Hey man,
I actually had started work on a GT4 series myself, using the MC GT4 as the baseline for car performance. I actually got as far as having all the cars spec'd out and really competitive with each other, I just didn't have the follow-through to get the series going.
I had all the cars that run the actual series that are in the game. I replaced the Ginetta with the TVR Sagaris, since the Ginetta is not in the game.
I'll give you my builds though, here they are:
Please note: There will have to be a rule forcing "spec" tuning of the front downforce to 100lbs on all cars, otherwise the Maserati has an advantage becuase it can more than double this on the front and there's no front for the 🤬 thing that doesn't have adjustable downforce, so all the cars have front aero to compensate. It's lame cause the real cars don't really have it, but whatever. Realism < Fun/Close Competition for me.
Anyways, here's the cars/upgrades.
2010 Maserati GranTurismo MC GT4
Upgrades: Race Differential
2010 Ford Shelby GT500
Upgrades:
Platform & Handling-
All Race (including weight)
Driveline-
All Race
Tires & Rims-
Race Tires
Front Tire Size Increase 285/35R19
Rear Tire Size Increase 315/30R19
BBS RK or equivalent weight rims
Aero & Appearance:
Race Front Bumper
Roush Rear Wing
Roush Rear Bumper
Ford Hood
Engine & Power-
Race Air Intake
Race Exhaust
Race Valves
Race Pistons & Compression
Race Flywheel
2011 BMW Z4 sDrive35is
Upgrades:
Platform & Handling-
All Race (including weight)
Driveline-
All Race
Tires & Rims-
Race Tires
Front Tire Size Increase 255/35R18
Rear Tire Size Increase 295/30R18
BBS RS-GT or equivalent weight rims
Aero & Appearance:
Race Front Bumper
Engine & Power-
Race Air Intake
Race Fuel System
Race Ignition
Race Exhaust
Sport Valves
Race Intercooler
Race Oil & Cooling
Race Flywheel
2008 BMW M3
Upgrades:
Platform & Handling-
All Race (including weight)
Driveline-
All Race
Tires & Rims-
Race Tires
Front Tire Size Increase 255/40R18
Rear Tire Size Increase 295/35R18
BBS RE or equivalent weight rims
Aero & Appearance:
Race Front Bumper
Engine & Power-
Race Air Intake
Race Intake Manifold & Throttle Bodies
Race Fuel System
Race Exhaust
Race Pistons & Compression
Race Flywheel
2010 Chevrolet Corvette Grand Sport
Upgrades:
Platform & Handling-
All Race (excluding weight)
Sport Weight Reduction
Driveline-
All Race
Tires & Rims-
Race Tires
Front Tire Size Increase 285/35R18
Rear Tire Size Increase 345/30R19
Gram Lights T57-RC or equivalent weight rims
Aero & Appearance:
Race Front Bumper
Engine & Power-
Race Exhaust
2010 Chevrolet Camaro SS
Upgrades:
Platform & Handling-
All Race (including weight)
Driveline-
All Race
Tires & Rims-
Race Tires
Front Tire Size Increase 295/35R20
Rear Tire Size Increase 315/35R20
Buddy Club P1 Racing QF or equivalent weight rims
Aero & Appearance:
Race Front Bumper
Chevrolet Hood
Engine & Power-
Race Air Filter
Race Intake Manifold & Throttle Bodies
Race Ignition
Race Exhaust
Street Camshaft
Race Valves
Race Pistons & Compression
Race Flywheel
2009 Lotus Evora
Upgrades:
Conversion
Aspiration Conversion: Twin Screw Supercharger
Platform & Handling-
All Race (excluding weight)
Sport Weight Reduction
Driveline-
All Race
Tires & Rims-
Race Tires
Front Tire Size Increase 255/35R18
Rear Tire Size Increase 275/30R19
WedsSport SA-70 or equivalent weight rims
Aero & Appearance:
Race Front Bumper
Engine & Power-
Race Air Filter
Race Fuel System
Race Ignition
Race Exhaust
Street Displacement
Race Pistons & Compression
Sport Intercooler
Race Flywheel
2009 Aston Martin V12 Vantage
Upgrades:
Platform & Handling-
All Race (including weight)
Driveline-
All Race
Tires & Rims-
Race Tires
Front Tire Size Increase 265/35R19
Rear Tire Size Increase 305/30R19
WedsSport SA-70 or equivalent weight rims
Aero & Appearance:
Race Front Bumper
Engine & Power-
Race Exhaust
2010 Nissan 370Z
Upgrades:
Platform & Handling-
All Race (including weight)
Driveline-
All Race
Tires & Rims-
Race Tires
Front Tire Size Increase 275/35R19
Rear Tire Size Increase 295/35R19
Volk Racing TE37 or equivalent weight rims
Aero & Appearance:
Race Front Bumper
Nismo Rear Bumper
Nismo Rear Wing
Engine & Power-
Race Air Filter
Race Intake Manifold & Throttle Bodies
Race Fuel System
Race Ignition
Race Exhaust
Street Camshaft
Sport Valves
Race Oil & Cooling
Sport Flywheel
2005 TVR Sagaris (replacing the Ginetta G50 that is sadly not in this game)
Upgrades:
Platform & Handling-
All Race (excluding weight)
Stock Weight Reduction
Driveline-
All Race
Tires & Rims-
Race Tires
Front Tire Size Increase 275/30R18
Rear Tire Size Increase 275/30R18
Ro_Ja R2-7 or equivalent weight rims
Aero & Appearance:
Race Front Bumper
Engine & Power-
Sport Air Filter
Race Intake Manifold & Throttle Bodies
Race Fuel System
Race Ignition
Race Exhaust
Sport Camshaft
Race Pistons & Compression
Race Flywheel
That should be it. Cheers!