Swapping final drive gears In automatic trans cars?.

[Racer45]

So I just recently installed comp cams 127400s and there great and have been an absolute game changer for me on track especially being able to run the RPMs up to 7350. Only down side is I greatly underestimated how much toque I’d loose down low. I tried tossing in a aluminum drive shaft a few weeks ago and it did help a tiny bit with getting the motor to spin up a bit fast but the car was still lugging coming out of hairpins and other tight corners. So now I’m tossing some 4:10s in at the moment to fully correct the issue.

So now I’m also thinking of donating my old 3.55 gears to my buddies 2008 GT that currently has 3:31s in it and doing the install in it for him to give him a little bump in torque. Only thing is….it’s an automatic. Does anybody know if he would even notice a difference with that slightly taller gear. Or would it just be diminished from the stall converter?

Question being is it even worth it?
Like Feeling wise?

I don’t mind the work load of it. That’s what beers are for right + given his mileage on the car I’m sure it’s all due for a rebuild anyways

 

[whitmanink]

people sometimes dont feel too much between 355 and 373 ,,
they will say go at least 410 to really feel it.

i beg to differ ..
but that said,
going 355 from 331 will yeild a little more get up and go,, but really not that much at all ,,
you may not really notice much off them two to be honest.

but at the end of the day it will be slightly quicker

 

[whitmanink]

altho i am wondering..

you go 7350 rpm?
the cams are good for 6700rpm?

do they fall out after 6700 or stay strong?

i went with mutha thumpers as they give about 1 hp less then yours ..

how are you going so high in the rev range?
what intake do you have?
what valve springs ? (i need to get yet)

i opened mine up to i think 6800rpm

 

[StockishS197]

So I just recently installed comp cams 127400s and there great and have been an absolute game changer for me on track especially being able to run the RPMs up to 7350. Only down side is I greatly underestimated how much toque I’d loose down low. I tried tossing in a aluminum drive shaft a few weeks ago and it did help a tiny bit with getting the motor to spin up a bit fast but the car was still lugging coming out of hairpins and other tight corners. So now I’m tossing some 4:10s in at the moment to fully correct the issue.

So now I’m also thinking of donating my old 3.55 gears to my buddies 2008 GT that currently has 3:31s in it and doing the install in it for him to give him a little bump in torque. Only thing is….it’s an automatic. Does anybody know if he would even notice a difference with that slightly taller gear. Or would it just be diminished from the stall converter?

Question being is it even worth it?
Like Feeling wise?

I don’t mind the work load of it. That’s what beers are for right + given his mileage on the car I’m sure it’s all due for a rebuild anyways

4.10s are usually what the auto guys use with good success. If you are going from 3.55s, I would recommend the 4.10s as that will get you off the line a lot quicker.

Usually with cams, you do want a looser converter to help prevent surging at low RPM but if you aren’t having that issue then I would start with gears.

 

[Racer45]

people sometimes dont feel too much between 355 and 373 ,,
they will say go at least 410 to really feel it.

i beg to differ ..
but that said,
going 355 from 331 will yeild a little more get up and go,, but really not that much at all ,,
you may not really notice much off them two to be honest.

but at the end of the day it will be slightly quicker

Well I appreciate the response, yeah I figured it would be pretty minimal difference. Its just my old gears would be going into a car with an auto trans. I just read some where you may need a higher stall converter to really reap the benefits of a gear change? I’ve personally never re-geared a cars rear end that had an automatic transmission. So I don’t really know what to expect. I may just toss em in his car anyways and experiment with it my self a bit anyways just to see what happens ‍

 

[Racer45]

altho i am wondering..

you go 7350 rpm?
the cams are good for 6700rpm?

do they fall out after 6700 or stay strong?

i went with mutha thumpers as they give about 1 hp less then yours ..

how are you going so high in the rev range?
what intake do you have?
what valve springs ? (i need to get yet)

i opened mine up to i think 6800rpm

Yea I use all the 7350rpm sometimes.

The cams do start to fall off a bit around the 6700rpm though.

Lito set the rev limiter to that and I didn’t question him about it figured he set up these cams multiple times so he knows what he’s doing. I told him it’s a race car and it’s to be used for racing my only requests to him was for a smooth throttle response and to turn off driver assists. Typically I would shift around the 6700rpm but on the track and im coming up to a corner at high rpms, being able to just hang in there a little longer without doing some awkward shifting or bouncing on the limiter befor mashing the brakes has been pretty nice. Enough so that ive smashed some PR’s by a couple seconds

Only down side is I’ve lost my torque to pass people on corner exits as hard as I used to hence me throwing the 4:10s in it now.

