myinfinitig20

Been researching final gear ratios for a while now and im still prett lost. Not really family with gearing and whatnot but from what i've read im guessing that a 3.9 gear from 350EVO might be too aggressive for my auto. SHould i just get a 3.5 gear from someone that is upgrading to a 3.9? What would be the acceleraton gains...we talking 10% faster acceleration, 15%?

Thanks for any info
Nick

Tfasano

iTrader: (1)

This is the million dollar question for us 5AT guys.
3.5 or 3.9?

ragtopz

not too sure about what the gains would be, but if you are going to upgrade your rear end gears, i would go with the 3.9. too bad there is no 4.1 out for it yet.

on my 88 5.0 mustang auto, it was running 2.73 stock in the rear. i upgraded it to a 3.73.... (numbers may be off a bit... been a while) and the difference in acceleration was huge. at 60 mph, i think there was only a 500-600 rpm increase.

many of the later model mustangs that come stock with 3.27 have been upgrading to 4.10 with no real negative effect.

of course it will drop your top speed a bit, for me that would be a non-factor.

booger

iTrader: (6)

Plus...I dont think I could get to the line in a 1/4 mile in 3rd....dont know...any one know ?

Redline350Z

I'm trying to figure out the same thing... did some math to see what the comparisons would be like. Check this thread to see what I came up with:

http://chicagoz.com/v-web/bulletin/b...hp?p=5681#5681

You definitely won't finish the 1/4 mile in 3rd gear with the 3.9 final drive. Even with the raised rev limiter you'd still need to shift to 4th gear.

For road courses, I'd love to have the 3.9 because it would make 3rd and 4th gear useful instead of just 3rd gear. But I do a ton of highway driving and I think a 15% gear reduction is just too much - the gas mileage would probably suck. It would make corner exits awfully quick though!

If there were a 3.7 final drive, it would have roughly the same effect as the 3.9 on a 6MT. I just don't think there is enough demand for it to justify anyone producing one.

booger

iTrader: (6)

Whats the stock FD ratio in the AT5 ?

shinz

iTrader: (3)

Stock is about 3.35 i believe. The 3.5 is not that big of a difference from stock 5at, so the increase in acceleration is not huge. As far as the butt dyno goes, you can definitely feel a difference from the stock 5at. IMO it is a worthwhile mod, more so than other small bolt on's such as cats, exhaust, pullies etc...
I had my mods installed individually and after every install, i could not feel a difference in my car as far as the butt dyno goes. They just made my car louder, and the pullies changed the way my car revs. After i had the 6mt FD installed, i could immediately tell that the car was a bit quicker.

Another point to consider is the price. I'm not sure how mucht he 3.9 fd is, but i think it's in the $1000 range, more or less. The 3.5 FD cost me $350 shipped from a member on this board.

As far as gas mileage, yes im sure it decreased but by how much i can't say. I never really payed attention to how much mpg i was getting before the FD. While on the highway cruising @80 the RPM's are about 3200, and while slightly on throttle just to keep up that speed, the RPM is about 3400-3500 i believe.

dwnshift

iTrader: (1)

We do have a customer that is wanting to install the 3.9 final drive into his Auto350Z. Its a dedicated track only car.
We came up with the 3.9 final drive last year after a lot of investigating, race data and thought.
This is what should have come on the "track model" 350Z.

myinfinitig20

shinz...have you tracked your car..im interested in knowing what kinda gains we're talking about with a 3.5 in our car and is anyone seling a 3.5, im down to buy for 300ish.

myinfinitig20

how hard is it to install these things? anyone down to help?

dwnshift

iTrader: (1)

It is highly recomended to leave these installs to a tranny or driveline shop or someone with lots of exp working on them.

Redline350Z

Here's a 3.5 for sale:
https://my350z.com/forum/showthread....threadid=89410

Q45tech

Acceleration is roughly a cube root function so a 15% higher rear diff gear would increase acceleration by ~~2.5%.........roughly 0.3 seconds decrease in quarter mile times not counting any increase time for any extra gear shifts.

As a case in point the old Q automatic had a 3.538 diff gear and with a JWT ecu [7300 rpm redline] could finish the quarter in 2nd gear at 14.8/97 mph[7200 rpm]. Changing to a 4.083 diff gear [15.4% increase] resulted in an improved 0>60 even 80mph but a decrease in quarter mile because 2nd gear ran out at 82 mph with 4.083 and created another 0.8 seconds lost in the tranny shift...........net net the quarter mile took 0.3-0.4 seconds longer with the 4.083!!!!! All the early speed gains were lost in the shift time.

The newer 5AT shifts a little faster so at absolute best it might be a WASH............try finding and fitting TEMPORARILY *a 22-23 " diameter rear tire [possible?] to get an ideal of what to expect.
* not safe to run on street due to lower load index of such a tire!!


A 15% higher diff runs the engine 15% higher rpm at same speed so it use 15% more fuel per hour/minute /sec at the same ROAD speed.............whether 2.5-3 mpg highway cruise loss is worth it?

Tfasano

iTrader: (1)

OK, so you suspect a wash with the 3.9 on the 5AT. What about with raising the line pressure in the tranny? Faster shifts should make up the loss in the extra shitf?

Z1 Performance

iTrader: (564)

I would recommend storngly against buying a used ring and pinion...it is a nightmare waiting to happen, as it will be nearly impossibly to get them to mesh the same way once they are taken apart. This came direct from my installer, who does high peformance drivelines for a variety of cars, but mainly Amercian drag cars. I actually ended up just tossing my factory rin and pinion in the trash after completing the 3.9 install on my 6MT.

The proper way to do it would be to buy a whole pumpkin from someone...but the install costs alone make it not worthwhile to swap this stuff in used IMHO

Tfasano

iTrader: (1)

Can anyone back this up with real life experiences?

Z1 Performance

iTrader: (564)

just ask anyone who installs the stuff all day long

booger

iTrader: (6)

I still have 3.5 ring and pinion for sale

shopdog

Not necessarily. Fuel consumption is a function of load. Higher RPM at a given road speed only increases load by the amount of additional pumping loss and drivetrain friction. This will cause some decrease in MPG at cruise, but not 15%.

In other words, the engine will be operating higher up the hp and torque curves, but the load doesn't demand more hp or torque, so you'll actually be driving the same speed with the throttle closed more than it was with the previous final drive ratio. Manifold vaccuum will be higher, so the cylinders will receive less mixture per stroke than if you were maintaining the same throttle opening as before the final drive ratio change.

maonisity

I'm driving 6MT and thinking of installing TECHNOS ECU and 3.917 Final Drive. Could someone give me some suggestion?

TECHNOS ECU: http://www.technosquareinc.com/350z.htm
FINAL DRIVE: http://www.350evo.com