The relationship between cylinder head airflow and horsepower is very nearly linear. You can replace the intake manifold, exhaust and camshafts, but ultimately, airflow through head determines engine power. Only replacing or re-working the cylinder head will allow your engine to reach its ultimate potential.
Instructions
1. Lay an old intake manifold gasket over your head's intake manifold mounting surface, then spray the assembly with a very light coat of white spray paint. When you remove the gasket you'll find a white ring around the port, which indicates the size of the ridge between the head intake port and manifold runners. Remove the area covered with white paint, and smooth the ridge to about an inch into your intake port. This gasket-matching procedure is fairly simple, and is the only one that you can perform with the head still on the car. If you're gasket-matching without removing the head, stuff a rag into your intake port with a dowel rod to keep metal shavings out of the cylinders. Just don't forget to remove it.
2. Repeat the gasket-matching procedure on the exhaust ports, but leave a 1/8-inch of white-painted material around the port. This step effectively forms an expansion chamber in the exhaust manifold, which creates a slight vacuum to pull expanding gases out of the cylinder.
3. Smooth the bowl area, which is the pocket behind the valve head. The bowl area contains an approximately 1/16-inch-wide ridge where the valve seat mates to the head. This ridge is a huge disruption to airflow, especially at low valve lifts where the airflow must stick close to the bowl wall to go into the cylinder. Run a cylindrical carbide cutter bit or grinding stone around the inside of the bowl area to blend the valve seat into the head. Do not polish this area completely smooth; the slight roughness in the bowl area will create a small amount of turbulence to keep fuel atomized as it enters the cylinder.
4. Remove between 3/16 an 1/8-inch material from the apex of the short-side turn, where the port starts to bend down toward the valve. Then, smooth the removed area about an inch and a half into the floor of port toward the intake port entrance. Smooth the short-side turn into the bowl area in a likewise manner, and repeat on the exhaust side.
