Offroad-Express

I just bought an RD28 Turbo engine to replace my RD28 engine.
The RD28 is running a 5 speed manual fitted in a heavy classic car(Mk2 Zephyr) and is most likely out of a Laurel.
Have been running with the RD28 and 5 speed for 12 years now I want to improve things if i can and was thinking about fitting a S15 6 speed gearbox behind the new engine, the 6 speed has a 0.767 top gear ratio which will give me a little more economy, longer legs is what my car needs.
My question is, does my current 5 speed have an overdrive top gear or is it 1:1 ?
I know that some vehicles have have 1:1 5th gears for the diesel models.
I suspect my current gbox has either 1:1 top gear or at best 1:8.54
I have found info that there are basically 4 types of 5 speed(S14) and two are 1:1 5th gear and two are 1:8.54 and 1:8.46
Also, will the S15 6 speed from a Silvia bolt in where an S14 used to live???

screw that stick a RB25de in it its got another 40 odd KW and it's not even turbo charged compared to an RD28et...the other reason i say that is the non turbo Rb25de are cheap as chips compared to anything with a turbo..

plus the 6 speed is weaker then the 5 speed.I doubt 3rd and the layshaft will last long shifting the zepher along with a turbo engine in front of it it was designed for the silvia (light in comparison)and very few of nissan's performance engines have a 6 speed behind it for that reason...

what you really want is a R33 rear wheel drive box but that depends on your budget. and being from a laurel which shares the same floor/ firewall and front chassi as the skyline and A31 cefiro it is all bolt in where as the silvia box will most likely require a new cross member where as the skyline box will bolt in to your already modified mounts.

Then if its road legal with the change in box and engine it will require re certification where as if you went with the skyline 5 speed and non turbo Rb25de you could get away with out it as it is the same family of platforms.

The box you have is most likely 1:1 4th, 5th being an overdrive.
Any gearbox that bolts up to rb series (all 20,25,26,30) will bolt up to the rd28. (the rd28 and rb30 block is the same casting). The 6speed will not bolt up as the sr20 it came out with has a different bellhousing. A custom adapter or the likes could be sorted, but would be a lot of effort for small gains.
The 6speed has the reputation of being weak, but when put behind a rwd turbocharged engine that the driver is clutch dumping etc any box will have a hard time.
The 'big box' that came out behind the rb25det would have all the strength you'd need and more, but at 4x the price of a 'small box' (like currently fitted) its a lot to pay when the small box will most likely handle your useage.
The small box comes out with a variety of ratios, but its near impossible to tell which vehicle had what, and even then know if the box is original to that vehicle. But playing with gear ratios, and diff ratios (if zephyrs have many options?) could get you your desired ratio spread.

R32 A31 C33 and r33(1989 to 1997 skyline cefiro laurel)

1st 3.214 2nd 1.925 3rd 1.302 4th 1.000 5th 0.752. Final drive 4.111

so yes 5th gear is over driven

dont quote me but its my understanding that the vl commy 5speeds behind the rb30e has a taller 5th gear than its nissan counterparts they were built to be more suitable for aus conditions etc just something to ponder on.

taranaki surf wrote:dont quote me but its my understanding that the vl commy 5speeds behind the rb30e has a taller 5th gear than its nissan counterparts they were built to be more suitable for aus conditions etc just something to ponder on.

only the Rb30ET had different ratio's to suit the torque curve of the turbo engine. all the N/A RB powered VL's ran what would be considered normal.

1 - 3.321
2 - 1.902
3 - 1.308
4 - 1.000
5 - 0.759

the ET

1 - 3.580
2 - 2.077
3 - 1.360
4 - 1.000
5 - 0.760

also the RB30ET VL ran the same diff as the 5.0 litre V8 which was a 3.45:1 compared to the standard 4.11:1 of the N/A VL

Coincidentally the only nissan box in the entire turbo range that doesn't have a 1:1 driven 4th gear is the silvia six speed.