Gear ratio for road course

[krishelnino]

I just found a used 3.73 Torsen diff with 6k miles off a 2018 manual car locally for around $600. I'm planning on doing more track days in the future, so will this be a good upgrade over my stock Trackloc 3.55 diff...considering the labor costs ?

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[EFI]

I just found a used 3.73 Torsen diff with 6k miles off a 2018 manual car locally for around $600. I'm planning on doing more track days in the future, so will this be a good upgrade over my stock Trackloc 3.55 diff...considering the labor costs ?

Yes, especially in the 2018s which only have 4 underdrive gears and are rather long compared to the 15-17.

 

[DB83]

You can, but you lose the option of a rear end swap because the torsen only came in 3.55 or 3.73 stock.

What do you mean i lose the option of a rear end swap?

 

[Performance nut]

What do you mean i lose the option of a rear end swap?

Having the ability to swap out fully assembled rear ends. You can find junk yard 3.55 or 3.73 or buy a brand new 3.55.

 

[DB83]

I don't see how that's the case. If i have my rear with a Torsen and a 3.31, and tomorrow i find a complete pumpkin with a 3.55 and trakloc, i can swap the entire rear end as the housings are the same and the input / output shafts are the same. No?

 

[EFI]

I don't see how that's the case. If i have my rear with a Torsen and a 3.31, and tomorrow i find a complete pumpkin with a 3.55 and trakloc, i can swap the entire rear end as the housings are the same and the input / output shafts are the same. No?

Yes, all 3.31 3.55 3.73 and 4.09 are interchangeable in the same pumpkin and differential (both track lok and torsen).

The only one that's not interchangeable is the 3.15

 

[Grintch]

I don't see how that's the case. If i have my rear with a Torsen and a 3.31, and tomorrow i find a complete pumpkin with a 3.55 and trakloc, i can swap the entire rear end as the housings are the same and the input / output shafts are the same. No?

Ford made no cars with a 3.31 and a Torsen. So the only people who have that combination will be those who paid to update their stock 3.3.1 traction lock to a Torsen. And they are unlikely to want to sell it.

By comparison it is easy to find Torsen 3.73 assemblies from people going forced induction who want a taller rear gear, and are drag racing oriented (where the Torsen's advantage is less significant). Especially from the 2015-2107 PP's, where in my personal opinion the 3.73 gear is too low. Or of course, from crashed cars.

I know of no compatibility issues across the stock pumpkins, though some worry about the durability of the aluminum ones.

 

[Performance nut]

Looking at the poll, I'm actually really surprised to see all the votes for 3.73. Interesting feedback guys, thank you for your votes. If you haven't voted, please do so. This isn't just for me but for future folks.

 

[boardkat]

I know of no compatibility issues across the stock pumpkins, though some worry about the durability of the aluminum ones.

can confirm that there is a compatibility issue with the 15-17 pumpkin on 18+ cars - i sold my '15 GT 6MT stock iron 3.73/torsen case to a friend with an '18 GT 6MT and he had to swap the pinion flange - i've also heard about MT/AT pinion flange differences for '15-17 but can't confirm through experience.

.. and as far as reliability goes, i can also confirm no issues after 5 seasons of autox and track abuse on both r-comp and street tires on various aluminum cases - but i imagine the restricted movement by having poly diff bushings and converting the case cover to bolt through (to take it out of single shear) played a role.

 

[Performance nut]

can confirm that there is a compatibility issue with the 15-17 pumpkin on 18+ cars - i sold my '15 GT 6MT stock iron 3.73/torsen case to a friend with an '18 GT 6MT and he had to swap the pinion flange - i've also heard about MT/AT pinion flange differences for '15-17 but can't confirm through experience.

.. and as far as reliability goes, i can also confirm no issues after 5 seasons of autox and track abuse on both r-comp and street tires on various aluminum cases - but i imagine the restricted movement by having poly diff bushings and converting the case cover to bolt through (to take it out of single shear) played a role.

Hey @boardkat . What is your vote?

P.S. Your build thread sort of inspired this thread. I really like the OS which sort of got me thinking rebuild with what ever gear I want. But @Grintch nudged me over on the prebuilt Torsen. Love to see your thoughts.

