Category Archives: GRB STI

Subaru Impreza WRX STI (JDM GRB)

JDM GRB STI: Rear diffrential – which type?

After conflicting information on the net I finally purchased el-cheapo endoscope camera and took some snaps of the differential.

Pictures are crap but enough to confirm that the diff is Torsen:



What you see is the helical gear shafts (I also confirmed that by spinning the wheel and observed the shafts spinning).
If it was clutch type it would had clutches, shims and cam mechanism visible instead.

In conclusion at least my JDM GRB STI came with Torsen (and it does not look like it was after market installation as all the bolts and seals are pristine).

Replacing Transmission and Differential oil in GRB WRX STI

The oil:
For gearbox/front diff and rear diff I use Motul Gear 300:

Oil Volume:

  • Gearbox and front diff: 4.1L
  • Rear diff: 1.1L

Requires 6L to purchase (or 4L can + left overs from last time), about $250 NZD for Motul Gear 300.

Before start make sure that car sits on four axle stands, and is levelled.
It is good idea to replace all the drain plug washers at same time.



Rear differential:

Relatively easy job.
Tools required:

  • Jack
  • 4x axle stands
  • 17mm socket
  • 19mm deep socket or 22mm deep socket
  • Hand pump or syringe like device

Assuming the car is already on axle stands and levelled…

To drain oil:

Remove drain plug (17mm bolt on the bottom).
Here is both drain plug and temperature sensor removed (used later for filling).

Here are plug and the sensor:

To fill oil:

Remove either 22mm nut with the sensor or sensor itself (19mm deep socket). I didn’t had 22mm deep socket so I removed the sensor. It is better to remove 22mm nut with the sensor as it is easy to over-tighten the sensor itself.
plug the drain hole and pump in the oil via temperature sensor hole, until it starts overflowing.
Remove the hose and let the excess oil drain.

Torque settings I use (found on internet):

  • 50Nm drain plug
  • 50Nm 22mm sensor nut
  • 10-20Nm??? sensor (not very tight!)


Gearbox and Front differential

A little bit harder job than rear diff…

Tools required:

  • T70 Torx bit (cannot do without it).
  • H10 Hex bit
  • 12mm socket
  • flat screw driver or some other device to remove clips (finger nails?)

Hand pump or syringe like device

Assuming the car is already on axle stands and levelled..

Remove the plastic guard (few clips and 2 12mm bolts at the front).
Although it is possible to change oil without removal (there is an inspection window), I removed the guard as it made easier to do so.

To drain oil:

There are two drain plugs, one for front diff (T70) and another for gearbox itself (H10):
Both of the drain plugs have to be removed.

Here are all plugs removed:

To fill oil:

Tighten both of the drain bolts.

Torque settings:

  • T70 Plug 70Nm
  • H10 Plug 50Nm

Unlike previous STIs the GRB STI does not have dipstick. Some might think this makes it worse, I think otherwise, as I don’t have to remove inter-cooler to change gear oil. To get correct amount of oil one must fill through side filler plug while the vehicle is levelled.
Here is the said filler plug (H10):

Using hand pump (or otherwise), oil is filled until it overflows through the filler hole:

another angle:

Once oil is overflowing remove filler tube and allow it to drain.
Tighten the filler plug to 50Nm.

Bolt/Clip on the plastic guard.

Tuning GRB STI for 95 RON

Due to scarcity of 98 Octane (RON) in New Zealand, beyond major centres, I got my WRX STI tuned for 95.
Tuning has been done by David Wallace ( I chose David because I had really good experience with him tuning my ST165 Celica GT4 (when he was with torque performance), as well as he has really good reputation.

Thanks to Subaru open ECU interface the process was very painless. The most difficult part of this tuning process is getting fuel tank empty to ensure it has 100% 95RON.

Basically David uploaded new tune at my place (via OBDII connector), and we went for a drive while David was monitoring the sensor output. After some minor adjustments the final tune was uploaded at the end of the drive.

The whole process took a little bit over an hour.

I am very happy with new tune, as not only I can safely fill up with 95 RON (although it is still better to run on 98 RON as ECU will take advantage of that), but it actually performs better.

Highly recommend doing this for every JDM GRB STI in New Zealand (even if it stock like mine).

GRB STI TGV deletion

Q. What is TGV?
A. Tumble Generator Valve.

The purpose of TGV is to create swirl/tumble of air-fuel mixture to improve emissions at certain conditions.
TGV consists of butterfly valves located before the injector in each intake runner. These valves when closed force air through a small passage.

