Choosing a gearbox conversion for your 180SX, S13, S14, or S15



Some gearboxes we acquired during R&D, from left to right: S15 6-speed, S14 5-speed, Z32 non-turbo 5-speed, R33 RB25 Skyline 5-speed, z32 twin-turbo 5-speed, R32 RB20 Skyline 5-speed, R34 RB26 GTR 6-speed


There will come a time when you either need a new gearbox right away (gearbox is broken), or you will want one before you break it. With enough resources you can adapt almost any rear wheel drive gearbox you want to your SR20... the question then becomes "What gearbox should I use?" Here are a few reasons when choosing:

  • I don't want to break a(nother) gearbox
  • I want it to be safe and reliable
  • It needs to be value for money

We considered selling pre-made aftermarket gearsets. We also considered manufacturing our own gearsets. After disassembling a number of SR20 gearboxes for R&D it was determined that a new gearset is an unviable option due to physical size restrictions of the SR20 gearbox housings, thanks to the power, torque and driving styles of the 21st century. We then moved our focus on to gearbox conversions - the idea is to borrow a strong gearbox from a fast car to meet our above needs. For that, we'll look at the high-powered flagship sports cars from Japan...

Analysing gearbox internals and gearsets



Nissan Z33 350Z / Z34 370Z with VQ35 / VQ37
Truth be told this was our first choice for a gearbox conversion for the Silvia. In fact we went the entire way with the R&D, engineering and manufacturing custom parts to make this beefy 6 speed gearbox fit and work. In the end it "worked" because we made a plethora of parts, and it "fit" because we hammered the chassis the whole length of the transmission tunnel - literally.

Z33 350Z / Z34 370Z 6 speed gearbox in a Nissan Silvia


In the Z chassis it's a great gearbox with 6 big and strong gears. On the bench it's still great, but in an S-chassis (and many issues also applies to the R-chassis) it's greatnesses are its own weaknesses - the gearbox is physically too big to fit in "properly", the shifter is too far backwards, the bellhousing is huge but the flange is thin, no speed sensor...

Too many details cover here, so check out these 3 articles:
Z33 + Z34 6 Speed Gearbox Conversion: Part 1 - Adaption
Z33 + Z34 6 Speed Gearbox Conversion: Part 2 - Making It Fit
Z33 + Z34 6 Speed Gearbox Conversion: Part 3 - Finale



Toyota Supra with 2JZ-GTE
Toyota's use of the GETRAG (V160/161) 6 speed gearbox was a great choice with many racers loving the over-engineered 2JZ engine and Getrag gearbox combination. Many racers are running 9 second quarter mile times on the standard gearbox, so this is one strong gearbox. 6 forward gears is also a huge bonus!

It is therefore a great choice, with good gear ratio's and great strength, but if we are to fit it to the SR20 we will require a custom clutch setup because the splines on the Nissan clutch plate do not match the splines on the Toyota Getrag gearbox.
Additionally the clutch release mechanism needs to be customised, the speed sensor will require adaption and of course the usual shifter, crossmember and tailshaft all require customisation as well. Did we mention the cost of used a Supra Getrag gearbox is between $3500 and $4500... if you can find one. A new gearbox costs over $6000. Because of the huge costs, lack of availability and major customisations required, the Getrag V160/V161 conversion is deemed unsuitable for our needs.


Mazda RX7 with 13B-REW
RX7's did not have a very strong gearbox from the Mazda factory so it does not meet the major reason why we're upgrading in the first place - strength. We scratched this gearbox off the list right away.


Nissan Skyline GTR with RB26DETT
The gearboxes in the R34 GTR is another GETRAG item. These gearboxes are also expensive starting from $3000+ used, very rare, and are designed for 4 Wheel Drive (4WD) - modifying the gearbox to be rear-wheel-drive-only is an expensive exercise. You are required to change the clutch release from a pull to push system and the usual speed sensor/crossmember/tailshaft/shifter needs customising. We chose not to go ahead with this conversion due to cost constraints as well as gearbox availability.

The R32 GTR and R33 GTR shares the same gearbox internals as the RB25DET-equiped Skyline and also the Z32 300zx, except they are in 4WD configuration... the money saved buying a 4WD gearbox you will then spend to modify it for RWD use, not to mention carrying the extra weight of a (now) un-used transfer case. Add on top all the headaches of doing this it's a smarter choice to just look at the rear wheel drive (RWD) equivalent RB25DET-equiped Skylines and the 300zx Z32 gearboxes. See below.

