Redline

It depends on valve train, piston speed and engine design. Diesels usually have lower redlines than petrol engines. Some cars show a variable warm-up redline. Valve float and high piston speeds are the main reasons for the limit. Some modern engines allow higher revs only after reaching full operating temperature.

Beyond the redline forces and valve motion exceed safe limits.

Avoid high revs with a cold engine. The rev limiter usually stops the engine passing the redline.