is there any dyno charts anywhere of before and after install? I would like to check out the torque curve.. or just discuss the performance enhancements.

 

the engine output doesn't change with the final drive, just gear ratios

 

That is my understanding as well. But changing the gear ratios well affect the output? which in return will affect dyno numbers?

 

IMO it will change power a bit but I don’t think nothing major at first I had my car tuned at 6.5 to 7 pounds with my old 2 1/4 Megan test pipes it choked the car a bit since I had dual 3 inch exhaust all the way back.

Then I changed to the AAM 3inch turbo back pipes with dumps I noticed the car woke up allot very nice power mod and sound also.

Then I changed my FD to the manuals 3.5 and yes I noticed a big difference the car is more aggressive and makes power more quicker.

When I had the 3.3 FD I race an rsx-s turbo on 10 ponds well he beat me by almost 2 cars well I changed my final drive. And a week later I ran a Lexus IS300 turbo on a t-76 at 16 pounds I beat him by a 1 to 1 1/2 and then he ran the rsx-s turbo that beat me and he beat it. So yea I think it made a difference on power and quickness.

 

Any idea why you can't or wouldn't want to use the 3.9 on the 5AT? I saw the for sale on a website and it said 6mt only. I know a few people have used the 3.5 but I don't see why no 3.9.

 

The 3.9 ring gear is a different size than the 3.3 so you can't do a direct swap. You need a 3.5 pumpkin as a base for the swap. If you want to put the 3.917 into a 5AT, you need to source a 6MT pumpkin first. You can swap the entire pumpkin.

Regardless of gear, the engine makes the same power. WOT at 5000 rpm in 1st gear is generating the same amount of power at the crank as WOT at 5000 rpm in 5th. Basic principles of gear reduction apply, so 1st gear applies more torque to the wheels than 5th.

On a properly calibrated dyno, you should see no change. Your torque curve will remain the same. If the dyno is improperly calibrated, you might see an overall increase in torque similar to running the dyno in a lower gear, just to a lesser extent.

 

the 5AT won't be able to shift fast enough in D mode. It'll just hit the rev limiter unless you have a valve body upgrade.

the 3.9 final drive puts the 5AT about even with the normal 6MT transmission only it has 1 less gear - plug the gear ratios into a gearing calculator if you don't believe it.

it has NO effect on power output, just torque multiplication which is not the same thing as engine torque/power.

it's about the same as if each gear became 2/3 what it used to be. So 1st gear would act like .67 gear, 2nd gear would act like 1.5 gear etc. Like you were running a lower gear, but not quite a full gear lower

yes it will kick you back in your seat 10% harder, but you'll be shifting 10% sooner

and because someone is bound to ask, YES it'll have an effect on your highway gas mileage. Not so much city mpg though cause you can just shift up into a higher gear, but you run out of gears on the highway.

And NO it has no effect on cruise control on 6MT, but YES it will have an effect on 5AT cruise control

NO it's not the greatest mod for a turbo - unless you're running slicks

YES it works great with centrifugal superchargers like vortech, HKS, and ATI, but probably a little more questionable with the stillen - would need very grippy tires to hook up

it has no bearing whatsoever on dynoing. It's the same sorta thing if you dyno in 3rd, 4th, or 5th gear - not much difference. The only difference being how much more efficient that particular gear is vs another one with it's location inside of the transmission. You will see no change unless you map out hp vs speed, not rpm

and before PMing me anything, run a search on this site cause the subject gets beat to death every single month

 

torque curves should be different with FD opposed to stock, shouldn't it? There should be more drop on the higher rpm, I would assume. different gearing should shift the power curves a bit. I wished there was a visual indication of where and how much the gains are (midrange).

 

k well here's a tq vs speed chart

tq or power vs rpm will not change though which is what your typical dyno chart is comparing

2nd chart is rpm vs speed

 

I made the swap and my cruise control works fine and I have a 5AT

I am actually planning to eventually run it with either Greddy TT or TN ST. As I understand this, the more power you put down, the taller gearing you need to hook up. I will simply achieve same acceleration on lower power. Another benefit of this (as I understand it) my cruising RPM in 4th at ~65mph is about 3500-3600, and the tach flies to 4k and slightest throttle tap. This way I would be able to keep the larger turbos fully spooled at cruise, and have an nice top end at 4th and 5th...

