I really like the innovation that the valve spring stiffness is tied to the cam value, reducing one slider that would need to be adjusted.
I also read about the sound issues with higher than 12,000rpm.
However, I would really really like to be able to make super light sports (and economy) cars with small high revving motorcycle type engines.
To do that, more cam would be needed. “100” typically maxes out power at around 9,500rpm.
It would be nice to be able to set the max rpm to 15,000 and have max power at 14,000 like the current 600-1000cc production motorcycle engines do. But it’s fully understandable that this might not happen due to sound generation issues. Even if we can’t have higher rpm, it would still be nice to be able to make power all the way up to 12,000 though 
This has been discussed many times and it will not happen, this is a car manufacturing game and not a motorcycle one. If you can show the devs a PRODUCTION car that has a 15k RPM redline then they might consider it but I do not know of such a car.
Also you can make power up to 12k but it takes a lot of tech points as it should, these kinds of engines are NOT realistic for production use, they would be very very limited runs in the game.
Most high-performance car engines top out between 8000-9000 RPM.
even tuned Honda engines don’t give any more power after 10’500rpm…
Bike engines do use a valvetrain technology that is not represented in the game (because they are not used in cars). Like pointed out above, 12k is VERY generous for cars, and it would have been equally valid to only let people rev engines to 10k, as you can reproduce 99.99% of all production cars ever produced.
