Mechanical limits

Rods, bearings, valvetrain, head gasket and turbo each have a limit. Tuning finds them if you let it.

• Cylinder pressure: follows torque per cylinder, timing near MBT and knock. Rods and bearings carry it. Low rpm under full boost (lugging) is the worst case because the piston is slow and the bearing sees peak pressure for longer.
• Valve float: above the spring's limit the valves stop following the cam. Power collapses, springs break, and a few hundred rpm later a valve meets a piston. Set the rev limiter below the float point; aggressive cams lower it, uprated springs raise it.
• Over-rev: inertia loads grow with the square of rpm. The mechanical limit is not far above the float point.
• Head gasket: sealed by clamping force against cylinder pressure; knock and overheating both unseat it.
• Turbo: shaft speed and turbine inlet temperature. Overspeed bursts the compressor wheel; sustained high EGT creeps the turbine blades.
• Rev limiter cut type: a fuel cut is kindest. A spark cut dumps unburnt fuel into the exhaust where it lights — EGT spikes and the turbine takes the heat.