 

taller gearing is not 100% better for everything though. Your 80-160mph speeds will suffer with taller gearing. People with turbos seem to run the same trap in the 1/4 mile but do it at a couple mph faster - and 1 less shift sometimes. But even with 1 less shift, their actual trap speed hasn't gotten all that better because the gearing is weaker and really shows its slower accelleration in the 2nd half of the 1/4 mile

In reality shorter gearing is usually always better for accelleration with the assumption that you have traction . Most people with turbos don't really have traction because they run small street tires - they can't use 1st gear fully. You need a serious tire and suspension setup to do it. That's why people want the taller gearing from the automatic - to get that 6% taller gearing to hook up easier in 1st, 2nd and 3rd gears.

 

Except I don't venture beyond 120 mph that often. And my car is primarily street driven. I'll see how this setup works. I spoke with someone who runs the TN ST on 6MT with 3.9 rear. He was absolutely raving about it...

To be fully honest, I made this swap in the midst of an "screw FI, my motor is weak" phase. I would like a good centrifugal setup, but they are so damn loud...

 

yeah, I think I'd go for the 3.9's with any setup myself.

some people really had nothing but bad things to say about the 3.9's and turbos though. 1st gear is usually worthless anyway, might as well make the other gears have more punch

 

That's how I am hoping for the things to work out. And on 5AT 3rd gear can go to almost 100mph. I think that 85-90 mph in 3rd is still not too shabby.

Unfortunately this is the usual my350z.com FI project progress:

1)Read the forums
2)Get obsessed with dyno numbers more then the butt dyno feel
3)Go for over 400whp on stock internals, b/c that's what all the cool guys have. Come on, 400 is the holy grail. Even 10whp less is not cool. Same mentality that makes you feel that something for $9.95 is a better deal the $10.
4)Blow up the motor. At this stage most of us run out of money. At this point there is a fork ith the path
5a)Throw in forged internals
6a)Well, not that you're build, don't be a wimp and go for 500-600 whp, like the cool guys
7a)End up with something unstreetable and nearly undrivable, but be everyone's hero.

But who cares, each of your update threads gets over a 1000 views and 5 pages of replies in less then 3 days. Not to mention that your tuner will absolutely love you and blow so much sunshine up your ***, you'll start believing it yourself (I know from experience. I dropped almost 7G in a month at one shop, the owner test drove my car, I drove back and he kept on telling me that I am such a great driver and should be playing with a ton on power ).

OR

5b)Put the car in the driveway

6b)Wait for god know how long to get the $$ together. The more you wait the hight # goals you dream up

7b)Proceed to 5a

 

yeah absolutely. A lot of the guys who have gone 600+whp have ended up saying the reality is that you rarely get to use all that power on the street. The car ends up being so loud that it'll get 20 people calling the cops on you and there just isn't a lot of opportunity in a city to go WOT without drawing major attention to yourself.

I might not be turboed, but my car is plenty fast as is for where I live (big city)

yeah that sequence of events is what happens to a LOT of people who end up with a money pit car and that starts taking the fun out of it when you start wondering what higher end car you could have bought. And no matter what, the Z's engine still is hardly anything but bombproof even with a forged block people are getting head lift and other weird things


the mods that give you just as much kick in the *** like gearing, flywheel, crank pulley, light wheels/tires, good traction, good suspension setup, good handling, less weight etc - people just don't care as much for that stuff cause everyone wants to see a dynochart and only care about that - as if that's all that matters

 

Another big my350z.com calculation is:

Corvette Z06 makes 500 hp. I make 600 hp. I should be able to beat it. Total disregard of difference in gearing, weight, suspension, etc...

It seems that every time I think about FI, I end up with a TT. I really need to figure out what I want before putting any more $$ into this car...

 

That kind of logic is pretty common on all automotive forums. Most people focus on easily tangible mods that add power and show up on a dyno or at the strip. Suspension, braking and other ways of making the car faster without adding power typically get glossed over.

 

Reading this thread makes me thankful that I just ordered the Central 20 gears for my Z

I'm an auto-x and road race kinda guy, so until I've got the money for the JWT TT setup and a built engine...which I PLAN to keep in the horsepower level of mere mortals, the gears will be just fine for me for now.

I think it realy comes down to what you want. If numbers on a paper and pulverizing someone in the 1/4 matter to you, than rock on with the 1000 whp race, but if you just want fun AND reliability, driveline mods rock!

Sorry... just my $.02

 

I just got the 3.9 done on my 5at....still have cruise control...and it does shift in D mode just not the best under wot....other than that best money I have spent on my car besides for my wheels

 

In my experience, you always LOSE some rwhp when you go to a shorter ring and pinion setup because of the increased friction. In the Camaro and Mustang world it was typical to lose 7 or 8 whp when changing out to 3.73 or 4.10 gears. Yes the car is faster overall due to the gearing change, but on a dyno you will actual drop a few ponies. Don't let this discourage you, your car is quicker with the shorter gears.