Changing the final drive, (Gearbox)

[Pritchard]

Afternoon,

 

Search is not my friend today. Found plenty of hits on the US forums, but not on the sane side of the pond.

 

Standard gear ratios take 6,500rpm DE engine to a theoretical maximum of 184mph. Thats 29mph over the electronic limiter, 84mph clear of an auto-ban (not to be confused with auto-bahn!) and 114mph past a speeding ticket.

In realistic terms, racing down the (private) road from a standing start and your in speeding ticket territory during third.

Point being - there is alot of 'wasted' mph in the top end that i will never use. why not put that to good use in acceleration instead.

 

so, theres 2 final drives that iv spotted for the 6speed box. the 3.9, and the 4.08.

The 3.9 drops the final by a nice 11% in multiplier. The theoretical maximum then becomes 164mph (-17mph).

 

MPG is relative to throttle position, rpm and speed. as i dont utilise the upper gears around town at the moment i still have them to select if the 3.9 allows me to do so, this being said MPG around town shouldnt be affected.

longer cruises will be, likely for the worse. but, then i didnt buy this car for its MPG qualities

 

so, anyone here fitted one of the aftermarket final drives?

What i am after really is a competent mechie to fit one. last thing i want is a berk who cant re shim a pinion properly.

 

Gearing is so often overlooked when modifying in my opinion. and a car with legs like this need to be curtailed to get me from A to B quicker

 

Pritchard

[Deano94]

I've also been looking in to doing this to mine. My main concern was loss of traction but couldn't agree more about it being overlooked. More than one way to skin a cat. From what I've read, 4.08 is the one to go for.

There is a topic on here here i found useful

http://www.350z-uk.com/topic/18090-39-final-drive/

[brillomaster]

its a good point, obviously Nissan selected gear ratios based on a number of things, but don't forget, the zed is a relatively heavy car, with a very flat torque curve, which is why it can push longer gears effectively.

 

yes around town mpg should be the same as you can drive in 4th when you would previously be in third, but as it is 70mph in 6th is exactly 2,500rpm, so I imagine that figure will go up to 3,000rpm at 70.

 

on a flat out acceleration run, you'd also have to consider the time spent changing gears - your 0-60 time will be dented if you need to change up to 3rd just before you hit 60, similarly 0-100 will suffer if you need to change to 5th say just before you reach that speed. it would be a negligible amount with a decent DSG gearbox, but on a zed you're changing gears the old fashioned way.

 

have you already done other mods to help with acceleration? a lightened flywheel and a throttle controller/uprev would both help a lot. if you're messing with final drives, you could also mess with wheel sizes - keep the 18s, but drop to a 40 profile all round which will have the effect of shortening (or lengthening - I can never remember which way round it is) the final drive.

[Pritchard]

yes i have a single mass lightweight fly already.

ECU is on the list too for the delimit of the lower gears.

 

i did punch it into the calculator last night with the gear ratios and tyre sizes, IIRC yes you are correct, 2,500rpm was 70. with the 3.9 it dropped to 66mph. but dont quote me, i didnt write it down! lol.

 

as for tyre size, yes i also looked at the lower 40% profile, this drops the gearing by 3.69%. a nice little drop, but i already have issues with speed bumps. i dont really want to lower the car, even if it is only 2cm

 

yea gear shifting is always going to be a bane. but for example:

6,500 rpm limit, 4th gear redlines at about 114mph with 3.5 FD.

6,500 rpm limit, 4th gear redlines at about 104mph with 3.9 FD

so in a race to 100, it still requires the same number of gear changes.

 

anyway, during the ECU remap the RPM can be slightly extended, so even though you may be beyond powerband, you can hold onto a gear longer than required to avoid another shift - say, nearing the finish line of a drag.

 

*edit., i decided to punch in me ratios.

3.5 FD - 2,500rpm in 6th = 71mph

3.9 FD - 2,750rpm in 6th = 70mph

Edited November 13, 2014 by Pritchard

[brillomaster]

ok seems reasonable to give it a go if you can change the final drive at a reasonable cost! but defo go for a throttle controller as well, no point doing anything if the ecu is still slowing down throttle openings in low gears.

 

if out and out acceleration is the target, you've got to look at making the car as light as possible - admittedly difficult on a zed without losing daily usability, but i'd definitely be looking at removing the spare wheel and swapping out the seats for lighter items, and getting some rays 18" wheels if you haven't already got some. after that it starts getting expensive and more compromised.

 

also if you're down in Kent i'd surely recommend Abbey Motorsport for any garage work you do decide on.

 

 

[Pritchard]

I'm already booked in with Abbey in a few weeks time. I'll be quizzing them then

 

In retrospec, the idea of dropping the tyre profile sounds like a nice idea - lower your car, shorten the tyre wall (cornering stability), and increase acceleration. sounds dreamy, doesnt it? lol.

 

of course, changing the final drive keeps your speedo accurate, or at least unchanged. where-as tyre changes mix things up a little.

 

out of curiosity i just ran a couple of tyre sizes. pretty easy to lower the car by 15mm and drop the final gear by 2.5%, while putting more rubber to the road:

 

Front:

225/45/18

to

235/40/18

drop of 15mm

 

Rear

245/45/18

to

255/40/18

drop of 17mm, and a 2.51% final drive reduction

 

bit off topic. sounds like an epic thread of its own. lol.

 

the 4.08 does sound awesome though.

theres some good clips of them on youtube. they look to suffer on grip, but then absolutely fly up the speedomoeter!

 

Pritchard

[brillomaster]

decent tyres will obviously help with traction off the line, get some MPSSs on there. problem with changing tyre profiles besides speedo inaccuracy, is increased arch gap - what you'll lose in ride height, you'll gain in arch gap, so you'll have to lower the car even more before it looks right, and then you'll have trouble with speedbumps and the live.

 

i'm looking forward to seeing our track car handles on lower profile tyres - we're going to drop from225/45/18 front to 245/35/18 front, and then on the rear go from 235/40/18 to 255/35/18 - drops of 15.5mm and 4.75mm respectively.

[GT4 Zed]

If you are going to change the FD I'll rather changed the lsd to Quaife ATB or clutch type diff. Will make a bigger difference to performance for about same price. But a 3.9 FD with an lsd will be awesome with a manageable 1st gear.

[docwra]

If you are going to change the FD I'll rather changed the lsd to Quaife ATB or clutch type diff. Will make a bigger difference to performance for about same price. But a 3.9 FD with an lsd will be awesome with a manageable 1st gear.

 

Hes right you know, its a lot easier to change the crownwheel in the diff than crack the gearbox open. I ran a very short Final Drive on my old Silvia and it felt just as quick as the S13 I had alongside it despite being about 130hp down on power, worth doing for sure