Special Section - Flow Dynamics & Valvetrain Science
The valvetrain on a pushrod, overhead valve race engine includes many components that must be selected and installed properly, or you can have a situation that costs you horsepower, durability, or just you (boom-boom engineering!). There are many valvetrain components in a pushrod system. So to help you discuss your needs with engine builders or component manufacturers, we've compiled a listing of basic information about the components that transform the shape of the cam lobe into movement of the valve.
Valves are similar for practically all internal-combustion engines in that they are made of steel, stainless steel, titanium, or other materials and come in variations of the head size, stem length, stem-to-head radius shape, and the angles machined into the face of the valve. The lighter a valve is, the easier it is for the valvespring to control it, so many exotic race engines run lightweight valves that last as long as the steel units but can operate at extreme rpm.
Valvespring Seat Shim
All the valvesprings in an engine should have the same seat and over-the-nose rates. To achieve this, valvespring shims are used to match all the rates. If one spring shows a low rate, shims are slid between the spring and the seat to bring it up. Care needs to be taken to not reduce the spring height so much that the spring binds when the valve is open all the way. On aluminum heads, a steel valvespring cup is needed to prevent the spring from digging in to the relatively soft aluminum.
The valvespring is connected to the valvetrain by the retainer/lock combination. The spring is compressed against the valvespring seat, and the valve stem protrudes up through the middle of the valvespring. The retainer is a flat, round piece of steel or titanium with a hole in the middle to fit over the valve stem. The locks rest between a slot cut in the valve stem and a step in the retainers as the spring is released from its compressed state. Different angles of the cut in the retainer/valve stem are used for different applications. Some valves have a hardened tip pressed on the end of the stem for added durability.
The valvespring is the hardest working component in the entire valvetrain. Its job is to keep the valvetrain following the shape of the cam lobe after the lifter goes over the nose of the lobe. Considering the high rpm that race engines are turning these days, this is a daunting task. A lot of development has gone into making these springs live and do a better job of controlling the valvetrain motion. Springs are usually made of H11 tool steel, chrome silicon, pacaloy, or a similar material. A damper spring plus an inner spring are often used in race engines to achieve the desire spring rate and eliminate harmonic resonance that can render a spring useless at various elevated rpm.
The seat the valve rests on when closed can be cut into the parent material of the head (cast iron), or an insert of stainless steel, beryllium copper, or some other exotic material that can be pressed into a machined step and cut to the proper shape. The valve seat must seal to the valve head tightly but also wick heat from the valve head. The angles cut into the seat are guarded by many head designers and engine builders as they can affect flow considerably.
The top ranks of Stock car racing use what is referred to as a valvespring oiler system to lubricate and cool the valvesprings. This system consists of some tubing plumbed under the valve cover to direct oil onto each exhaust valvespring. These are needed because the dry-sump oiling systems are designed to pull all the loose oil out of the engine, and some of that oil would normally slosh around in the top of the cylinder heads to cool and oil the valvesprings. The oil spray increases valvespring durability considerably.
If an engine didn't have tight-fitting valve-stem guides, we probably wouldn't be able to see the cars on the track through the oil-smoke haze. On cast-iron heads, the parent material is machined to the stem outside diameter, plus a clearance to hold the valve stem solid. On aluminum heads, like these shown here, valve guides are installed with an impact hammer-some shops chill the guides in liquid nitrogen to shrink them so the distortion from installing them is minimized. Small Teflon or Viton valve-stem seals are installed on top of the guides to practically eliminate oil consumption. The valve lift should be checked with the seals in, since most performance applications require that the valve guide be cut down to prevent the retainer from hitting the seal at max lift.