my question is whether i would be better of with a 3.7 or a 3.9 final drive in my '06 350z mt... i have pretty much all the bolt ons you can get and nitrous might be an option also. I am eventually planning on building the engine but no s/c or turbo... I basically want something i can drag race with... either at the track or wherever. I have DR's that i use to race with so im thinking traction probably wont be a problem. so that will be the main objective here... i have heard lots of different things from different people, but i wanted some more input before i decide to do anything. thanks

 

3.9 will improve performance more than the 3.7

not sure what you're looking for here... there are ratios above 3.9 now available I believe - so that also is a consideration.

 

def. 3.9 kinda like puttin 4.10's on a mustang pretty much. 10% in each gear is what i hear. You will lose a mpg or so. Top speed will still be over 160 i think so no need to worry in that department good luck and post a log if you do the install etc.

 

If your mainly concerned with tracking your car, go with 3.9. You may not see much of a difference going from 3.5 to 3.7, but 3.5 to 3.9 should be pretty significant.

 

i was kinda leaning toward the 3.9 myself, but i heard some people say that it would rev up to fast on the track and i would not like it as much as if i had the 3.7... true? or no?

 

no suggestions???

 

I like my 3.7 fg, but probably could have been ok with the 3.9. How much highway driving do u do? Do u have a lightweight flwheel?

 

3.9 final drive was the best NA mod I have done. Period.

 

who are these "some people" and what do they drive?

the 3.9, or the 4.1, would be an AWESOME upgrade, especially on an modded NA Z...I know, I own one

 

well these some people are just friends i talk to that drive z's also... and to answer a question above, the freeway driving is not much of a problem cause pretty soon my car is going to be only a track car or fun car... (other means of transportation to come)... but it just makes me think. how much faster does it make the car feel with like a 4.0 or 4.1... since it revs up so much faster? i just hope with something like that i would be able to keep traction on the drag strip
And no i dont have a lightweight flywheel

 

Gears are a multiplier. A 3.9 is roughly 10% shorter than a 3.5, so accelerates 10% quicker in every gear. Conversely, top speed per gear is reduced roughly 10% as well, which is why it's a great combo for those with increased redline. I can tell you on my car, the 3.9 + an 8200 redline = a good time

 

Adam,

I've been trying to do some research in final gears, running simulations, etc. The flip side of the equation is that longer gearing allows you to stay in a lower gear longer, and thus put down more torque than someone who has to shift because they have run out of rpms.

What is your take on this?

Doing many simulation runs w/ a 500 hp/tq car, I found the 1/4 mile time dropped significantly only below 3.133 final drive ratio. I was thinking that is the sweet spot for high hp?

(Sorry for going OT bring higher hp into the equation - did the simulations using the torque curve from a 700bb kit on a built motor - but would appreciate your input)

 

Its a bit of tradeoff when you're comparing different final drive ratios.

On a stock 5AT, the 3.357 lets you finish the 1/4 in third pretty close to redline. I'm not sure what your torque curve is, but dropping to a 3.133 would probably let you finish around 5000-5200 rpms. Unless you have a huge power drop over that point you wouldn't gain anything from a wheel torque perspective.

Unless you run a setup with the 3.357 that makes you shift out of 3rd, I can't see how going to the 3.133 would make the car faster in the 1/4 unless it minimizes wheel spin and improves launch. Since neither setup leaves third, I don't think the extra time in 1st and 2nd would offset the lack of acceleration you have in all 3 gears as 1-3 are where the FD difference will be most predominant from an acceleration / wheel torque perspective.

I say this mainly because up through third, the shorter gearing will be pulling strongly. It isn't until you get up to 4th or 5th gear where the differences in wheel torque are small enough and I think you'll really be able to capitalize.

My perspective is for the 1/4 maximize gearing to your engine to finish in the top of 3rd on the 5AT, which would be the 3.357 on stock diameter tire setups. The 3.133 will really shine on 100-150 runs, 1 miles runs, etc...

Unless you run a short DR or slick, then the 3.133 is ideal.

 

does anyone know who makes and sells a 4.1 final drive, and what would be the expected power gains...~15%?

 

Great analysis helldorado - and you're right I found the 3.133 to be slightly slower than 3.357 (but by hundreths of a sec) - using this site to simulate runs: http://www.nightrider.com/biketech/accel_sim.htm (yes it is for bikes, but I figure just comparing rear diffs, it'd be ok, and I entered all the parameters based on my car's specs: wheelbase, coefficient of drag, etc).

What I was thinking was that the 3.133 as you mention provides some advantages for higher speed runs as well AND I think would give a boost to fuel economy as the top speed cruising rpms are lower (though admittedly I don't know what the optimum rpms are for cruising at 75 mph for the VQ in terms of fuel efficiency - I know if you go too low on the rpm scale, you lose efficiency).

I found it interesting that putting in different ratios into the simulator above brought gains (I tried 5.0!!!), but not of the magnitude I was expecting. Maximum Gs increased significantly, but 1/4 miles times improved only slightly. Of course that simulator takes traction into account and I didn't know how to change coefficient of friction for different tire choices. Anyway it is interesting to play with...

 

Yeah, what ends up killing the faster gear ratios, is the fact that you have to shift into 4th. Pretty much, once you end up in 1:1 or overdrive, heavy acceleration is gone and it puts more strain on the engine.

That calculator isn't too bad. Its a hair slower than real world, but overall, the numbers seem pretty accurate for speeds, shiftpoints, etc... but its most useful if you dump the results into a chart to help give a graphical representation of the numbers. What I did find interesting was how it backed up some of the conclusions I had drawn after doing my own gearing swap.

 

its funny... with all this info, its still really hard to try and make a decision and knowing for sure its the right one for the application. lol

 

i bought 3.9 central 20 but i recently removed the gear............extremely bad gas milage i get 15 avaerage driving civilized...........also first gear is completely useless i didnt even use it to get of the line.....always second

 

Your the first person that i have see to say anything babd about the FD swap!

Can you give any more to your answer ? i want to do the 3.9 FD swap but may not !!

 

exactly - I have over 300 NA hp, at the wheels, and have had the 3.9 final drive for a few years now. 0 change in gas mileage from pre install to post (I get 320 to the tank, on the nose), and first gear was anything but useless - in fact, it ripped

If you are going NA, the 3.9 or the 4.08 (4.1) is absolutely the way to go, no ifs ands or buts about it. If you are using a centrifugal supercharger, the 3.9 or 3.7 is for you, and if you are staying turbo (single, twin, whatever), go with the 3.3 from an auto