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I am looking at purchasing a 3.9 final drive for my 6-spd Z. However I have some questions about performance and drivability...

1) What kind of an improvement have people seen in 1/4 mile times? I have heard of the autos gaining .5 sec by switching to the 6-spd gearing, wondering if the gain would be similar for the 3.9 swap...

2) Drivability? a) How much higher does the engine rev at highway speeds? b) How much was your gas mileage affected?


This sounds like a perfect mod for the car, but want to make sure the gains are worth the inital cost of the parts and the daily driving trade offs.


Thanks,

Joe
Fort Wayne, IN
 

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Joe,

I don't have one in my car now, but have been also thinking about it. The problem I have with it is that it will shorten your gears. In other words you will need to shift sooner in each gear. Now in most cases that is not a bad thing except for first gear. First gear on the 6MT is so short already. I am running a Vortech SC with a 9lb pulley and I need to shift before I really get off the line. I can't imagine what the 3.9 final drive would do.

Obviously the choice is yours...I think if your not running FI you should be fine. But I would think long and hard about using it with FI. I am.....
 

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Discussion Starter #3
I see what you are saying about the first gear thing and the added horsepower of forced induction...

Seems like this mod would be great if it lowers your times by .5 sec in the 1/4 mile. What else out there can you buy for under $1000 installed that will effect your times by that much?

Come one guys!!! anyone out there with the 3.9 final drive? Some feed back would be great!
 

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if you plan on staying NA, then it's definitely the way to go!!!

if you plan on adding FI for 300-450rwhp, you should stay with the 3.53 MT gearing

if you plan on going above that you will probably want to swap for the AT gears (you'll be shifting before you hit full boost if they're any shorter)


but as I said, the 3.917 gears are probably one of the BEST mods out there, especially if you do a lot of road racing... that's what they're geared towards (worst pun EVER), not the 1/4 mi. track. you will feel like you have a whole new car on the street, the stupid-grin factor will definitely come back :D
 

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3.917 all the way. It's only a 10% reduction. You'll actually use all of 5th gear, and still have a descent rpm at cruise speeds in 6th.

It's an excellent way to make your Z faster. Just like changing gears on a bike.

This is def. in my short list of future modifications. How often do I really need to go more than 160mph anyway??

Won't affect your gas mileage much unless you include the new flogging of your "new" car. :irock:


I believe you are only dropping max mph in each gear about 2-4 mph. Not sure however. Need to research (math) those numbers exactly.
 

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I lost almost 10mph @ 2500rpm in 6th gear, iirc the top speed loss is something like 20mph, but all above the stock speed limiter

there's a comparison graph @ my350z.com from 350evo
 

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its such a good mod. It doesn't place any more strain on your drivetrain or engine, just means that at all speeds you're closer to the powerband.
 

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This is a mod that bang for the buck...you will feel a difference. When I first tested it ..I was like holy ____! This is the gearing that should ahve come on the track model cars in the first place.
Contrary to what people think...it does not make first gear too short. It has no effect on the speedo as the wheel speed are taken from the drive flange rings on the outside of the rear housing. It is a great imporvement in acceleration.
Cheers
[email protected]
 

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I've been talking to some guys about doing this as well.

So your RPMs will be about 300 higher on regular interstate cruising speeds?
 

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I live in Atlanta GA...where can I take my Z and have this done???? gear and installation ..How much?
 

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Originally posted by Dwnshift@Aug 5 2005, 09:51 AM
This is a mod that bang for the buck...you will feel a difference. When I first tested it ..I was like holy ____! This is the gearing that should ahve come on the track model cars in the first place.
Contrary to what people think...it does not make first gear too short. It has no effect on the speedo as the wheel speed are taken from the drive flange rings on the outside of the rear housing. It is a great imporvement in acceleration.
Cheers
[email protected]
[snapback]141936[/snapback]​


I'm about to do this MOD if I can find a doc as to your statement of where the speedo works from. My speed shop and I are concerned with ABS and computer readings.

Any more info would be great. (What happened to the old days of just throwing 4.56's in your Chevelle and going street racing?)
 

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Originally posted by jkenefic@May 13 2005, 05:01 PM
I see what you are saying about the first gear thing and the added horsepower of forced induction...

Seems like this mod would be great if it lowers your times by .5 sec in the 1/4 mile.  What else out there can you buy for under $1000 installed that will effect your times by that much?

Come one guys!!!  anyone out there with the 3.9 final drive?  Some feed back would be great!
[snapback]103530[/snapback]​


$1000 installed? Where at? Im in Southern California, here they charge like $1600.
 

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The speedo/ABS is not effected. The wheel speeds are taken from the out put flanges/ABS rings on the rear end housing. Look at where the axles connect to the outputflanges. If there was a problem with it....there would be 100 people complainign about it.....I can tell you that. The only mod that will effect your Speedo is going wit a larger diameter wheel/tire package.
The only way this will effect for ABS is that the shop that installs the housing .... forgets to move the sensor wires and crushes them when they re-install the unit causing them to short out and then you will get an ABS light.
We have sold over 100 of our 3.9 ring and pinions to 350Z and G35 owners
 

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has anyone verified how much quicker this set up is.is it really .5 seconds? i read that this mod has about a %10 gain.so on a stock engine that would be the same as 30 hp,right? that would be about .3 second in the 1/4 mile wouldnt it?still execelent bang for the buck.just curious if anyone has any numbers. also if you add 100 hp would this gain .4 ?
 

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1/4 mile times are where it becomes a gray area. When you drop in the 3.9 gears, it may cause you to shift an additional time going down the strip. This might actually make your time worse.
 
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