What Does The Valve Job Do?

Before getting into what angles work best for what applications, it’s easiest to work on the basics. First, the valve must seal. A valve that “talks back” or pops out the intake or exhaust manifold is obviously not sealing. The causes could include a burned valve, trash or carbon buildup on the seat, worn valveguides, worn valve stems, bent valve stems, carbon buildup on the valve stems, warped seats, warped valves, and probably a few others. Any one of those would warrant tearing the heads down for a freshening.

Secondly, the valves must control heat. Typical IC engines are less than 20 percent thermally efficient, which means that for every bit of energy released during the combustion process, less than 20 percent is converted to energy propelling the crankshaft. The remaining 80 percent is wasted as heat energy into the engine. Consider that on average, in the middle of the stroke, the valves account for over 10 percent of the surface area inside the combustion chamber and cylinder, and they must be able to dissipate this heat somehow. A percentage of that heat is passed along through the guides, but the rest must be transferred from the valve to the seat and on to the coolant passages before the valve is overheated and “tulips,” or burns.

Finally, the valve job must facilitate the movement of air and fuel into the chamber and be able to get the exhaust out with a reasonable amount of efficiency. It is this final job that, though mechanically is the least important, engine builders and cylinder head specialists focus their efforts on in order to gain that last almighty horsepower.

Parts Of The Valve Job

On the cylinder head, the valve job consists of the seat (which is the portion that actually contacts the valve), a top cut, a bottom cut, and sometimes a secondary bottom or throat cut. Setting the diameter of the throat or “venturi” is also critical. Kaase says: “If that’s not close to being the right size, no matter what you have for angles, it’s not going to be that good. I think maybe too big is probably a little worse than too small. On a race engine, it’s usually 91 or 92 percent of the valve size. If you go .050 bigger it probably wouldn’t be very good. Plus once it’s too big, you’re screwed and you’ve got to put metal back.” On street engines, that number is usually closer to 87-89 percent. For the vast majority of engines, a 45-degree seat angle is used, though old Pontiac fans will remember the 30-degree seats on late-’60s and early-’70s V-8s. The location and width of the seat cut is paramount to the performance of the head. Narrow seats, .045-inch or even less on the intake side, tend to flow better, but aren’t suggested for the exhaust side since they only have about 75 percent of the surface contact area of a typical .060-inch-wide exhaust seat of a given valve diameter, and the exhaust needs that extra area to dissipate heat.

On the valve, there is the seat, which again is the part that contacts the head and is usually accompanied by a back cut. The margin is the small area between the seat and face of the valve aiming into the chamber.