone mentioned AutoX where a stock geared car might not take advantage of all of 2nd gear - it depends though, on a larger course a stock geared car might not have to shift to 3rd where a 4.8 car would in some spots, there the 4.1 car would be faster to the end of the straight (again not even factoring in shift time, which is something else to consider). Another scenario might be like the post above this one, where a lower FD might optimize your acceleration corner to corner on a track for a specific track/situation, but you have to be careful you don't shoot yourself in the foot in other places on that track (or other tracks).

Considering most well tuned FI cars can't put power down in first, that gear is a waste because you can't even use it from a standing start to get you up into the power band, you have to feather the throttle. IMHO for acceleration you'd be better off spending the FD money on a smaller pulley plus flex fuel (or meth injection if you don't have access to ethanol).

One last thing to point out, FD changes do not change your gear spacing - if you are looking to keep your tach higher in the rev range in between gears like the OP mentioned, you need a new gearset.

Apologies for the novel, probably too much coffee this morning

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[new2subaru]

Here's a good thread on FD's


https://www.ft86club.com/forums/showthread.php?t=111688


It's a good read

[DaveB.inVa]

Do like others have suggested and find a 4.30 rear end from a manual 2017+ car. The code on the 4.30 differential is Y58.

I found one from a 2019 that had 4800 miles for $460. It was a definite improvement and more gearing would be nicer but it's hard to beat for under $500.