GTSt modifying guide

by Fuggles
11 years ago

Words Andy Butler Photography Zoe Harrison

Although there is a plethora of Skyline models, apart from the well-known GT-R, we’ve narrowed the focus of this guide down to the R33 GT-S25t turbo variant.  It’s the most plentiful of the rear-wheel-drive Skylines in this country, and it’s the most tuneable version for the money.  If you wanted big power from any of the normally ­aspirated cars, you’d probably start off by adding a turbo, anyway.  Though for this article we have focused on the R33 with much of the ‘equipment’ being common on the R32 and R34 it still makes a useful insight and a good place to start if you want to tune up your R32 GTSt or your R34 GTT

Tuning a GT-S25t can be an expensive business if you want to go for a large chunk of power. Although the turbocharged GT-S has only one turbo (the GT-R has two), there are still six pistons, six rods, two cams, 24 valves and so on. And you can probably guess where this line of reasoning is going. If you want really big power, you’re probably better off starting with a GT-R and going from there. But, if you just want to make your GT-S a bit more lively, read on.


Before you start tuning a GT-S25t, you need to know about the two major hurdles that can cause problems. The static compression ratio is quite high, at around 9.0:1, and the standard turbo will only boost safely to 0.8bar. The ceramic exhaust wheel doesn’t like running high boost, so tweaking up the pressure too high will result in explosive damage. These two facts don’t have much bearing on a first stage of tune, but they will become more relevant once you want a bit more power, which you can achieve by swapping some of the breathing apparatus.


As we know with lots of other Japanese cars, the first thing to do on a Skyline is let it breathe in and out more efficiently.  As well as a cat-back system and a de-cat pipe, the front pipe is very restrictive and should be replaced at the same time as the rest of the exhaust system for best results.   So budget for a quality large-bore exhaust system, de-cat section and front pipe.  Just be aware that some are more resonant than others, so do some research into sound levels to make sure you can live with the noise and, in this day and age, that you can actually get on track!

On the intake side, a panel filter is probably better (and a lot cheaper) than fitting an induction kit, because keeping the original air trunking means that the filter gets a good cold-air feed instead of sitting in the engine bay gulping less dense warm air.  If you do want to fit a cone filter kit, it’s important to make up a cold-air box around the filter to give it a chance to perform well.

But, if you want a full replacement induction it the extra power from the exhaust and filter mods should net you around 20-30bhp and, crucially, give improved throttle response.


With gas flow improved, a set of performance spark plugs as a next step, along with a boost controller.  This is a must for the next stage of development, as it gets around the GT-S25t’s two-stage boost system, which runs 0.65bar normally and can now be wound up for something higher.   The new boost controller allows higher pressure, giving stronger performance at lower revs.   Power output at this stage would be up to 40bhp more than standard at a 0.8 bar setting, a useful increase. The standard output is 220-240bhp.

Going beyond here and maybe you want an ECU upgrade.  The reasoning behind an engine manage­ment change is to compensate for possible det­onation problems that might otherwise occur should the boost be wound up beyond 0.8bar.  The cushion of safety provided by the ECU upgrade makes it a sensible option, and the swap pays dividends should you want to switch injectors or go for more power at a later date, as the re-mapping flexibility is there already.

The next jump in performance gets quite a bit more expensive, because the turbo has to be changed for something capable of blowing harder, while the engine needs some work to address the high compression ratio.  Also, a front-mounted intercooler would be a good idea to help lower intake charge temperature.  But now the standard ECU will be out of its depth, so this will have to be upgraded if that hasn’t already been done. You can sometimes find an R33 GT-R intercooler second-hand or go for a new aftermarket one.  Remember bigger isn’t always better.  The bigger the cooler the more air it needs to fill and the longer it takes to fill al that space with air to put into the engine.

Changing the turbo for a hybrid turbo is a good way to go.  But turbos are a very personal decision and you should decide where you want to get to and whether you want to step up in one big leap or take your time and change out turbines as you go.  Again, bigger is not always better.  The bigger the turbo the greater the lag.

