Puffs of dark smoke when starting an engine or clouds of smoke when the throttle is engaged after a period of idling are often a sign of worn cylinder head seals or valve guides and a signal that the cylinder heads of the engine need reconditioning. Reconditioning is also a standard step when rebuilding an engine for improved performance to help maximize airflow and power.
Worn cylinder head seals and valve guides result in oil consumption due to oil leaking into the engine cylinders, which also causes the telltale emission of smoke from an engine in need of reconditioning. In addition, worn valve guides fail to seal the combustion chamber of the cylinder properly, reducing its internal pressure and combustion efficiency. Reclaiming the horsepower lost from the engine due to poor sealing of the cylinder head is one of the reasons cylinder head reconditioning is important when modifying an engine for increased horsepower.
Tuners who wish to push engines beyond their stock horsepower configuration as part of cylinder head reconditioning projects can accomplish this by cutting the cylinder head valves to achieve better airflow. A common method of cutting the valves is known as a back cut and involves narrowing the angle of the valve face to reduce the amount of work required to lift the valve. Cutting progressively steeper angles into the valve seat in the cylinder itself can also help improve airflow and engine power.