Domed Valve Heads
In racing valvetrains, there is always an imperative to reduce reciprocating mass, as it allows the valvetrain engineer to be more aggressive with his cam profiles in an attempt to improve the valve lift curve, by increasing lift, increasing the area under the curve and so on. This will generally require greater valve acceleration or deceleration.
If we are constrained to using the existing valve design, we may be able to cope with increased forces and stresses due to increased acceleration. But we will need to increase the valve spring load if we are to maintain the same factor of safety against loss of control at the cam nose. This may require a new spring design.
If we can lower the mass of the valve and its associated reciprocating components, we might be able to achieve our aims using the existing valve spring. There are many ways to do this, and one of the most common has been to have domed or dished valve heads.
This solution has been common in both racing and series production engines for some time, and there is little doubt that this measure is effective in reducing valve mass. There are a couple of very good reasons why we might want to reduce valve mass at the head.
The first is that it is the valve head - specifically the seat surface - that we are looking to control. When we design a new cam profile, what we are really looking to do is control the lift of the seat surface of the valve. Decreasing the mass of the head, of which the seat surface forms a part, is therefore an effective way of improving control.
The second reason is that the valve head is the largest part of the valve, and therefore has the greatest scope for mass reduction.
Many in racing, however, have now turned away from domed valves and are looking to other solutions for lighter valves. So what are their reasons for looking for other solutions?
In the case where compression ratio is hard to find, as in Formula One, where the bore-to-stroke ratio is extreme, losing volume from the combustion chamber due to a dished valve means there is often some consequential effect in having to reclaim this volume. Often - and this has also been the case in road applications - the solution is to design the piston to have a raised portion in the centre of the valve pocket.
This makes the piston slightly heavier and leads to higher stresses in the piston. There is also a consequent increase in the stresses in the con rod and crankshaft, albeit a very small one.
If we provide both a dish in the valve and a lump on the piston, we have maintained the combustion chamber volume but increased its surface area. However, this is known to be bad for engine performance, leading to lower efficiency due to higher heat transfer from the combustion chamber.
There is also some evidence from combustion simulation studies to suggest that the combustion in the dish of the valve head is not complete, and combustion efficiency can therefore be diminished by the use of a dished valve head.
Fig. 1 - A high-performance two-valve head, recently rebuilt with dished valves
Written by Wayne Ward