warning long post
Havent tried them yet but intend too helps when the wheel spin has given me another daytona well at least the cash to buy more
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Some more "non tuning" tips i just don't have time to tune atm due to being back in uni but these always help get people started
Forza 6s tuning lounge section on Forza.net is always helpful due to tuning guides being available and open sources builds that will inevitably pop up plus if you are stuck there is no harm in asking for help
Due to having so many builds adding Raceboy77 or any popular tuner from the tuning lounge on forza.net as followed on sharefront can be useful as you can view the parts they have used as you as you did not purchase others parts before applying it. This can act as a good reference in getting the idea behind how a car should be built for the various classes and specified tracks as when tuning a good build for the car is the first big step before tuning as there is no point in tuning a car that shows little promise (unless for fun for course)
When building I always suggest to at least upgrade tyres compounds a level followed by tyre widths as wide as possible when possible as the most important upgrade regardless of build as the base cars tend to lack grip on stock tyres this does tend to shoot up the PI forcing higher class usage but its is worth it.
All handling upgrades are next besides roll cage should be added as many as possible to allow for tuning with priority on brakes, roll bars and suspension. Roll cage depends on the car at hand a more modern car can normally go with out whereas a classic muscle car will likely need it to stop being such a land boat.
adjustable Aero even on speed builds is always handy as you can just lower the down force as this provides much needed stability if you are ok with making your car look ugly that it.
For transmission it depends on the car the clutch upgrade should be ignored unless there nothing else to add, transmission depends on your car and its current gear ratios some car with older trans like muscle cars heavily benefit however due to the extra gears provided increasing overall top speed . driveline should wait until there like 1-5 PI to spare for chucking out extra weight and diff is mandatory
For engine upgrade ignore camshafts unless nothing else is available as these in the previous games at least it only provides its extra power on very late RPMs. Always upgrade exhaust as its chucks out a lot of weight whilst providing power. ignore ones that add weight like oil and cooling (except aspiration like superchargers) they are not worth it unless they are all you can add left.
Finally swaps are dependant on the car but what I can say is engine swaps will depend on the type of build as higher class builds and power builds will benefit greatly from them if there is PI to spare and same goes for aspiration as you get a lot of power for the PI increased.
Hope that helps