Most cars i garage cannot upgrade engine at all even if it says clearly lower power than limit for class. More than 50 kW in some cases up to limit.
And many other things also regarding upgrades, immediately shows a warning rectangle that car will exceed class or similar.
The car class thing is a blur, really.
It drains the fun if you cannot use it fully.
Thanks.
Have you tried different combinations of engine components?
For a couple if cars I went through every component not just engine stuff.
Some model I can extend with ARB front or rear.
Some I think allowed some filter upgrade.
I think this was a bunch of C-450 and maybe a few others.
Not knowing how it works you just drop upgrades. But maybe upgrades just make car compete in another class and no other issue. It’s not explained anywhere I found.
I’m not fond of the homoglobation stuff, that I cannot control.
Better with some different presets of upgrades you save and can recall with car competing in different classes.
Most cars in the game will come with a pre-installed upgrade package for whatever homologation division that car is in. Example: If you buy a Lotus Cortina, it’ll come already equipped with the parts necessary to participate in Vintage Sport Compact races. There’s usually not much you can add to the car without putting it over the limits for that Division.
If you want to make your own build from scratch, you’ll need to go into the Tuning and Upgrades → Upgrade Shop → Setup Manager and select Reset Default Upgrades. That’ll put all the real-world factory parts back on the car and reset everything to its default settings. From there, you can upgrade however you want. Keep in mind that if you’re building for a specific Division (Vintage Sport Compact for the Cortina) tire compound, tire width, power, PI, and weight all need to be within the requirements before you can use the car in events for that Division. If you’re just building to a Class (example: D400), you can use whatever combination of parts you like as long as it’s under that PI.
If you ever want to re-install the Forza build/setup, just reset the car to default and choose Quick Homologate from the Upgrade Shop menu.
Where are such things explained one wonders?
I was afraid to buy that homologate thing because just felt it would drop own control over car setup.
Part of the fun is really to do these things too, upgrading, tuning and so forth.
Why isn’t tire pressure and ARB free to upgrade or tune?
Being better and learn to tune with these really nice telemetric available in FM7 is progress and part of the fun. A skill to learn to be top of the board, well TBH upper half of the board in my case as a start. 
I read somewhere, I forgot where, about the numbers in class like C450 it has something to do with performance on some generic track.
I think they went overboard with homologation making each car identical, making it all about driving. I think tuning is a skill too, to be honored!
For people that have been racing every sequel in the Forza it probably says these things you shared in this post at one time or another. But I looked in current FM introduction and the stuff there and it says nothing about these things so you can use it to your advantage. I even looked at FM6 explanations about tuning and did not see this explained as elequently as you did here.
Leaving people in ignorance is just making them bored more quickly.
- people get hooked when they understand how it works
Still loads to learn
Let racing be about all dimension like in real life
Not being so uptight about homologation brings more dimensions to racing like in real life.
- general driving skills
- tuning skills to master car
- optimizing car for a particular track
At least a lot of events driven by a more free classification. But also fun with event that require all american cars, all japanese cars and similar. But don’t drop so many dimensions that it quickly become boring.
Thanks again for taking the time explaining this. 