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Redline

Car culture term

What is Redline?

The maximum safe RPM for an engine, marked in red on the tachometer. Exceeding redline can cause engine damage.

Redline is the maximum safe RPM for an engine, typically marked in red on the tachometer. The redline indicates the upper limit of safe sustained operation; momentary excursions above redline are typically tolerated by modern engine management systems but sustained operation can cause engine damage including bent valves (from valve float), damaged rotating assemblies, or detonation issues.

Different engines have different redlines based on design philosophy. Honda VTEC engines (especially older ones like the B16 and B18C5) typically redline at 8,000-9,000 RPM. The Honda S2000 F20C revs to 9,000 RPM in stock form. Diesel engines typically redline at 4,500-5,500 RPM (lower stress per power output). The Lexus LFA V10 famously revs to 9,500 RPM. Most modern naturally aspirated four-cylinders redline at 6,500-7,000 RPM.

Modern engines have rev limiters that prevent operation above redline. Two-step rev limiters (with a separate launch RPM limit and a redline limit) are common in performance applications, allowing the driver to launch with the engine held at the optimal RPM. Aftermarket rev limiter modifications (raising the redline) require careful evaluation of valvetrain capability, oil flow, and rotating assembly limits.

Related terms

Forged pistonsConnecting rodsCamshaft

Frequently asked questions

What happens if I rev past redline?
Modern engines have rev limiters that prevent operation above redline. Without a rev limiter (or with an aggressive limiter), valve float can occur (valves stuck open), which can result in valves contacting pistons. Rotating assembly failures (broken connecting rods, spun bearings) are also possible. Single brief excursions above redline are typically not damaging; sustained operation will damage the engine.
Why do some engines have higher redlines?
Engine design factors. Lighter rotating assembly (lighter pistons, rods, crankshaft) allows higher RPM safely. Better valvetrain (stiffer springs, lighter valves, dual overhead cam) allows higher RPM without float. Smaller displacement typically allows higher RPM. The Honda S2000 F20C (high redline) is a small-displacement, light-rotating-assembly engine designed specifically for high RPM.
Can I raise my redline?
Sometimes, with caution. Software (tune) can raise the rev limiter. Mechanical capability of the engine determines whether the engine can survive at higher RPM. Common upgrades to support higher redline include valve spring upgrades, lighter valves, and rotating assembly inspection. Without supporting modifications, raising the redline can cause valve float damage.
What is a soft redline?
Soft redline (also called fuel cut redline) is a rev limiter that gradually reduces fuel before completely cutting it. The transition is smoother than a hard redline (which abruptly cuts fuel). Soft redlines are more refined for daily driving but allow brief excursions slightly above the marked redline. Hard redlines are more aggressive and protect the engine more strictly.
What is a launch RPM?
Launch RPM is the optimal RPM for launching the car (typically used in drag racing or quick acceleration starts). The driver holds the engine at launch RPM with the clutch engaged or by using a launch control system. The exact RPM is engine-specific; turbocharged engines often launch around 4,000-5,000 RPM to keep the turbo spooled. Stock cars typically don't have separate launch RPM controls.

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