Variation of redline
The acceleration, or rate of change in piston velocity, is the limiting factor. The piston acceleration is directly proportional to the magnitude of the G-forces experienced by the piston-connecting rod assembly. As long as the G-forces acting on the piston-connecting rod assembly multiplied by their own mass is less than the compressive and tensile strengths of the materials they are constructed from and as long as it does not exceed the bearing load limits, the engine can safely rev without succumbing to physical or structural failure.
Redlines vary anywhere from a few hundred revolutions per minute (rpm) (in very large engines such as those in trains and generators) to more than 10,000 rpm (in smaller, usually high-performance engines such as motorcycles and sports cars with pistonless rotary engines). Diesel engines normally have lower redlines than comparatively sized gasoline engines, largely because of fuel-atomization limitations. Gasoline automobile engines typically will have a redline at around 5500 to 7000 rpm. The Ariel Atom 500 has the highest redline of a piston-engine road car rated at 10,600. The Renesis in the Mazda RX-8 has the highest redline of a production rotary-engine road car rated at 9000 rpm.
In contrast, some older OHV engines had redlines as low as 4800 rpm, mostly due to the engines being designed and built for low-end power and economy during the late 1960s all the way to the early 1990s. One main reason OHV engines have lower redlines is valve float. At high speeds, the valve spring simply cannot keep the tappet or roller on the camshaft. After the valve opens, the valve spring does not have enough force to push the mass of the rocker arm, push rod, and lifter down on the cam before the next combustion cycle. Overhead cam engines eliminate many of the components, and moving mass, used on OHV engines. Lower redlines, however, do not necessarily mean low performance, as some skeptics sometimes assume. For example, a Supercharged Buick 3800 V6 with a redline anywhere from 5500 to 6000 has a torque curve that peaks at 2600–3600 rpm, yet the engine is a strong performer from takeoff all the way through to the redline.
Motorcycle engines can have even higher redlines because of their comparatively lower reciprocating mass. For example, the 1986–1996 Honda CBR250RR has a redline of about 19,000 rpm. (Though due to regulations in Japanese motorcycle manufacturing this was later lowered to 18,000). Higher yet is the redline of a modern , but during the 2006 season, engine speeds reached over 20,000 rpm on the Cosworth engine