After doing alot of reading both here and on my350z, I have decided that my next mod should be the 3.9 gears. Do you guys think it would be a worthwhile mod for an AT?

Should I expect to see similar gains as the manual cars do?

Since my 2005 already has the LSD, do I need to purchase a full pumpkin or just the gears?

EDIT: Looks like just the gears should work: http://www.350evo.com/catalog/produc...7b99660a54f029

 

Dang Boris, I see the mod bug hasn't let go yet...

 

$900 for gear/seals alone!!!!!!!!! Holy living crap!!!!!!!!! We get screwed for having imports because gears for an F-body/Stang/domestic is $150-250. $900 is highway robbery. You should also plan on $300-500 for install.

Keep in mind that a 3.9 gear is really steep for this auto's internal gearing. Here will be you max speeds in each gear assuming the stock 6600rpm limiter:

1st 33mph (stock 41mph)
2nd 55mph (stock 65mph)
3rd 87mph (stock 101mph)
4th 127mph (stock 149mph)
5th 152mph (stock 179mph, yeah right)

The biggest drawback is you most likely won't be able to use cruise control. 5AT guys with the 3.5 swap have reported that the cruise won't work at speeds above 75mph, the crusie kicks off. I imagine it's because the TCM reads the rpms only and thinks the car is actually 85mph (cruise control speed limiter, I think).

 

yeah this will be my next mod also... that is a lot for the gears though. wish it would come down in price.

i really feel like it will help a lot though, i am going to the track tomorrow, i will probably run high twelves with a 100 shot, but when i hit fourth gear it just seems like it doesnt do much, i think with a ECU flash, the gears will be great,

just a higher crossing point and being higher in the rpms, i really think you will tell with the new gears

 

Dave, now that you put the speeds into perspective, I may have to rethink this approach. I can get the 3.5 gears for $400 from 350evo, and probably even cheaper used from a 6MT 350z...

The truth is that even the $900 although steep, doesn't seem like a waste of money for .5 sec gain in the quarter mile. I don't think any bolt-ons, or combination of bolt-ons can net these same results.

Do you think spending $250-$400 for the 3.5 gears is worth the negligible .1-.2 sec advantage in the quarter? The cruise control means nothing to me- I think I've used it once in this car so far.

My buddy swapped gears in his e46 330ci from an e46 325i, and it made a world of a difference in his cars power output, and transformed it into a beast. There was suddenly plenty of go from a stop, and every time you switched gears @ redline you would be in the engine's sweet spot. It reminded me of the VTEC engaging on my old rice rocket!

 

that much diffrence? if u do decide please post and if anyone does or have this, if you have F.I. does it effect it in anyway?

 

IMO, the 3.9 gearing is pretty aggressive for street use, but the car will be far more lively in 1, 2, 3. Gears dramatically improve seat of pants acceleration, usually more so than what's reflected on timeslip.

I guess the first thing I'd do is verify if that .5 second gain is legit and if it was on an NA car. I don't think anyone with an NA 5AT G has this gear set. You might want to confirm the gains with the Z crown. .5 seconds seems too good to be true. A lot of cars make great gains from gears. Adding a 4.33 gear in place of the 3.27 on a 98-01 Cobra is good for a legit .3 and 2mph and that's considered really good. But look at how much more torque multiplication is being added. You also have to consider the NA Cobra with it's small V8 and 3.27 gears was woefully undergeared from the factory. 1st is good for about 50mph, 2nd to 75mph, third to 110mph, etc. Most imports, like the G, already come from the factory with pretty deep gearing from the get go. Domestic V8s typically are slightly undergeared to keep MPGs up and to keep thier drivers from putting the cars in ditches.

The 3.5 gear would be a wise consideration. You can buy the whole "pumpkin" and swap it in within a couple hours. If you don't like it, you could probably sell the peice for no loss.

I have run some really short drag radials on my G35 which gave me the equivalent of a 3.5 gear. According to the slips I was no quicker with the short DRs compared to my OEM 17s, but my MPHs were 2mph slower on average because I was forced to grab 4th gear 150' short of the finish line instead of being able to hold 3rd. The car definitely felt more more lively though. I would think the 3.9 gears would probably force the 4th gear shift about 400' short of the finish (~87-88mph). Assuming you could get 96-97mph in the 1/4 mile, you should be crossing the finish at around 4800-5000rpms. Ideally, you want to be crossing the finish line at peak power (~6100-6300rpms).

