BRAKES: The first thing to know about braking in Horizon, is that braking distances are really short, which means brakes aren't as important as they could be.
The main reasons to change your brakes are reducing weight, and adding adjustability.
If you're driving something that just won't stop, it's either your brakes or your tires fault.
There are up to 3 available upgrades.
Street: Fancy new parts that are almost stock.
These rarely change vehicle weight, if ever, they simply offer better braking performance, if only slightly.
A good upgrade in-game, not so strong you over-brake, but better than stock.
Sport: Sometimes reducing weight, other times adding weight. (Particularly old cars with terrible brake systems)
Offers a little more braking than level 1.
Most time I think it's better to go with racing brakes or street, because of the PI system.
The street kit should have most cars stopping fast enough that you're really looking for adjustability beyond this kit.
The racing kit usually doesn't cost much more PI, and significantly reduces weight and adds adjustability.
Race: Almost always coupled with weight loss (20-60lbs typically)
These brakes usually reduce a decent amount of weight, while providing maximum stopping power, they also give you the ability to fine-tune your system.
Keep the weight loss in mind when you see a big chunk of PI taken by them.
The only downside is most of the time your brakes are good enough already, skipping brakes is definitely an option sometimes to use PI elsewhere.
SUSPENSION:
A tricky upgrade, it's easy to dive on the racing suspension and call it a day. But is it worth it, and what sacrifices (if any) must you make to use it?
Street: A basic improvement over anything that doesn't have it, this works pretty well off-road too, so if you're crossing country, or just make mistakes and wind up in the dirt, it's a pretty capable kit for all-purpose.
Sport: Similar to street, an obvious in-between street and race. Better on pavement, still not bad off-road.
(This should have a few adjustable settings including springs, dampers, and camber, but it doesn't)
Race: Fully adjustable, and the only adjustable of the road suspensions. If a car or truck doesn't drive well, you almost have no choice, plus there's a pretty good chunk of performance in a racing suspension, right?
Rally: Fully adjustable, but starts you off with the full "Baha treatment". Ride height jacked, everything soft as a pillow, usually low camber and sometimes low caster.
Drift: It's like racing, but if you purposely make it really hard to drive.
More on that, later.
SWAYBARS:
Another good PI filler is swaybars, but don't underestimate these just yet. I've found many occasions where adding swaybars kept a stock or street suspension in line, keeping the handling tight without sacrificing PI on more suspension.
If you're really maximizing PI, you'll likely run into a circumstance where you can't quite install both swaybars, or can't upgrade both to fit the PI. In these cases, select which to upgrade further based on car characteristics. If oversteer is a problem, you can keep the rear stock and add a racing bar up front. Or, you can keep the front stock and use adjustability on the rear to soften it significantly.
Keep in mind, on rare occasions the rear swaybar takes a bit more PI than the front. (Accurately reflecting the performance gain since the rear bar has a larger effect than the front)
CHASSIS:
Roll cages? I'm not gonna lie, I usually tell myself I can drive out of chassis twist, so more on that, later.
Weight reduction is good. Weight reduction is your friend. But is it overrated in the PI system, like every other game where light builds dominate almost every leaderboard?
Obviously it depends on the vehicle and road layout which is best, but what's best overall, on average?
Is there any "type" that's usually best? (Lightweight build, medium build or heavyweight builds)
TRANSMISSION:
How good your car's factory transmission is will likely play a part in determining which transmission you're getting, but many cars have a transmission option that improves performance, and sometimes allows adjustment, that decreases PI.
Street: Usually just adds an extra gear, other performance is the same. If the engine received camshaft upgrades, the gears may be too long. Stock gears can also be too short, especially if camshafts haven't been upgraded, or large power gains have been made.
Sport: Decreases shift time, adjustable final drive. This means it doesn't matter if the car has long or short gears, as long as an adjustment will fix them you don't need to spend PI on the racing transmission.
Sport often lowers PI or comes with no PI added.
I like the sport option, particularly on cars that take a large PI hit from racing transmissions.
Racing: Fully adjustable, this solves any gearing problem and speeds up shifting more than any other upgrade.
Great for tight tracks, or anywhere lots of shifting is required, and off-road applications. Often, if it doesn't help performance it doesn't hurt the PI.
If the racing transmission doesn't use too much PI, it's always on my car.
Clutch: Aftermarket clutches can save a lot of time in racing, especially for drivers running automatic, or even plain manual without a clutch.
It's a good modification to make, but I wouldn't prioritize this over a better transmission, which usually offers both, quicker shifts and better gears.
Driveshaft: Should reduce drivetrain weight significantly, putting HP gains to the wheels. With a very low PI cost, it's probably not a bad way to go, but works great as a PI filler.
(For example, if you're already at 800PI, you can usually sneak a lighter driveshaft in)
Limited slip differential: At no PI cost to you (none at all) why not?
Especially on 4wd, this can really make your car or truck drive exactly how you want it to.
On older cars, it's almost a necessity to keep the inside wheel from spinning into oblivion.
AERO PARTS:
Aero parts have finally taken a PI hit in FH4, which is exciting. Most cars gain significant performance from added downforce, and the PI system has attempted to address the issue by adding significant PI to many cars, making the decision between speed or handling less one-sided.
Front bumper: Either for looks or for downforce, and sometimes advertised with reduced aero drag.
Adjustable front downforce is key in high speed handling. This can add significant PI, so make sure you'll be driving fast enough to make use of it.
Rear wing: This adds downforce, usually. If a car already understeers it might not be worth it, but for high power RWD cars, it can make a very large difference.