300ZX Z32 VG30 - checking the standard shifter position - too long and will require cutting of the chassis to fit into a Nissan Silvia

300ZX Z32 VG30 - If installed, the STANDARD Z32 shifter comes out here: you must cut out metal



Nissan Skyline R34 with RB25DET
These are the same as R33 RB25DET Skyline gearboxes except the clutch is pull-type as opposed to push-type. Nissan Silvia's use a push-type clutch, so this gearbox is not the best choice, especially when you consider the exact same gearbox is available in push-type form, as found in the R33 RB25DET Skyline, so the R34 version is also scratched off the list.

Skyline R33 RB25DET - checking the shifter position - gearbox is too short and will require cutting of the chassis to fit into a Nissan Silvia

Skyline R33 RB25DET - If installed, the RB25 shifter comes out here: you must also cut out metal



Nissan Skyline R33 with RB25DET
Nissan 300ZX Z32 with VG30DETT

These are internally the same as the R32 and R33 GTR gearboxes without the 4WD transfer case and are known as the FS5W30A. There are numerous drag racers with these cars that consistently run 10 second quarter mile times, proving it is a very strong gearbox. They use the same push-type clutch design as the Nissan Silvia so you can run your standard SR20 flywheel and clutch combo. The speed sensor output is compatible with the Silvia and small (yet important) things like the clutch release and gearbox mounts are all interchangeable. These gearboxes are ticking all the boxes! Now should we go with the SKYLINE R33 or the 300ZX Z32 gearbox?

300ZX Z32 VG30 gearbox with ALPHA OEMGA parts - checking shifter position - excellent!

300ZX Z32 VG30 gearbox with ALPHA OEMGA parts - comes right out of the shifter hole :)



Why Use a 300zx/Z32/VG30 Gearbox Instead of a Skyline/R33/RB25DET Gearbox?
Our research and development has produced a conversion kit that places the shifter in the standard Nissan Silvia shifter hole with the 300zx/Z32/VG30 gearbox whilst having the input shaft inserted and spigot supported, and with the clutch splines engaged better than the rest.

The clutch side is THE most important and also the most difficult area of a gearbox conversion - it is the major reason why we choose to use the Z32 gearbox and retain your flywheel, flywheel bolts, clutch, and clutch release/clutch slave cylinder. No flywheel spacers used and no modifications to it is required.

R33 (RB25) gearbox input shaft VS Z32 (VG30) input shaft


This is a snippet of Alpha Omega Racing's CAD model of the two main contenders; R33 RB25 and Z32 VG30 gearbox input shafts. You can see that the RB25 gearbox is short by 6mm, which doesn't sound like much to most people but to an engineer that is a HUGE amount! Splines are also shorter so they don't fully engage into the clutch plates. What does this mean in the real world? This article has more pictures and real world examples of the problems: Z33 + Z34 6 Speed Gearbox Conversion: Part 1 - Adaption

SIDE NOTE: Clients have come in with 6-9mm thin RB25 gearbox adapters that "do not need bellhousing machining" wondering why their clutches are wearing out after a few months of use, the gearbox is getting louder and louder, selecting gears is getting harder and harder, etc etc... now you know part of the reason is that the RB25 input shaft is naturally already too short AND THEN they space it out another 6-9mm! Although it's more expensive, these are the reasons why we use 20mm thick billet aircraft grade alloy and ask you to machine the bellhousing (our S15-specific kits do not require bellhousing machining).


We also choose the 300zx gearbox because it's cheaper than the Skyline gearbox but features the same strong internals, the gear ratios are the same, 300zx Z32 gearboxes are the same in both non-turbo and twin-turbo variants (our kits suit both) and Z32 gearboxes are generally in better condition, especially the ones from non-turbo 300zx's. The VG30 input shaft length is superior to the RB25 and we have CAD designed and CNC manufactured a shifter setup that puts the shifter in the standard shifter hole - no cutting of your metal :)


In summary, the 300zx/Z32/VG30 is the ideal choice for a Nissan Silvia gearbox conversion when used with the ALPHA OMEGA kit based on:

  • Value for money
  • High strength
  • Excellent fit in an S-chassis
  • Outstanding finish and quality
  • Reliability and safety

ALPHA OMEGA gearbox - 300ZX Z32 VG30 full conversion kit all ready to go (Version 1 shown, now shipping Version 5)



All gearboxes were purchased and analysed in our facilities for R&D except for the two Getrag gearboxes due to their rarity. This article is written in relation to the Nissan Silvia/180sx/s13/s14/s15/200sx/S-chassis.


Copyright ALPHA OMEGA Racing. ALL RIGHTS RESERVED. No part of this article can be reproduced without explicit written consent.