 

I guess it depends on which FI. With a centrifugal blower, the car tends to make stronger gains as the rpms increase. With the VQ, the biggest gains with a Vortech/ATI are above 5000rpms therefore adding a set of deeper gears would allow you to get into that band quicker and to stay in the band on the shifts.

Turbos on the other hand are different. They have better volumetric efficency in the lower rpms therefore they pump up the overall power curve dramatically especially when compared to a Vortech/ATI SC. Once the turbo starts to spools up (usually around 2500rpms, full boost at 4000rpms), the power curve basically elevates across the board. This is why a turbo G making 350whp is usually significantly faster than a 350whp Vortech/ATI G. The turbo has a lot more power under the curve. With all this extra power in the low to midrange plus tons of topend power, the turbo car doesn't really need all the torque multiplication from deeper gears. Actually, most twin turbo Z owners are swapping in 3.3 gear sets because it actually makes them quicker.

The Stllen Roots style blower is a bit different. It makes lots of low to midrange power, but due to the massive heat generated with twin screw blowers, it doesn't make huge gains in the upper rpms like the Vortech/ATI/turbos do. With that said, a G with a Stillen SC probably would be fine with the stock gears.

 

yeah i am smoking everyone off the line and to the 1/8th mile, i just that top end when i am getting caught. still putting down decient times, just a low mile an hour, i mean stock i ran 14.2 at 95. i have a 04 G35 sedan.


so if i can get that power in the right place i should be able to knock off a lot more, also all i ever do is drag race, occasional drifting and some light to light every once in a while, i am thinking about welding the diff, and putting in the 3.92 all at once.

on what dave said, i have had a couple offers in switching for my 3.3 gears also, so i might just get the gears and keep someones oem lsd. another option cause i know the welded diff is pretty extreme. although fun....

 

So, once you've purchased this part, where would you have it installed? Can you trust any speed shop to install this correctly? I'm guessing no.........

 

The 3.9 gearing is really expensive. U are going to have to spend at least 1200 dollars in parts bc the auto lsd will not work with the 6mt gearing therefore u cant just buy the 3.9 gears. Then, u have pay somewhere around 600 for putting the gears into your orginal pumpkin and installing it in your car. The down time is at least a few days to a wks due to the fact that you wont know what kind of shammys u have (will have to order them) unit u take the pumpkin apart. If u have the downtime and the money to spend somewhere around 2g then all the pw to u. You should see somewhere from .3-.5 off your quarter mile but also see around 3.5K rpm at 80mph.

The cheapest way is to find an used 6mt 3.5 pumpkin with vlsd (+400) and install that for around 150-350 and that may see improvements from .1-.2 off quarter miles (depending on mods) and cost and labor for under 1g.

 

If you're really considering this, you owe it to yourself to read the thread in the Reviews section.

There is a lot of info., first-hand experience, cost involved, etc. that you might be interested in.

 

oh yeah i am trading my friend with TT for his pumpkin before i do this..

 

I changed my 5 AT pumpkin for a 6 MT. I gained .34 in the 1/4 mile. I also changed the pulley on my Stillen 282 rwhp to 295 rwhp. I calcuilated I gained .15 with the gear change. Here are the hp # and times
rwhp gears tires
266 13.8 3.32 245/ 45/ 18 bridgestoned
282 13.4 3.32 275/ 35/ 19 (13.52 with 245/45/18)
295 13.1 3.53 275/ 35/ 19 toyos

Had to shift to 4 th gear and trapped out at 108.1. The car feels faster and my best trap was 102.1 with the 3.32 gears. It puts you back in the seat and the excelleration is awsume! With the 245/18's and 295 rwhp the tires spin in first and second gear. (bridgestones) tire is useless. At the track have to launch at 750 rpm or the toyos just spin.
Will post when I put on 26/10/15 slicks. The gear change should make a big difference then.

 

Those times are alittle hard to believe without the papers but it's nice to know that it's possible for an auto to pull times like a 6MT.

13.8 with what mods and the 3.5 gearing?