Generally under normal operation these valves are open. The main reasoning of TGV removal is small performance gain due to removal of restrictions from open butterfly valves. As bonus it will also increase reliability by making the system simpler.

One thing I noticed: since removal (even before tune) the “hole” around 3000rpm during warm-up has disappeared.

The TGV system on GRB STI consists of following parts:
Two TGV position sensors.
Two TGV actuators.
Four butterfly valves connected in pairs on each bank.

It is located under manifold, in the spacers between heads and the manifold. The cars equipped with TGV can be recognised by having sensors and actuators attached to the sides of manifold spacers.

Here is the TGV in all of its glory (the black spacer is Zerolift TGV deletion “kit”):

The exercise of TGV removal (aka TGV deletion) for me was a “by the way” thing as I was getting my car retuned anyway (I needed to run 95 octane safely as in New Zealand beyond main cities 98 octane is very scarce).

Due to difficulty of locating the genuine spacers without TGV (these exists and normally found on 2004-2007 WRX STI), I settled for plastic Zerolift TGV deletion kit.
A bit of warning on Zerolift TGV deletion kit: because they are plastic THEY MUST BE TIGHTENED TO CORRECT TORQUE! Which is very low, and feel little bit tighter than finger tight. They will crack otherwise!
Once I reach 100,000kms and have cam belt due, I will replace them with genuine spacers.
Another thing is because my STI is JDM the Zerolift kit didn’t fit correctly (the middle holes didn’t line up), I had to modify the manifold to fit (slot the holes):

The Zerolift TGV comes with O-rings, and these must be installed evenly (a bit of exercise for fingers). Do not use manifold gaskets!

At the same time I decided to do the spark plugs, I settled for LFR7AIX (one range colder, iridium). Note: unlike previous EJ20, EJ207 (at least on WRX STI) has long reach spark plugs!

Now to the process (it was my first time working on STI)…

Intercooler and battery out:

Remove BOV:

Remove earth lead and fuel lines:

Remove coolant bottle:

Remove air pump hoses:

Remove air box:

Remove alternator:


The main loom will need to be disconnected from the plug near firewall, and then after unbolting and unclipping numerous hoses the manifold comes straight up (almost):

More of TGV:

Now for the spark plug change.
The access if fairly limited but no need for special tools or jacking or pushing the engine:

The coil is held by 12mm bolt and comes out with a bit of wiggling.
The coil:

The spark plug:

Back to the TGV.
Assembled manifold with TGV deletion kit:

Make sure the PCV valve is connected back (easy to forget as it is right under the throttle body):

All together (bar alternator):

Since spacers are plastic I decided to add an earthing wire. I have also moved original earthing wire to the head (just on top of the AVS solenoid). I had to drill a hole on the A/C pump bracket and cut thread as I found no suitable place for the new earth wire:

Starting the car without TGV brought up numerous non-TGV related error codes. To test that they are false positives I have connected the original spacers back:
It turns out that they definitely were false positives. Interesting that ECU would only trigger TGV error codes if TGV was partially missing.

To get it ready for new tune I had to drain the fuel tank. The reason for draining as it was filled up with 98 RON, and I needed for tune to be safe for 95 RON. Unfortunately I found the hard way that you cannot simply remove the return line and run the car until it stops, due to shape of the tank. It would only empty the fuel pump side this way (there is a siphon between sections which is feed by return line).
To drain the tank completely I had to remove fuel pump and the cover on other side. Then I had to siphon both sides (very difficult on such low car), as GRB is missing drain plug on the fuel tank.
Here is what fuel pump (housing) looks like:

Enabling side vents on GRB WRX STI

The GRB WRX STI comes with side vents from factory.

Unfortunately they are not functional and there only for aesthetics, as you can see on this pic:
The vent cover is plastic, held in place by clips. It can be pulled of with minimal disassembly by unscrewing/unclipping inner guard on the bottom, and unclipping the side skirt in the vent area.

I used three power tools to expand and create holes:
1) Air powered nibbler (bought it from supercheap):
2) Dremel with metal cut-off disk:
3) 13mm drill bit (for starting point for nibbler).

All of these could be done with just dremel, but making it clean would be difficult.

Here it is cut out and cleaned up:



Not pictured: inner side of plastic vent cover has a rubber flap, I removed it as well.


In the end I am happy with this little mod, even by having radiator fan on on stationary car is enough to feel warm air coming from the vent.