370z LSD differential swap into R34 GTT

[shortyboy]

Hello, new user here and new owner of a '98 ER34 4 door. Located in Hawaii. I've tried searching the web but only found some old info on 350z diff swaps. I see that GKTech offers diff conversion bushings for S/R chassis to 350/370z diff. The main reason Im looking to swap is because in 5th gear, going 110-120km or 60-70mph, rpm is at 3k or more. I believe the gtt manual diff ratio is 4.11. The diff that is available locally to me is a 370z diff w/lsd, so ratio is 3.6 ( need to check if auto or manual). Has anyone performed this swap? Do the stock axles bolt on? I know I need to drill diff flange to match driveshaft. Thanks in advance

[robbo_rb180]

Will need the s14/15 gktech bush kit, 370z axels and gtr/z32tt hub/bearing.

The diff will sit lower than current setup and depending how low your car is axel angles will be weird and may damage cv's.

I think my old rb25 would sit around 2800-3000rpm doing those speeds with 4.11 gear so nothing wrong there. A s14/15 diff may be a better swap as a 3.69/3.7/3.9 ratio and will bolt in pending whether you have abs/non abs car and 5x1/2x3 axels.

[GTSBoy]

Do shit properly. Put the gears you want onto the centre you want in the housing you need to use, with the stub axles that will work in your car.

Whole Z3x series swap is lazy and wrong, as Rob describes.

[shortyboy]

1 hour ago, robbo_rb180 said:

Will need the s14/15 gktech bush kit, 370z axels and gtr/z32tt hub/bearing.

The diff will sit lower than current setup and depending how low your car is axel angles will be weird and may damage cv's.

I think my old rb25 would sit around 2800-3000rpm doing those speeds with 4.11 gear so nothing wrong there. A s14/15 diff may be a better swap as a 3.69/3.7/3.9 ratio and will bolt in pending whether you have abs/non abs car and 5x1/2x3 axels.

 

50 minutes ago, GTSBoy said:

Do shit properly. Put the gears you want onto the centre you want in the housing you need to use, with the stub axles that will work in your car.

Whole Z3x series swap is lazy and wrong, as Rob describes.

 

Thanks for the input. Just saw the diff available locally and was wondering how much effort needed to fit. Since I was looking to add an lsd. s14 diffs would be easier to source than s15 in Hawaii. Car has abs.

[GTSBoy]

R34 GTT ABS uses wheel speed sensors that are tucked in close to the diff housing, reading tone wheels that are mounted on the stub axles. So, whatever "other" diff you obtain, needs to have the same. This reduces your options quite a bit - there's a lot of variability between models, early and late, manual and auto, etc etc. So do not assume that any other diff you can get whole will work.

You also need the right stub axles. If your R34 has a VLSD, then you could and should be able to put its stub axles into say, an S14 diff. That is provided the spline count is correct, which also changed across the years. The VLSDs have one quite short and one quite long stub axles (inside the diff centre), because of the design of the VLSD (which is shit, by the way). You cannot put those stubs into other types of diff, and vice versa.

If your R34 has a helical LSD (later S2 R34s had these, they are as rare as fcuk), then you already have a great LSD.

You can buy new CW&P gear sets. If you had a helical, I would merely be spending a couple grand (aussie) on a 3.7 or 3.9 gear set and put that onto your existing centre. Should be able to get it all done (the setup work is fiddly) for less than $US2.5k.

If you don't have a helical, then you can also get a Nismo GT Pro LSD centre that will come with stubs, and you can put the good gears onto that. That's another, probably, $1500.

Worth it, in all case, over the fcuking lottery that is Nissan diff tetris, trying to buy something 2nd hand that has possibly already been frankendiffed by someone (like me) who is willing to take apart several diffs to put the good bits together in one.

[soviet_merlin]

By the way if you haven't seen it there was this discussion around R34GTT diffs and diffs in general the past days. I feel like I learned a lot from it.

