Late 90s/early 00s JGTC Regulations

[Phantom Lord]

Hi there!

I've been watching some older Super GT and JGTC races on YouTube and I've got a few queries about the regulations back then. Incidentally, stuff like this is well worth watching!

As you probably know, the current GT500 regs are aligned with DTM - silhouette racers with no real similarity to their road-going counterparts beyond styling cues.

So, my first question is: in the older JGTC era, to what extent were the GT500 cars related to the production models?

Secondly, with GT300 we currently have a mix of mother chassis cars, FIA GT3 cars and JAF-GT300 cars. For the latter category, my understanding is that these are also silhouette racers, mechanically very different from the road cars.

Same question applies here: were the older cars more closely related to production models? I had this idea that back then, smaller teams used to buy actual road-going sportscars and then convert them to race-spec, but I may have that totally wrong...

I've had a bit of a Google on this but not found any definitive resources. Any info would be much appreciated!

[ss_collins]

Yes the GT500 and GT300 cars were in theory based on the production models up to the introduction of the DTM based regulations, but the rules were rather loose, to the point that by the end very little of the production car was actually left. The JAF GT300 cars (Subaru BRZ, Toyota Prius) still run to these rules.

To highlight how loose the rules were - the Honda HSV010, there was never a production version.

There is loads of archive content coming up here: https://www.youtube.com/channel/UCy-...klAlOq-chcI4GA

Hoping to put out some spotlight videos later in the year detailing some of the classic JGTC & Super GT cars.

[Japanese Samurai]

I don't have 90s/00s JGTC tech regulations.
But, in 2003, JAF-GT500 tech regulations were greatly revised.
*It was accepted that the front and behind of the monocoque could be pipe framed, and you can freely decide suspension style (inbord suspension is allowed).
*Flat bottom is obligatory for underfloor shape.
*You can freely decide where to mount the transmission (transaxle gearboxes are allowed).
*You can freely rotate the engine position as long as it is within the range of the engine room (NSX can use vertical engine).

Therefore, earlier JGTG cars were very similar to production cars.

[canaglia]

Originally Posted by Phantom Lord

Hi there!

I've been watching some older Super GT and JGTC races on YouTube and I've got a few queries about the regulations back then. Incidentally, stuff like this is well worth watching!

As you probably know, the current GT500 regs are aligned with DTM - silhouette racers with no real similarity to their road-going counterparts beyond styling cues.

So, my first question is: in the older JGTC era, to what extent were the GT500 cars related to the production models?

Secondly, with GT300 we currently have a mix of mother chassis cars, FIA GT3 cars and JAF-GT300 cars. For the latter category, my understanding is that these are also silhouette racers, mechanically very different from the road cars.

Same question applies here: were the older cars more closely related to production models? I had this idea that back then, smaller teams used to buy actual road-going sportscars and then convert them to race-spec, but I may have that totally wrong...

I've had a bit of a Google on this but not found any definitive resources. Any info would be much appreciated!

For sure japanese-samurai will be the most expert about this, but as far I remember:
late '90 -> 2002, JGTC gt500 cars were based on street chassis, basically it was some kind of japanese GT class comparable to early '00 GT class of fia GT.
From 2003 -> 2013, during these years, gt500 evolved in a proper silhouette-car prototype class with progressively chassis and aero huge development freedom, coming to the mandatory use of spec and bespoke racing engines as well.
From 2014 -> 2019, gt500 dropped NA engines adopting the 2L turbo NRE and applied to DTM specs, with more or less aero development freedom according to seasons and some waiver like the MR layout for NSX.
This year both DTM and GT500 apply to class-one specs with identical technical specs, infact despite appearences NSX is FR layout.

During years also homologated FIA GT1 specs were allowed in GT500, but just one of them was succesfull (LARK mclaren f1) in late '90.
During 2003-2013 years just few FIA GT1 like murcielago GT1, 550 maranello and DBR9 cars raced, but despite a higher power output, were nowhere close to gt500 cars due much better aero of japanese cars. Not to consider that teams fielding GT1 cars usually had to rely to shitty yokohama or not so great dunlop tyres since bridgestone was dealing only with some gt500 teams.


About gt300, since 2012 gt300 class is basically made by FIA GT3 cars and JAF-GT mother chassis based silouhette cars. Before that season, since late '90 gt300 has always been a mix of FIA gt1/gt2/gt3/gtn cars + various types of prototypes like the mooncraft shiden or the IS350 and customized street based cars like amemiya rx7's or that ford gt with a '90 f1 engine... basically cars without any real spec/regulamentation, it was just bop who kept everything togheter (somehow).

[Phantom Lord]

Thank you all - some very useful info!

[Japanese Samurai]

Sorry, I was misunderstanding.
The inboard suspension was introduced from the beginning of JGTC.
However, since the side members of monocoque remained, it seems that the suspension geometry was limited till 2002.
In 2003, the subframe was changed to a tubular frame, so the suspension layout became more flexible.

[MagVanisher]

By the way, I was wondering how and when GT500 evolved from production-based racecars loosely based on FIA GT1 to silhouette prototypes?

[chernaudi]

I think pre-2003 cars had to be fairly production based. 2003-08 the had to run production car unit bodies between front and rear firewalls. Fore and aft was pretty free as far as using tube subframes with pushrod inboard suspension becoming common.


2009-2013 only the roof and A-pillars had to be production sourced and everyone started running carbon tubs.


2014-present is the current Class 1 rules.

[MagVanisher]

I see. On the other hand, I wonder why JAF decided to allow widebodies in GT500 class in the first place?

Even before GT500 moved to prototypes based on road cars, the cars themselves have wide fenders.

[chernaudi]

I don't know the specifics about that. GT1s were wide body from the GT2 days to now. Even the GT2s were allowed to be 2000mm wide and they were from when they became GT1s, and now GT3 and GTE cars are allowed to be 2050mm wide from 2012.