Variation of Redline
Engines with short strokes can handle higher rpm because there is less force in reciprocating motion. Lighter components can increase the redline as well, since they have less inertia and decrease forces present in the engine.
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 ten thousand 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 current Mazda RX-8 has the highest redline of a production rotary-engine road car rated at 9400 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 5500-6000 has a torque curve that peaks at 2600-3600rpms, 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 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 Formula One car. Regulations in 2010 limit the maximum engine rotation to 18000 rpm, but during the 2006 season, engine speeds reached over 20000 rpm on the Cosworth engine.
Nitro engine for RC cars has a "redline" (but in this case is the maximum RPM) of 39500 RPM (it's a 5.9 cc HPI engine).
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