Cylinder Head Lift: What it is, what causes it, and how to prevent it
Cylinder head lift is a phenomenon that occurs when the cylinder pressure in an internal combustion engine causes the head to lift away from the deck surface of the engine. In minor cases, this can result in combustion gasses entering coolant passages. This pressurizes the cooling system and pushes coolant out of the cooling system, which will lead to overheating. In more severe cases, head gasket damage can occur, leading to head gasket failure.
The illustration here depicts a cylinder head that is lifting during the combustion cycle. The red area above the head gasket is the gap created by the lifting. This gap fills with hot combustion gasses and the corrosive air-fuel mixture.
Cylinder head lift is caused by excessive cylinder pressure, which can result from several things, including detonation and/or preignition, improper fuel type, over-advanced ignition timing, improper air-fuel ratio, vacuum leaks, malfunctioning sensors and increased boost pressure.
Detonation, also known as knock, is one of the most common. During normal combustion, the spark from the spark plug ignites the air-fuel mixture, which leads to a controlled burn and normal cylinder pressures. When knock occurs, another flame front occurs outside of the spark-induced flame front, causing uneven burn and creating shock waves. Cylinder pressures spike when this happens, exceeding the pressures the cylinders and head gasket were designed to hold. (Note: If the wrong grade of fuel is used, or fuel has gone “bad,” its resistance to detonation will be lower.)
Over-advanced ignition timing can also cause knock. Modern engines have electronically controlled timing systems that can reduce timing advance if knock is detected. If the wrong grade of fuel is used and the vehicle’s computer does not adequately adjust the ignition timing, knock can still occur, and cylinder pressure will still spike.
High compression ratios and forced induction (by the means of supercharging or turbocharging) are ways to increase either performance or fuel economy. Unfortunately, head lift is a common problem for these applications too. When air is compressed, it heats up. Hotter air entering the cylinder increases the likelihood of detonation. Cylinder pressures are also greatly increased when boost pressure is increased to make more power, even if detonation is not occurring.
Cylinder head lift can vary, dependent on the engine and cylinder head design, and the causes as mentioned above.
To combat head lift, Fel-Pro gasket engineers perform extensive testing on applications that are known to lift heads. Using extremely sensitive instruments, they take precise measurements of cylinder head movement, then design the gaskets to compensate.
As an engine builder, to reduce the possibility and severity of head lift:
- Use the appropriate block and heads for the application, and make sure the surface finish matches the specifications called for (based on gasket material).
- Use the appropriate head fasteners. Always replace Torque-To-Yield head bolts with new T-T-Y bolts. If the engine is going to be used in a performance application, consider the use of high strength head studs or bolts. Always follow the proper torquing instructions for head fasteners. This ensures optimal clamping load.
- Avoid cylinder pressure spikes. You can do this by always using fuel that meets the minimum required octane recommendation, especially in high-compression or forced induction engines. Also, make sure that replacement spark plugs are the correct material and heat range for the application. And if modifying an engine to make more power, take appropriate action to support these modifications.
- Use the right gasket designed for the application.
Fel-Pro PermaTorque®MLS head gaskets are designed on an application-specific basis to combat head lift. They offer excellent recovery—that is, the sealing embossments act as springs to maintain a reliable combustion seal even against head lift. In addition, the LaserWeld™ stopper layer incorporated into many PermaTorque MLS head gaskets offers the strongest combustion seal available. LaserWeld™ also allows for exact compression of the multiple layers of the gasket, reducing the chances of head lift. Over-compression of the embossments is prevented too, allowing this gasket to maintain its seal under the most extreme conditions.