 

[robbo_rb180]

Also there is this page
So much info on nissan diff ratio, stub patterns and teeth. I think this info really needs to be copied and a thread made and sticky in Drivetrain section. 
https://www.eatsleepboost.lt/tech-info/nissan-tech-info/nissan-r200-diff-ratios/

Can use the part numbers on amayama to for your vehicle
https://www.amayama.com/en

[joshuaho96]

8 hours ago, shortyboy said:

Hello, new user here and new owner of a '98 ER34 4 door. Located in Hawaii. I've tried searching the web but only found some old info on 350z diff swaps. I see that GKTech offers diff conversion bushings for S/R chassis to 350/370z diff. The main reason Im looking to swap is because in 5th gear, going 110-120km or 60-70mph, rpm is at 3k or more. I believe the gtt manual diff ratio is 4.11. The diff that is available locally to me is a 370z diff w/lsd, so ratio is 3.6 ( need to check if auto or manual). Has anyone performed this swap? Do the stock axles bolt on? I know I need to drill diff flange to match driveshaft. Thanks in advance

Factory R34 GTT should be at ~3100 rpm indicated at 75 mph (120 kph). Easy way to tell if there's actually any room to run a taller (lower number) final drive is look at manifold pressure at highway cruise steady state. If it's already pretty close to atmospheric leave well enough alone. On an RB26 the ITBs obscure some of this but there's really not a lot of room to drop engine RPM. 

[GTSBoy]

An RB25DET Neo should be able to pull a 6th or a ~10% taller diff ratio without blinking. No need to worry about putting it into boost.

[joshuaho96]

On 3/14/2024 at 9:27 PM, GTSBoy said:

An RB25DET Neo should be able to pull a 6th or a ~10% taller diff ratio without blinking. No need to worry about putting it into boost.

My math did put it right around 3.7 final drive being about ideal for highway cruise but it doesn't really consider anything else. I thought about doing similar on an R33 GTR but I find myself using pretty much all of the reserve torque in fifth quite often. The spacing between 3rd and 4th is also kind of strangely small while the gap between 2nd and 3rd is huge. The other thought I had was final drive ratio changes requiring speed sensor recalibration so I mostly gave up on those thoughts.

PAR engineering also offers a 0.7 5th gear. I still don't really know if their ratios are the way to go or the more common stuff like PPG/OS Giken/etc.

[GTSBoy]

1 hour ago, joshuaho96 said:

but I find myself using pretty much all of the reserve torque in fifth quite often

You mean all of the reserve NA torque? But for a Neo with a better inlet manifold and VCT, there is both more of that available, and also....meh. Use a little boost, change down a little more frequently, etc etc.

All I can say is that 1st gear would be a hell of a lot more useful with a taller diff ratio. I used to have 4.3 (because it was a 2L) and I had that in there when I first did the Neo swap. After the change to 4.11 gears, the car was infinitely better to drive. The old mantra from the 80s, when adding a turbo was, put in taller diff gears, use the torque adder that you've just bolted to the exhaust manifold as it was intended. Yet, for some reason, Skylines have 4.11 gears and bloody Silvias have 3.7s! Never understood that.

[joshuaho96]

5 hours ago, GTSBoy said:

You mean all of the reserve NA torque? But for a Neo with a better inlet manifold and VCT, there is both more of that available, and also....meh. Use a little boost, change down a little more frequently, etc etc.

All I can say is that 1st gear would be a hell of a lot more useful with a taller diff ratio. I used to have 4.3 (because it was a 2L) and I had that in there when I first did the Neo swap. After the change to 4.11 gears, the car was infinitely better to drive. The old mantra from the 80s, when adding a turbo was, put in taller diff gears, use the torque adder that you've just bolted to the exhaust manifold as it was intended. Yet, for some reason, Skylines have 4.11 gears and bloody Silvias have 3.7s! Never understood that.

Right, I'd need to be able to log exactly where the stock ECU is in the fuel tables to see where it's actually going into high load enrichment but it doesn't take much of a grade in my experience to start getting into positive pressure. God forbid you're closer to 80 kph, the reserve torque margin gets very thin around 2000 RPM, 2800 IIRC is min BSFC according to Nissan's dyno data. The R34 GTR 3.545 is to some extent compensating for the shorter 6th gear, roughly equivalent to a 3.7 with the FS5R30A. 

[GTSBoy]

I drive the Neo in 5th at 50 km/h. You don't want to floor it, because it does grumble a bit, but it is flexible enough to roll around down that low. At 60-70km/h it is perfectly driveable in 5th.