Hey brother. Decided to hop over from the other post and offer some thoughts, as I’ve been tuning and racing for years, and indeed the way Forza ratings work can be challenging.
The main thing to remember is that this is a game based very largely on numbers, and they usually don’t lie. I’ll go into number translation later, but suffice to say that tuning (note: tuning, not upgrading the parts) is at least 50% as important as the upgrading itself, and in many cases more important (I believe this is one of them). As well, power (engine parts) and weight aren’t technically at the top of my list when it comes to upgrading. I’ll admit that perhaps I’m wrong on those being less important, but my approach has worked well for me for about a decade, and I used to play the Forza Motorsport series on a fairly high difficulty. At the moment, I play FH4 on Highly Skilled, but I can play at a higher difficulty. I just don’t, and it’s mainly due to there not being a way to effectively playtest and tune a car in real-time on an actual track.
HINT TO TURN 10: ADD THAT FEATURE FROM FORZA MOTORSPORTS
Anyway, the first things that I change are drivetrain parts for better shifting and acceleration (especially the differential), then brakes and sway/roll bars, rear spoiler (better handling, plus lowers Performance Index slightly), then tires (mainly just the compound if it doesn’t push PI too high, and tire width), and THEN I’ll upgrade engine parts until I reach the desired PI. I don’t usually touch weight until later on. My reason for this approach is that my primary concern has always been addressing handling first–namely, I’ll put up better lap times by going into and getting out of turns faster (also note that manual gear shifting can assist with deceleration while turning). This approach started as a way of offsetting my inexperience with different drivetrains (AWD vs FWD vs RWD), although that distinction is far less relevant now than it was back in FM2/3. Plus I’ve noticed that it’s just a great general approach to tuning.
There are obvious exceptions, such as upgrading for Rally, in which case tires and suspension are first. That said, try that approach out and see if you have better luck in those low-end races. Sometimes, though, some cars are just better. /shrug
Also, before I even get into the car I start by adjusting the final drive 0.05 at a time to maximize acceleration (I’m watching to see if the numbers improve and by how much), then adjusting the brakes. After I drive it around a bit, I’ll make tweaks to the differential, damping, anti-roll bars and springs (usually in that order) to tweak understeer vs oversteer.
When it comes to looking at the specific index numbers, I just focus on those numbers. I almost never look at the actual weight, HP, torque, displacement, etc, and I don’t even look at the speed/braking numbers until I actually go to tune the car.
As far as losing to a WRX, it’s difficult to say why you’re losing to them, but it is a good car. In general, I think the computer usually (or mostly) just adds raw power to get the PI up for a race. I could be wrong, but I don’t know how to check it reliably.
I’d probably add that there’s nothing wrong with using someone else’s tunes. In fact, it can help to give some clarity about what to try with your own tunes (assuming you can actually see the specific tune specs, so if I’m wrong, then nevermind). Tuning is pretty time consuming, and it can be difficult to notice what might be wrong when it comes to understeer/oversteer. And again, given that there’s not a great system for tuning while testing like there is/was in Forza Motorsport, and given that we get so many cars in FH4, manually tuning isn’t always practical for everyone.
Moving on, the relationships between power & weight are what roughly become the index ratings, but the index ratings are broad descriptions.
Power/HP: the raw engine horsepower, as calculated by the engine size, aspiration, and any engine upgrades
Speed: a general rating for the top speed of the car
Handling: a general rating for the ability to turn, as well as weight distribution
Acceleration: how quickly the car will acceleration from gear to gear
Launch: the “off-the-line” rating starting from 0
Braking: general braking efficiency
Something to note is that the index ratings do not necessarily translate from car to car. Lots of high-end cars will have a launch and acceleration of 10, but can have fairly different 0-60 and 0-100 speeds, as well as the top speed. There’s a similar line of reasoning when looking at, say, handling from car to car, because drivetrain can have a moderate performance on the actual handling–which, as said above, will be more significantly affected by how you tune the car.
Anyway, all of that to say: focus more on the index numbers rather than the raw power, torque and weight. These will tend to only be important once you’re in much higher end cars (S2 and X). Some cars won’t be very good at lower index ratings, and some will. Much of the differences can be offset by proper upgrading and tuning.
Hope this helps!