I've been meaning to post this for about a year now, and I'm pretty sure I haven't. My poor tired head.
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Anyway, race modification should have a few levels to it.
Level 1 Racemod would take your car and make it compatible with serious low level racing such as with SCCA Professional League.
Level 2 would spec a car for Rally, BTCC and similar leagues, and would cost more.
Level 3 would go all out and be the most expensive, including wheel base widening, bodykits and chassis adjustments to make the car compatible with JGTC/Super GT and the like.
Its a nice idea, so you can gauge what level of modification you want. If you want to give a car a few improvements e.g sports suspension, a small turbo, and a quick weight reduction you would want a level 1, to get a 'cup car' standard,
If you are more serious you want a real racing machine with a bigger turbo, semi racing suspension, slicks, maybe even a customizable transmission, you want Level 2, so you have a serious kit to match you serious car.
If you want to turn you car into a hard core, heavy tuned racing monster, with racing/original suspension, full weight reduction, full N/A or turbine kit, the works, you want to got for the Level 3.
+rep, for a great idea.
I would like to elaborate on that one a little further.
there are 5 different types of kits all together but not all cars are elegable.
They all have certain attributes which may help them for that specific area, but ultimately some are better than others.
Cup car: Cheap, a few stickers, a roll cage, some interior parts removed, maybe a small rear wing for slight downforce,
stage 1 weight reduction required first.
Touring car: Expensive, full livery (BTCC,GT300,cars of similar stature), new body kit with subtle but important changes, rear wing, many interior parts removed, roll cage.
Stage 2 weight reduction required
GT/Super touring car: Very expensive. The best kit for racing full stop. Full livery, (GT500, GT cars etc.) Full body make over, light new body, with big rear wing producing massive downforce, Splitters on front for downforce, wide arch kit (including increased track) for increase stability and improved load transfer, rear diffuser at the back underbody of the car for more downforce, completely reworked chassis, air intakes, all non essencial interior parts removed, etc. If you want the fastest lap time, this is the one.
stage 3 weight reduction required
Tuner car:Fairly cheep. Not as quick on track as the race modifications but perfect for 1/4 mile and top speed cars, Light weight interior, streamlined body kit to reduce drag, livery according to tuner, spoon, mopar, Alpina, etc.
Lightweight stage 2 required
Note: not all of the kits are designed for top speed this is just an example, kit properties vary from tuning companies
Rally car Expensive. Suitable rear wing for downforce, lightweight body kit, most interior removed, Rally livery.
Stage 2 weight reduction required
You choose specific kits for your car e.g.
Honda Civic (any current model)
Cup car: lightly tuned car.
Touring car: a more seriously modified civic, the civic BTCC would be a perfect example although there are probably GT300 cars which would also be of this standard.
Tuner car: A car civic built for power rather than lap times would go well with the tuner car kit, this would probably be similar to a spoon kit.
Rally car: While I haven't seen a Civic rally car there are no doubt many of them knocking about, so if you have tuned your civic with off-road in mind then this is the perfect kit.
GT/ super touring car: N/A the civics probably wouldn't have this kit since you are very unlikely to find a civic capable of these speeds.
Dodge Viper GTS:
Cup car: Would be a normal GT-S with a few stickers and a small rear wing.
Touring car:N/A
GT/Super touring car: This would be the kit found in the GT series
Rally car: N/A
An additional special kit may be available for certain cars, e.g. a Silvia may have an a special drift kit.
A certain rally car may have a special Pikes peak style kit,
A car like the R390 road car may have a special LMP upgrade to make the car to a Le Mans standard.
These special kits may have Excessive prices that come with the kit but are unique to the car, or only available to a select group of cars.
That is what I would like to see as Race modifications in GT5.