 

[boardkat]

your setup and mine..... differ substantially

what i can speak to is my own experience trying 3.31/3.55/3.73/4.11 and both torsen (OEM) and clutch-type (OSGiken) diffs on a built MT82 transmission (w/ GForce gearset) at varying levels of N/A and F/I power for both autox and track applications.

TL;dr my vote doesn't really matter, you need to figure out what you're trying to accomplish, and what compromises you're willing to accept.

2015: 3.73 w/ OEM torsen @ stock power w/ 7000 limiter. reasons for this: waiting for parts
feedback: torsen was a bit of a pill on the bumpier sites i ran at, with power down difficult at times. not what i'd consider an apples-to-apples comparison though, because i immediately put the OSG in when it arrived on a pallet from japan, along with all the other parts that started arriving around then.

2015-2018: 3.55 w/ OSG @ 450rwhp/380rwtq w/ 8250 limiter. reasons for this: usability of 1st/2nd gear for autox (with a much higher rev limiter to avoid a shift to 3rd on higher speed courses).
feedback: this was a great combination for autox - could still pull out of lower speed corners in 2nd gear, but was long enough to rev high. OSG 1.5-way did exactly what i was hoping it would do - unlocked off throttle (helping with rotation on corner entry), and locked up on power (helping with rotation on corner exit and power down everywhere else). OPG eventually shattered and the engine cratered, necessitating rear end setup changes.

2019: 4.11 w/ OSG @ 650rwhp/500rwtq w/ 7500 limiter. reasons for this: ability to start in 2nd for autox, with a short shift to 3rd (where it stays the rest of the run - powerband here was 0.75-0.8g thrust from 25-85mph), but was expected to be a compromise on track (shift to 5th needed most of the time)
feedback: tried a 2/3 autox gearing strategy that worked great - as you'd expect, boost fixes the lack of torque at lower RPMs, but the 3rd/4.11 strategy was damn near perfect for power down and having a usable gear EVERYWHERE on course. never got it on the track because the engine detonated, taking out a ring and cratering again :(

2020: 3.31 w/ OSG @ 750rwhp/600rwtq w/ 7350 limiter. reasons for this: usability of 2nd/3rd gear for autox, and longer 4th gear for lower speed tracks (negating the need to shift to 5th).
feedback: a dedicated autox machine this car is not! lots of throttle mapping revisions required to used 2nd and even 3rd with the additional power and (especially) torque. powerband in 3/4/5 is great for the track though, and more usable power is possible (pending me catching up to the car and dialing in the setup). fwiw i did try an event with the 4.11 - couldn't even use 3rd gear at an autox! more power, more problems

 

[boardkat]

addendum to all of that: i don't know enough about AT setups and your gearing/upshifting penalties. but in my experience, i try to limit the amount of shifting needed, since there is a (time) cost on a 6MT each time you do it outside of heavy braking/downshifting. short gearing works great on cars with lower power, which is why i've gone longer. and having the ability to bump your rev limiter between sessions is a great real-time workaround if you a.) get caught between gears and b.) have an engine that can handle it (or the bolder blind/misguided confidence i adopted, lol)

 

[Performance nut]

@boardkat thank you very much for that. I think that is perhaps the most varied and thought out review of gearing I have seen in a really long time. The no maintenance thing the Torsen looked nice but I have heard folks have complaints about them. Even though I'm not professional, I do strive for the best which is why I followed your build for awhile after you got the OSG. At that point, it really didn't matter what gears I used so long as I don't use what I got. With your write up, I think you definitely allowed me to narrow it down quite a bit.

Thank you very much sir!

 

[boardkat]

no prob! i used to spend a lot of time agonizing over setup changes, but in the end, i've found just trying different things is the best way to vet out the theory. of course, there's a cost to this, which is why i've learned to do most of it myself (or find people i trust that can help). my advice for you is follow what alesandro is doing closely , since he's quickly ramping up the curve and runs his 10AT pretty hard.

btw, i'm still posting updates, but mostly on my FB racing page now - my build has gotten a bit nutty since i stopped posting here!
have dabbled with smaller updates over on TMO too, since most of the racing folks live over there.

 

[Performance nut]

my advice for you is follow what alesandro is doing closely , since he's quickly ramping up the curve and runs his 10AT pretty hard.

I'm not sure if I have seen any of his posts or not. What is his screen name?

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