Gran Turismo 4 Modding Discussion

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So thanks to some developments over the past couple of years by some very talented developers which have given us the ability to unpack and repack GT4's file system, and some more recent research from myself into engine sound formats, the modding potential for GT4 is really starting to come to light. I've been toiling away on my own mod in the background for a while now, and I figured that there's now enough progress to start one of our modding threads for GT4.

I'll kick things off by giving a brief overview of my as-yet-unnamed GT4 mod. I'm basing mine on the NTSC build of GT4 Online, as it's packaged on a single layer disc and has a few nice enhancements compared to the base game, including but not limited to proper 16:9 scaling for fonts in widescreen mode, the online functionality which is now restored thanks to others' work on custom servers, and other minor bugfixes which have been discussed over the years.

To avoid any piracy concerns and keep download sizes small, I'll be delivering my mod as a patch file that users can apply, using xdelta, to their own disc image to make the modded version. All you'll need to do is ensure your disc image is dumped to Redump standards (i.e. an MD5 hash of 3306538778dda2ded87ceaf52c944a98), make a backup, and apply the patch. This system should also let me bundle multiple versions of the mod to deliver some optional aspects that some folks might not want changed in their games, which I'll touch upon below.

Content Porting/Restoration:
  • Mazda MX-5 '05 added and implemented into the Mazda new car dealer
  • Many previously-unused engine sounds have been utilised where appropriate to give cars more realistic sounds (e.g. Autozam AZ-1 was a 4cyl sound before, now using a more realistic 3cyl sample that nothing else used)
  • (Planned) re-adding the cut Roadgoing Formula Cup to the Exterme Hall
  • Recreating* the cut Driving Missions 35 through 40


Tweaks and Fixes:
  • Realistic automatic shifting (e.g. Nissan GT-R Proto, Mercedes SL500) applied to more cars with suitable auto transmissions
  • Some UCD selections and all licence, race, and mission awards changed to shake up the typical career paths a little**
  • (Optional) Endurance races normalised to a more accessible length, e.g. 1-2 hours max
  • (Optional) Removal of those pesky loud horns and crowd cheers on tracks
  • (Potentially) Changing up some race opponents' cars and skill to increase the challenge a little


* - A little background on the cut driving missions - while there are text strings left over describing mostly how these events would have worked, there is no actual data for them left in the game. You may have seen entries on TCRF showing the Chase Battle missions, which describe a game mode where the player has to reduce the time delta between themselves and a faster car to win. As far as I know this isn't actually possible in the versions of GT4 we have, so a few people have made their own interpretations of how these missions would've worked which you may have already seen on Youtube. What I'll be building into my mod is my own interpretation of that set of missions, complete with some sort of prize car for completing 35-40.

** - Mainly to strike a good, enjoyable balance - not like GT4 allowing the classic CLK/RSC cheese strategies, but also not like GT7. Expect things like "National B Silver and the early days' UCD will both offer cars that are good for the beginner/professional halls", things that will change the game up but also not force the player down one specific ideal path.

Sounds:
If anyone has played the GT6 Spec II mod then you'll know the deal here - I want to work through as many cars as possible in GT4, giving them all the most accurate sounds I can find. I'll be sourcing sounds from a number of different places, such as creating porting entirely new sounds across from other games, utilising unused sounds as mentioned above, or even just remixing and reassigning existing GT4 sounds to cars that are more appropriate for them. Bear in mind though that these sounds won't be perfect - I'm working with tiny 100KB per sound PS2-era limits at the end of the day.

I've cooked up a little preview video of what I've got so far:
 
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This is awesome! I'm surprised I hadn't seen this before.

Right now I'm following @pez2k's progress modding GT2, which has fixed many problems I didn't know existed. Some issues, like engine torque data being offset by 500rpm, persisted from GT2 all the way to GT4. I know there are lots of other mistakes, like the gearbox of the Mitsubishi Eclipse. Are you planning to address these kinds of issues, and if so, do you have a list of existing issues, or would you like help compiling one?
 
Finally able to add in regional versions of cars, so I've spent some time adding the Korean cars and the JP-only Mini:
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Mini has to go into the new showroom for now as I'm not sure if the legendary area can easily be hacked in for this dealer.
 
What about adding the Mini to the UCD instead (or in addition to the main dealership)?
 