Extra fuelling in the form of an uprated fuel pump and new larger injectors would be required to cope with the higher boost levels.  It would also be advisable to think about stripping down the top end of the motor to fit a thicker 1.6mm steel head gasket before the boost is raised much higher.  With the extra fuel and more boost, around 300bhp would be available with safety, making the GT-S25t a very lively drive, indeed.

That’s about as far as most people will want to go, because attaining further increases in power and torque will start to get very expensive.


Apart from the clutch, which will need upgrading when asked to handle much more power than stock, the GT-S25t transmission seems to be very robust. The rear-wheel-drive system and gearbox can probably handle more than you can afford to throw at it, so you shouldn’t be in for any big bills unless something has been incorrectly serviced or seriously abused.


To tighten the handling response and lower the car a little, maybe fit a set of uprated and lowered springs which work fine with the stock dampers.  After this level you could go to a more complex damper kit a full replacement set of coilovers.  Just be sure you can stand the increased stiffness of the ride when you aren’t driving in attack mode.  Also, don’t make the mistake of going too low on adjustable suspension, and always, always have the geometry accurately reset after fitting.


A set of uprated discs and more aggressive ads should do the job nicely for most types of use.  Or you could go for bigger disks and callipers, often opting for the GT-r standard variant as a away to go.  But if you want more than that maybe bigger rims would give you more room for much bigger disks

Apart from a little work with a drill and tap-and-die set to enlarge the mounting bolt holes, it should be a simple enough fit GT-R disks.  Please note however if you are starting with some models (such as the R33 GTS) it may have four wheelstuds instead of five to hang the disk on to the wheel.  The cost of upgrading brakes depends on how lucky you get at sourcing the second-hand parts. Of course, this leaves you having to pay higher prices for bigger and better pads and discs later on so, if you’re heavy on brakes, it might be worthwhile biting the bullet and going for something like an aftermarket conversion.  This will set you back a fair chunk of cash, but will mean cheaper service items like pads and discs. Oh yeah, and if you want to drive fast you’d better make sure you can stop equally fast!


With big wheel arches to fill and 16in rims as standard, the GT-S can accommodate up to 18s or 19s without problem. Now that wheels and rubber are getting bigger, the prices of 17s and 18s are coming down and choice is good. Wheels are obviously a personal choice and there are plenty of good tyres out there to choose from.

Skylines aren’t that tyre-hungry unless there is something out of whack on the suspension geometry. Just be aware of the increased inner tyre wear on a bigger tyre because of the non-adjustable camber settings. Also, make sure that you get wheels with the correct offset – or you’ll have all manner of problems.


Bearing in mind that a GT-S will probably be running quite a bit less power and torque than a modded GT-R, not to mention less grip, it would be worth losing a bit of weight from the interior if you want to do some decent numbers on the track or strip. Lightweight competition seats will save many kilos over the standard Nissan perches,   I although this might also be going outside the car’s natural character, which is much more grand tourer than balls-out racer.

1 Comment

  • Nizam Amin says:

    TQ so much for this great and easy to read information. I hv been looking for this kind of info since i am in the process of upgrading my R33 GTS25T not to turn it into a huge horse powered ride but more into a lovely and nice ride.

    I have done or ready to do up my car in :
    -full exhaust system (exhaust manifold will be upgraded)
    -induction kit (after market)
    -APEXI boost controller
    -ECU (none) but alternative APEXI AFC Neo is in place
    -front mounted intercooler system already in place(intake manifold will be upgraded)
    – bolt on turbo range 430bhp will be installed
    -fuel pump 044

    As i am still doing the upgrading process, I am wondering what size of the injectors would you suggest for this type of upgrade and if you could suggest the brand for the steel head gasket mentioned.

    Again thanks for a very informative read and looking forward for yr reply. Much appreciated.

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