Turbulated or de-turbulated

Bentley, 25/30 and Wraith: Rolls-Royce described the cylinder head fitted to these models as being either "turbulated" or "de-turbulated". The question that is often asked is what is the difference; what is the meaning of "de-turbulated"?

The shape of the combustion chamber in the cylinder head on these engines is a ‘bath-tub’ shape, the size being dictated by the size of the valves, which are arranged side by side, and the size of the cylinder bore. The difference between the "turbulated" and "de-turbulated" cylinder heads lies in the shape of the side walls of the bath tub when viewed in a vertical cross-section.

The earlier cylinder heads were specified with a "turbulated" combustion chamber. In this configuration the width of the combustion chamber was reduced at the mouth of the combustion chamber, a lip being created on the gasket or fire face of the cylinder head. The intention of the lip was to create an element of swirl and turbulence in the incoming charge of the fuel-air mixture on the intake stroke of the engine.

A side-effect of the lip on the combustion chamber, which effectively reduced its cross-section at the fire face, was to shroud part of the inlet valve, and to slow down the rate of flow of the gas entering the cylinder.

For the "de-turbulated" cylinder head the lip around the mouth of the combustion is deleted, leaving it straight-sided. In the case of the Bentley 4 1/4L this had the effect of reducing the compression ratio from 6.8:1 to 6.4:1. In addition the enforced swirl was also lost. However the cylinder filling was substantially improved, and this more than compensated for the potential loss in performance due to the lower compression ratio. By all accounts the loss in combustion efficiency due to the loss of swirl is negligible; it is debatable how effective this was as an enhancement to the engine performance.