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Getting Started



It is recommend to mod Gran Turismo 4 Online Public Beta as opposed to retail, as retail uses dual-layer discs which cannot be built easily.

Extracting the game

All PS2 GT titles use the volume system, which will need to be extracted in order to gain access to game contents.

If you haven't already, extract the .iso file using 7-Zip or by any other means. You should be able to see a GT4.VOL file.

Using GT4FS, extract the volume file as such

Extracting GT4.VOL
GT4FS extract -r GT4.VOL

This may take a while, but when complete, you should be able to see a new extracted folder that follows this file structure.


This section currently only applies to GT4 Online US (SCUS-97436).

HostFS is a feature normally available on PS2 devkits which allows loading files loose without a volume file. This avoids the need of normally creating a new volume file and ISO file for every change you want to make.

PCSX2 supports HostFS, but games needs to have explicit support for it.

As of 30/09/2023 it is possible to patch GT4 to implement and use HostFS thanks to Nenkai.



Ensure that you have the volume extracted into a VOL_extract folder, next to the ISO file before starting this section.

Creating a patched executable

First, use PDTools.GT4ElfBuilderTool to decrypt and decompress CORE.GT4 into its own game executable.

Converting CORE.GT4
PDTools.GT4ElfBuilderTool.exe CORE.GT4 BASE_SCUS_974.36

Then, use PS2 Plugin Injector to inject GT4Hook.elf into the game executable as such:

Injecting GT4Hook.elf
ps2plugininjector.exe -i GT4Hook.elf -o SCUS_974.36.elf BASE_SCUS_974.36

You should now have a file named SCUS_974.36.elf. Make sure that it sits next to the main game ISO.

Setting up PCSX2


  1. Add search directory to the ELF file
  2. Actually rename the elf file to have the elf extension to show up i.e SCUS_974.36.elf
  3. Right click on the elf entry, properties
  4. Summary -> Disc Path -> Browse -> Select ISO file
  5. Emulation -> Tick "Enable Host Filesystem"


Select your ISO file

  • CDVD -> ISO Selector -> ...

Then, boot through the ELF file (important!)

  • System -> Run ELF... -> SCUS_974.36 (example)

If the game boots, you are good to go and can begin editing files.


Almost every file should now be available to edit loose, but there are exceptions; you can not edit streamed files such as:

  • pss files (videos)
  • es files
  • sqt files
  • ins files
  • ads files
  • Anything in carsound

These sound/movie files needs to be streamed from the PS2 IOP, they will be specifically loaded from the volume still. Build a volume if you need to edit any.

Additionally, the SpecDB is fully loaded at boot, you will need to restart if you are making any changes there.

Packing a volume


Only use the following sections if you are not using HostFS, if you're making sound modifications, or publishing a mod.

After making edits to game files, you'll need to repack the volume. There are two ways described below, the first being the most convenient.

Method 1 - Appending to the volume

You can "pack" a new volume containing your new content by appending the changes to your existing volume. This avoids having to wait for a full VOL build for what may only be a few changed files.

You would need to create a new folder with the edited files, while preserving the original paths. An example:

Packing Example

├─ MyMod/ # Always preserve game paths
  └─ rtext/
     └─ US.rt2
├─ GT4.VOL
├─ GT4FS.exe

Using GT4FS, you would then run the following command:

Appending to the Volume
GT4FS pack-append --read GT4.VOL --append MyMod


Make a backup of your original volume file. It will be edited.

Method 2 - Building a brand new volume

This is the slow method, building an entirely new volume from an extracted folder.

Using GT4FS, you can run the following command to build a volume from a game directory:

Building a brand new volume
GT4FS pack --read <path to extracted volume with edited contents> --output MyNewVolume.VOL

Building an ISO


This part covers games that uses one disc layer such as GT4 Online. For building dual-layer isos, use this section then refer to the next section.

For every change to the VOL, the ISO needs to be built. As this can be a lengthy process, building ISOs on a SSD is highly recommended.

As mentioned previously, GT4 Online Test Version is the best candidate for ISO rebuilding, as it is a single-layer disc and is significantly smaller than a retail copy (2.59 GB vs 5.57 GB).

Two methods are provided:

CDVDGen (Slow Method)


  • ISO Tools, for building ISOs - Should include cdvd2iml5.30, ISOLayerMerge.
    • Install cdvd2iml5.30. This is the tool you will be using to build ISOs.
  • cdvdgen_20 - Cannot be linked directly here. Hint: -> "PS2 SDKs", "Show All" look for CD_DVD-rom Generator ver2.0.
    • Once you have the zip, extract it, and also download UniExtract. It is required to extract the cdvdgen setup, as the setup file breaks on modern windows.
    • Extract with it. You can delete UniExtract afterwards.

Building an IML

First, open cdvdgen.exe (in the data1/Program_Executable_Files folder). Create a new project, select DVD-ROM Master Disc, and drag the game's files onto it with the following layout:

File order
SYSTEM.CNF     # System Configuration file
SCUS_974.36    # Bootstrap executable, may differ per game!
CORE.GT4       # Main game executable, always after bootstrap executable
DNAS310.IMG    # Only if present, GT4 Online versions
IRX            # Folder
NET            # Folder
EPSON          # Folder, optional unless you're.. printing
GT4.VOL        # Always last!

In the Volume tab on the top left, set Disc Name to the proper game code, i.e SCUS-97436 for GT4 Online. Fill the rest with whatever you want.

In File, press Export iml file.


You will have to remove/re-add GT4.VOL to the project and export an IML each time to rebuild the disc.


You should also save the project to speed up future rebuilds.

Actually building the ISO

Open cdvd iml2iso and open the newly created IML file with it.

Press iml2iso and iso creation should start. It takes one minute or so with GT4 Online on a HDD.

Congratulations! You now have an ISO ready to go.

UltraISO (Fast Method)

You can use UltraISO which natively and properly supports building PS2 ISOs. It is however a paid product, so acquire it however you want.

Open your original ISO (preferably a copy of it, always have a backup of the original) into UltraISO, remove GT4.VOL.

At the top left, in Actions, go to Add Files and select the newly built GT4.VOL file. Then simply just save. It'll take a few dozen seconds, but after that, your ISO is ready to go.

Building a Dual-Layered ISO


This is untested.

This applies to retail GT4. Read the "Building an ISO" part first.

The second layer contains all the videos/movies. You are free to extract it with GT4FS if you want to view/edit anything, otherwise leave it as-is.

You will need Apache3 AND UltraISO to build dual-layered discs.

  • Select the original ISO, and extract all the files. On top of the original file structure, you should also have GT4L1.VOL file on top of the original file structure.
  • Move GT4L1.VOL into it's own folder. Using UltraISO, create an ISO of the folder containing it.
  • Create the first layer with the process described before.
  • Using ISOLayerMerge, merge both iso files into one.