What about adding the Mini to the UCD instead (or in addition to the main dealership)?
Definitely possible, I'm planning on shuffling at least some parts of the UCD around (not everything since that'd be a ton of work). I'll probably have it in the new dealer as well though, just so people have a chance to try it out rather than waiting and possibly missing it.

EDIT: Never mind, was actually just an oversight on my part, legendary dealer works fine:
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Since we can now more reliably edit the game's data I've started branching out into making fixes and adjustments for certain cars and their upgrades, with some help from fellow car/accuracy nuts like pez2k and Submaniac.

The first of which isn't really a visual change, but now the 1963 Corvette C2 has some more realistic transmission options:
  • The stock 3 speed gearbox has had its ratios made more accurate as PD seemed to be using rather rounded numbers
  • The first upgrade level ("Close" in-game) is now set to the exact ratios of the M20 4 speed gearbox that was available as an option
  • The second upgrade level ("Super Close") is now a 5 speed manual that matches the ratios of the Tremec TKO, a popular transmission swap for muscle cars
  • Full Customise is the same as the game's generic 5 speec with generated customisable ratios

As for a slightly more visual one, the infamous bug where the 1.8T-engined Audi TT behaved as if it were FWD has now been fixed. As a quick demonstration I did a 1000m run before and after the fix, showing a full throttle launch with no TCS:

As you can see the original version lights up the front wheels like crazy, whereas the fixed version now correctly launches with a ton of traction.
 
Wondering if those added/restored PAL cars be used by AI or player exclusive only?
Definitely possible, I'm planning on shuffling at least some parts of the UCD around (not everything since that'd be a ton of work). I'll probably have it in the new dealer as well though, just so people have a chance to try it out rather than waiting and possibly missing it.

EDIT: Never mind, was actually just an oversight on my part, legendary dealer works fine:
View attachment 1238607
 
Re-added and working just fine:

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Apparently it was never set up to show up in the Mini manufacturer event either, so I've added it there too - can't be that much less competitive than the Mini One that could already show up.
 
Excuse my ignorance, but I'm not familiar with the capabilities of modding. Are you mainly swapping files around? Guess what I want to ask is this: Are you able to modify the physics? Seems GT4 doesn't really account for inertia at the rear end of the car. Is it possible to make the vehicle dynamics closer to GT5 or 6 or even 3? Or is that simply too involved or "hardwired" into the game, thus unchangeable without extensive knowledge and time investment?
 
Excuse my ignorance, but I'm not familiar with the capabilities of modding. Are you mainly swapping files around? Guess what I want to ask is this: Are you able to modify the physics? Seems GT4 doesn't really account for inertia at the rear end of the car. Is it possible to make the vehicle dynamics closer to GT5 or 6 or even 3? Or is that simply too involved or "hardwired" into the game, thus unchangeable without extensive knowledge and time investment?
Unless it's something that's controlled by parameters like chassis flexibilty or tire slip angles, it's likely hardcoded and not editable.
 
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Subtle change with a little bit of car trivia attached to it.

This clip of some odd gearing on the MK5 Golf GTI was posted in the modding Discord by Meister:


Some research from @ddm found that it has some strange looking stock ratios - 4th is 1.110, and 5th is 1.100. These values are rounded but also in real life they're the other way around - 4th is actually 1.100 (1.097) and 5th is 1.110 (1.108). The reason for this is that, like some other cars, the MK5 GTI's gearbox actually switches over to a separate output shaft with a different final drive ratio for 5th and 6th gear. This means that gears 1-4 have a final drive ratio of 3.944 while 5-6 have a ratio of 3.086. This explains why the ratios are peculiar on paper but in reality they behave pretty much like any other car, as the change in ratio and FD work in tandem.

The weird part is that GT, including GT4, actually simulates this - a lot of cars such as the Audi A3 and Golf R32s have it set up properly with no weird ratios. Since it's already supported in game and there's plenty of documentation for the gearbox online, I just plugged in the second FD ratio (and used more accurate un-rounded values for each gear while I was there), and it behaves as expected:



The 2006 Eclipse also has a similar issue but its cause is a lot simpler and funnier - PD accidentally applied the second FD to the last four gears as opposed to the last two, so the shift happened at the wrong time and didn't increase the separation between 4th and 5th as expected, so I've fixed that too.
 
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