Valve Technology
Beyond the labor-intensive, intake and exhaust port finessing, there are also numerous considerations that involve each particular cylinder head’s valve arrangement. The unique size, angles, and spacing of the cylinder head’s valves are all factors that must be taken into consideration during the porting process as airflow must also be finessed in the violent realm of the combustion chamber as well. The spacing of the intake and exhaust valves relative to the ports is unique to most cylinder heads and the grinding techniques that best unshroud each valve for optimal airflow are also different for each cylinder head.

The shape and angle of the valve seat is also very critical in the effort to improve airflow. In the interest of speed-of-manufacture, most factory valve-jobs feature a single 45-degree angle cut, or a two-angle cut on the seat. Multi-angle valve jobs are expensive for mass production engines that are normally not expected to engage in ultimate performance contests. The three-angle valve job, the universal, performance-engine standard, utilizes an initial throat cut that is about 60-70 degree, which leads to a 45-degree seat cut on the valve. The third and final cut is the top cut that is generally about 10-20 degrees and is made immediately after the valve seat.

The angle of the top cut helps reduce valve shrouding and improves airflow past the valve as the valve begins to lift off the valve seat. In more extreme cases a 5-angle valve job can be employed that uses additional, shallower cuts for the entrance and exit to further enhance airflow. In addition, the surface of the valve head can be swirl polished to help further enhance airflow. The machined swirl pattern is effective in a similar way that dimples help a golf ball travel further, the slight swirl grooves help reduce the boundary layer of air atop the valve and reduce drag.

The valve job is a critical element toward improving airflow at lower valve lift, and performance lower in the rev-range. The basic performance valve job is a highly- effective means of improving airflow when the valve is near the seat, such as at lower valve lift situations. An effective valve job will improve power throughout the rpm range without any drawback or potentially negative effects on durability. Larger valves can often be installed into a head, but the potential gains might be mitigated by a reduction in flow velocity, a thicker, more obstructive valve stem or the fact that the edge of the valve is now closer to the combustion chamber wall.

Another key to executing a successful valve job is the quality of the tool used. The most effective grinding machines represent a significant investment, but produce far more consistent, and precise results, that are balanced between each combustion chamber.

If the valve guides are worn (see page 6-43), replace them (see page 6-44) before cutting the valve seats.
Renew the valve seats in the cylinder head using a valve seat cutter.
Carefully cut a 45o seat, removing only enough material to ensure a smooth and concentric seat.
Bevel the upper edge of the seat with the 30o cutter and the lower edge of the seat with the 67.5o cutter (Intake) or 60o cutter (exhaust). Check the width of the seat and adjust accordingly.
Make one more very light pass with the 45o cutter to remove any possible burrs caused by the other cutters.
Valve Seat Width:

Intake:

Standard (New): 0.85 - 1.15 mm
(0.033 - 0.045 in.)

Service Limit: 1.60 mm (0.063 in.)


Exhaust:

Standard (New): 1.25 - 1.55 mm
(0.049 - 0.061 in.)

Service Limit: 2.00 mm (0.079 in.)




After resurfacing the seat, inspect it for even valve seating: Apply Prussian Blue compound (A) to the valve face. Insert the valve in its original location in the head, then lift it and snap it closed against the seat several times.




The actual valve seating surface (B), as shown by the blue compound, should be centred on the seat.
If it is too high (closer to the valve stem), you must make a second cut with the 67.5o cutter (Intake) or 60o cutter (exhaust) to move it down, then one more cut with the 45o cutter to restore seat width.
If it is too low (closer to the valve edge), you must make a second cut with the 30o cutter to move it up, then one more cut with the 45o cutter to restore seat width.
NOTE: The final cut should always be made with the 45o cutter.