Advice Wanted - S15 Helical Diff Centre Info R34 Gtt

[david.r34.gtt]

I have read lots of threads on this forum, plus plenty of information elsewhere, and as a result am about to take the plunge on an upgrade on the stuffed viscous 4.11 diff in my 1998 R34 GTT Series 1.

My plan is to buy a complete Aus S15 helical diff unit (ie front housing, 3.67 crown & pinion, helical diff centre) and provide that along with the diff unit from my R34 to a differential specialist. They will remove the S15 helical diff centre and rebuild the R34 diff, keeping the std 4.11 gear set and reusing the side flanges to suit the R34 GTT drive shafts.

Once they are done, I reinstall the rebuilt diff unit in my car using the existing drive shafts.

Please let me know if there are fundamental errors in my plan before I go and splash the cash.

To answer the inevitable questions:

- Yes, I know a R34 GTT Series 2 diff is also helical, and would be an easier replacement, however, I have been looking for one unsuccessfully for the last 2 months and given up

- Yes, A 1.5 or 2-way would be better for the track/drags, but this is primarily a road car that will see the occasional track day, and a helical/torsen diff is better suited to that use.

- Yes, I could change to a shorter diff gear set (eg 4.3:1), but this is a road car. See above answer.

Thanks

David

Edited July 23, 2015 by david.r34.gtt

[Kinkstaah]

Can be done. I had a S15 helical in the back of my R34.

When that ended, I got an actual VLSD from the new donor R34.

The VLSD felt much better. Would not go back to Helical ever again. Only because people typically get more power in these things.

With more power, dat inside wheel single spinning is a problem.

2/10 would not do again/glad I no longer have it

[david.r34.gtt]

Can be done. I had a S15 helical in the back of my R34.

When that ended, I got an actual VLSD from the new donor R34.

The VLSD felt much better. Would not go back to Helical ever again. Only because people typically get more power in these things.

With more power, dat inside wheel single spinning is a problem.

2/10 would not do again/glad I no longer have it

Thanks for confirming that the S15 helical can work in the R34 diff.

I have a helical (aka torsen) diff in another car, and have been very happy with it's performance, although that car does produce a lot less power than a GTT Skyline.

My experience is that the sort of problem you describe with the inside wheel spinning usually happens when the suspension allows that wheel to unload. With appropriate sway bars and springs, that should not happen.

[GTSBoy]

I concur that there should be nothing wrong about helicals in powerful cars. My brother-in-law's Supra is about 400rwkW and the helical in that (and in every other Supra that has one) is excellent.

I have one (S15 helical) in my R32. Actually I don't. It's out at the moment because there's something wrong with it and hopefully the diff shop has worked out what it is by now. Anyway, I loved it when it first went in.

BUT.....here's the problem with your plan David. You cannot use your original GTT 5 bolt stub axles. The helical centres need one of the shafts to be a different length to those used in the viscous diffs. If you can't find an R34 helical, then you are likely to have as much trouble finding the stub axles to suit. What I did was use the 6 bolt S15 stubs and some S chassis driveshafts. Less than ideal because they are nowhere near as strong. But it works.

[david.r34.gtt]

Thanks for the feedback "GTSBoy"

When you say "The helical centres need one of the shafts to be a different length to those used in the viscous diffs", are you saying

a) the 5-bolt and 6-bolt stub axles are the same length, but that the helical centre is a different length to the viscous centre, so one of the stub axles will be offset in the diff housing, and so the drive shaft on that side also needs to be a different length, or

b) the stub axles are a different length, and then because you have to use the 6-bolt stub axles you then need the matching axles?

Either way, my plan is stuffed without using the 6-bolt drive shafts. My S15 diff includes the stub axles.

Anyone want to buy an S15 diff? :-)

[GTSBoy]

What I'm saying is....

Any 5 bolt stub axle from a viscous diff is the same length as any 6 bolt stub from a viscous diff, and

Any 5 bolt stub from a helical diff is the same length as any 6 bolt stub from a helical, but

You can't use the stubs from viscous in helical, or vice versa, because at least one of them needs to be a different length (I can't remember which).

So yes......if you use 6 bolt S15 stubs, you need to use 6 bolt driveshafts to match (either from an S chassis car or one of the pissweak NA Skylines).

Depending on what the news from my diff guy is when I get home next week (I'm working in the US this week), I may well be interested in buying your diff. Depends on whether my centre is the problem, or if it's the CW&P that's crook. I should be home on Monday.

[Kinkstaah]

In addition (and in my previous setup) my R34 had a S15 diff, stub axles, drive shafts, AND the ABS computer and wiring from a S15.

You need ALL of it to work.

And when it does work, it works ....okay.

Source, coming into a corner with a good setup under power.

Mind you, power = 400kw

Can literally see from video the traction go from two wheels to one. It was at that point that the lack of consistency and feel made me happier to have a more consistent but less absolute grip of a mechanical diff.

[david.r34.gtt]

Thanks Kinstaah.

Too much mucking around and compromise for me, having to use the S15 stucks and axles. Being slightly OCD, I would forever be concerned about breaking them.

So I have decided to just go with a Nismo 1.5 Way and put up with the less than ideal on-road characteristics.

Thanks everyone for your great feedback