I've been looking into this for the past 6 months and I'm no expert but this is what I can gather are all of our options for mission upgrades:
1. S13/14 box which is still regarded as slightly stronger than the S15 six speed but I'm not totally convinced. If anything like Andeep has said it's just these boxes are more readily available and cheaper than S15 version.
2. Nismo gearset upgrade - apparently totally not worth the money and provides no more robustness than the stock S15 gearset. You just get closer ratios.
3. OS Giken gearset upgrade - I believe this is slightly stronger but not much. Again for your power figures complete waste of money. It's not even dog teeth (think they are still helical in design) so there's no more efficiency in the tranmission after fitting and costs £k's!!
4. RB25 box conversion. This is what I'm going for early next year. As a guidance for the UK, I've been quoted by Garage-D around £1,500 to get the whole lot done including parts etc. I don't know the exact amount of power the RB box can take as a rule of thumb but it's definitely higher than the S15 6 speed and it's definitely more than enough for what I need (sub 400 bhp @ fly). You shouldn't need to mess about with welding an S15 bell-housing to the RB main box cos there's adaptor plates out there which will allow you to mate these together. As long as it's relatively thick and well made you'll be ok with this. Other considerations are mating the RB first half of the prop to the stock S15 second half of the prop if using S15 diff etc. Also new crossmember required to fit RB box as stock one doesn't work. Transmission tunnel needs bashing a bit to accomodate the RB box (it's bigger lol) and finally an adaptor needs to be fabricated to move the gearstick position further back as an RB box will be too far forwards if you just plonk a gear-knob in situ within an S15 interior... other option is to cut your centre console away!! :wack:
5. Z32 box conversion. Wing told me about this as he is able to sell you adaptor plate for this. Very similar to the RB box conversion but there might not be a need to bash the tunnel as I think the Z32 box fits ok. Also like the RB box, the Z32 is going to be stronger than the S15 6 speed but not sure if it's stronger than the RB item? I didn't want to go down this route in the end cos Z32 manual boxes are gonna be much harder to source in the UK as my impression is 99% of Z32's here are ****-tronic.
6. Dog gear set - I looked into a proper PPG dog gear set which could withstand high power/torque and be great for eliminating transmission losses. But firstly the cost is not cheap at all. Apex Performance quoted me around £3k+ for an entry PPG dog gear set and I would have to buy an S14 box to put this into as don't think any of these companies will make a gear set for the S15 6 speed. Also as a daily driver I'd end up f*cking up the use of these all out dog sets and I'm sure they will make my life feel like it's not worth living when I pop down to the shops. Although for race application they look immense as you can 'flat shift' and they can take all the abuse (although not abuse from bad shifting technique!)
