system
Given the following post, respond as if you're commenting on Reddit:
human
How is redline on a car chosen? Why do some cars rev-up to 6k while others go to 9k before hitting redline?
gpt
Comparing two engines with the same capacity, the one with more cylinders allows for lighter moving parts and thus a higher redline.
On a 4 stroke usually the first mechanical limit is the spring load on the valves, the valves not being to return to their position quick enough they might hit the pistons. This is where the rev limiter is placed for safety.
Talking about engineering choices, since power=torque*revolutions, higher rpm mean higher power can be produced (simply because you are burning more fuel per time unit). The engine is designed accordingly, usually there is a trade-in between maximum power and efficiency and fuel consumption. A higher-revving engine can produce more power but when you are driving at low speed, which is most of the time, basically you are keeping the engine in a less efficient part of the torque-rpm curve. This is why for average car engines the engine is designed to have maximum torque (thus efficiency) at the typical cruise speed. Sports engine sacrifice efficiency for performance. As a thumb rule, car engines have maximum torque at approximately 75% of max power rpm.