Plus when I attend autocross events, I would typically be in top 10 for raw and the past few events I’ve been finishing in around 30th place. And makes the car not feel as fun as it did befor just bogging the car around in second or again doing some awkward shifting into first trying to find some power anywhere on the course

Still love the cams though don’t get me wrong I prioritize over all track gains over anything else but I still want to go kick some ass at autocross between events

 

[Racer45]

4.10s are usually what the auto guys use with good success. If you are going from 3.55s, I would recommend the 4.10s as that will get you off the line a lot quicker.

Usually with cams, you do want a looser converter to help prevent surging at low RPM but if you aren’t having that issue then I would start with gears.

Appreciate the response. Yeah the automatic car I want to throw my old 3:55s in ain’t cammed. Its just got a cai + tune.
Was just wondering if it would even make a difference in a auto car going from 3:31 to 3:55 gears being with the stall converter n all that
I’d imagine he would get atleast some marginal gains from it even with the stock converter. But I have no idea that’s why asking but I’ll probably just throw em in anyways and see what happens

 

[Squirrely]

I have a 2007 GT auto. Just recently had a 3.73 installed plus cold air and tune. It’s a different car. Much more responsive than the 3.31

 

[Racer45]

I have a 2007 GT auto. Just recently had a 3.73 installed plus cold air and tune. It’s a different car. Much more responsive than the 3.31

Thanks for the input. I’m guessing Your still using stock stall converter and it still made a noticeable difference. Awesome! Thanks you for sharing that’s pretty much what I was looking to hear.

Where you get your tune from though?
And what gas you running?

 

[Squirrely]

I got my tune through Alternative Performance (Lidio) Mustang God
3.73 was perfect. I run 93 octane and that’s what it’s tuned for. It’s really a blast to drive.

 

[Squirrely]

Oh and yes a stock converter. Next year long tube headers with H pipe and keeping the American Thunder mufflers.

 

[asdrew]

FWIW on my '07 V6 auto I swapped in an '09 3.73 GT axle and it made a huge difference right away, even more so after tuning it.
I now also have an '06 GT Lito tuned auto and when I get around to it the 3.31 stock axle with be replaced with the 3.73

I would think moving your 3.55 gears to your buddy's GT will be worth it, especially with a tune to adjust for the new ratio.

 

[Pentalab]

people sometimes don't feel too much between 355 and 373 ,,
they will say go at least 410 to really feel it.

i beg to differ ..
but that said,
going 355 from 331 will yield a little more get up and go,, but really not that much at all ,,
you may not really notice much off them two to be honest.

but at the end of the day it will be slightly quicker

3.55 / 3.73 = .95 (5% difference)

3.31 / 3.55 = .93 (7% difference)

3.31 / 3.73 = .887 (11.3% difference)

3.31 / 3.90 = .849 (15.1% difference)

3.31 / 4.10 = .807 (19.3% difference)

2nd gear in the 5r55s is already 14% lower vs 2nd gear in the 3660 manual tranny. A 3.73 is required in the manual to equal 2nd gear and oem 3.31 in the auto.
3rd gear in the 5r55S is already 16.5% lower vs 3rd gear in the 3650 manual tranny. A 3.90 is required in the manual to equal 3rd gear and oem 3.31 in the auto.
Both the manual and auto are 1:1 ratio in 4th gear.

IF a 3.55 is installed in the auto, it will pull just a tiny bit better vs the 3.31 oem rear gear. Top speed will be reduced in each gear. Top speed is 223 kph in 4th gear (@6 krpm) with the 3.55 . It's just (.93 x 240 kph = 223 kph)......... vs 240 kph in 4th gear (@ 6krpm) with the oem 3.31 rear gear.

Put a 4.10 rear gear in either the auto or manual....and top speed in 4th gear (@6 krpm) is now just .807 x 240 kph = 194 kph. (and 238 kph @ 7350 rpm)

 

[Autokyrios]

Others have already mentioned it, but changing rear gears isn't the whole puzzle. It's one component. Tire diameter, gear ratio, transmission ratio, RPM. It's a hunt for the sweet spot depending on how you want to drive. Knowing how you want the car to behave in your usual zone is key to picking the right gear set.