Improving the induction system, e.g. fitting throttle bodies, can greatly increase the airflow into the engine, but beyond a certain level of engine tuning this is no longer enough. As an engine is more highly tuned, and particularly as the CR is raised, it becomes increasingly important to improve gas flow through the head. This doesn’t just mean work on the inlet and exhaust ports but also means changing the valves.
The standard K series cylinder head
The K series engine was first developed with a 1400cc capacity and very few changes have been made to the cylinder head since then, even as the engine capacity has increased to 1600 and then 1800cc. Since the 1400cc engine has 27.7 and 24.0mm valves maths suggests that an 1800cc engine should have 31.4 and 27.2mm valves*. Unfortunately as standard there isn’t enough room. Our larger than standard (29mm inlet, 26.4mm exhaust) valves are the largest that can usefully be fitted on standard seats in front of a standard shaped port. The QED inlet valves also have the further benefit of a wasted stem. This reduces the diameter of the valve stem where it would normally interfere with the gas flow and incidentally makes the valve slightly lighter.
The VVC/VHPD K series cylinder head
There are obviously differences between these two cylinder heads but for the purposes of discussing valve sizes they are the same. In both cases Rover presumably realised that for a high performance engine they would need a different head casting and so the heads for these engines have larger port sizes and come with larger valves as standard. Since the standard VVC sized valve is already big enough* our valves are no bigger, just a different design. As with our 29mm valves the last portion of the inlet valve stem has a reduced diameter to improve gas flow, but since these valves are larger, and with consideration of the stresses on the valve train at high rpm, we have also worked to reduce the valve weight. In the case of the inlet valves, for example, the QED valve is almost 10% lighter than the OE item.